What Is Yeast?

Yeast are a group of unicellular fungi a few species of which are commonly used to leaven bread and ferment alcoholic beverages. Most yeasts belong to the division Ascomycota. A few yeasts, such as Candida albicans can cause infection in humans. More than one-thousand species of yeasts have been described. The most commonly used yeast is Saccharomyces cerevisiae, which was domesticated for wine, bread and beer production thousands of years ago. See Yeast (baking).

Yeast physiology can be either obligately aerobic or facultatively fermentative. There is no known obligately anaerobic yeast. In the absence of oxygen, fermentative yeasts produce their energy by converting sugars into carbon dioxide and ethanol (alcohol). In brewing, the ethanol is used, while in baking the carbon dioxide raises the bread and the ethanol evaporates.

An example with glucose as the substrate is

C6H12O6 (glucose) →2C2H5OH + 2CO2 Yeasts can reproduce asexually through budding or sexually through the formation of ascospores. During asexual reproduction a new bud grows out of the parent yeast when the condition is right, then after the bud reaches an adult size, it separates from the parent yeast. Under low nutrient conditions, yeasts that are capapable of sexual reproduction will form ascospores. Yeasts that are not capable of going through the full sexual cycle are classified in the genus Candida.

Yeasts for leavening bread may be produced commercially or caught from the environment. Many yeasts can be isolated from sugar-rich environmental samples. Some good examples include fruits and berries (such as grapes, apples or peaches), exudates from plants (such as plant saps or cacti). Some yeasts are found in association with insects.

The use of potatoes, water from potato boiling, eggs, or sugar in a bread dough accelerates the growth of yeasts. Salt and fats such as butter slow yeast growth down. A common medium used for the cultivation of yeasts is called potato dextrose agar (PDA) or potato dextrose broth. Potato extract is made by autoclaving cut-up potatoes with water for 5 to 10 minutes and then decanting off the broth. Dextrose (glucose) is then added (10 g/L), and the medium is sterilized by autoclaving.

Yeast fermentations comprise the oldest and largest application of microbial technology. They are used for beer and wine fermentations and bread production. Beer brewers classify yeasts as top-fermenting and bottom-fermenting. This distinction was introduced by the Dane Emil Christian Hansen.

Top-fermenting yeasts (so-called because they float to the top of the beer) can produce higher alcohol concentrations and prefer higher temperatures. An example is Saccharomyces cerevisiae, known to brewers as ale yeast. They produce fruitier, sweeter, real ale type beers. Bottom-fermenting yeasts ferment more sugars leaving a crisper taste and work well at low temperatures. An example is Saccharomyces uvarum, formerly known as Saccharomyces carlsbergensis. They are used in producing lager-type beers. Brewers of wheat beers often use varieties of Torulaspora delbrueckii.

Winemakers use a variety of different yeasts depending on the type of wine and the condition of the grapes. Too high a sugar or alcohol concentration slows the growth of yeast, so for very ripe grapes with lots of sugar he or she would use a yeast tolerant of those conditions. If the yeast dies before all the fermentable sugar has been converted to alcohol, the result is a "stuck" fermentation. Some yeast is chosen because it tends to develop certain aromas, such as the distinctive "banana" smells of Beaujolais from Georges Duboeuf. Wild yeast are naturally present on the skins of grapes, so grape juice will spontaneously ferment unless the wild yeast are arrested by cold temperature or sulfates. Depending on the strain of indigenous yeast, the result may be unpalatable or possibly more complex than if a single cultured strain were used. In general, natural yeasts are riskier than cultured, and tend to be used by tradition-oriented, Old World-style winemakers.

Saccharomyces cerevisiae is also known as budding or baker's yeast. It is used as a model organism by biologists studying genetics and molecular biology (in particular the cell cycle) because it is easy to culture but as a eukaryote, it shares the complex internal cell structure of plants and animals.

Saccharomyces cerevisiae was the first eukaryotic genome that was completely sequenced. The yeast genome database [1] (http://www.yeastgenome.org/) is highly annotated and remains a very important tool for developing basic knowledge about the function and organization of eukaryotic cell genetics and physiology. Another important S. cerevisiae database is maintained by the Munich Information Center for Protein Sequences [2] (http://mips.gsf.de/genre/proj/yeast/index.jsp).

Another important experimental model is Schizosaccharomyces pombe or fission yeast.

Candida albicans, a diploid sexual fungus (a form of yeast) is the causal agent of opportunistic infections in humans, the most common being oral and vaginal infections. Systemic fungal infections have emerged as important causes of morbidity and mortality in immunocompromised patients (e.g., AIDS, cancer chemotherapy, organ or bone marrow transplantation). In addition, hospital-related infections in patients not previously considered at risk (e.g. patients on an intensive care unit) have become a cause of major health concern.

Among the many organisms that live in the human mouth and digestive tract is the yeast Candida albicans, which under normal circumstances lives in 80 percent of the human population with no harmful effects. However, overgrowth results in candidiasis. Candidiasis is often observed in immunocompromised individuals such as HIV-positive patients. Candidiasis also may occur in the blood and in the genital tract. Candidiasis is commonly known as "thrush", and is a common condition that is usually easily cured in people who are not immunocompromised. To infect host tissue, the usual unicellular yeast-like form of Candida albicans reacts to environmental cues and switches into an invasive, multicellular filamentous form. This switching between two cell-types is known as dimorphism.

In a process that superficially resembles dimorphism, Candida albicans undergoes a process called "phenotypic switching", in which different cellular morphologies are generated spontaneously. One of the classically studied strains that undergoes phenotypic switching is WO-1, which consists of two phases - one that grows as smooth white colonies and one that is rod-like and grows as flat gray colonies. The other strain known to undergo switching is 3153A; this strain produces at least seven different colony morphologies. In both the WO-1 and 3153A strains, the different phases convert spontaneously to the other(s) at a low frequency. The switching is reversible, and colony type can be inherited from one generation to another. While several genes that are expressed differently in different colony morphologies have been identified, some recent efforts have focussed on what might be controlling these changes. Further, whether there is a potential molecular link between dimorphism and phenotypic switching is a tantalizing question.

In the 3153A strain, a gene called SIR2 (for silent information regulator) has been found that seems to be important for phenotypic switching. SIR2 was originally found in Saccharomyces cerevisiae (brewer's yeast), where it is involved in chromosomal silencing - a form of transcriptional regulation in which regions of the genome are reversibly inactivated by changes in chromatin structure (chromatin is the complex of DNA and proteins that make chromosomes). In yeast, genes involved in the control of mating type are found in these silent regions, and SIR2 represses their expression by maintaining a silent-competent chromatin structure in this region. The discovery of a Candida albicans SIR2 that is implicated in phenotypic switching suggests that it too has silent regions controlled by SIR2, in which the phenotype-specific genes may perhaps reside.

Another potential regulatory molecule is Efg1p, a transcription factor found in the WO-1 strain that regulates dimorphism, and more recently has been suggested to help regulate phenotypic switching. Efg1p is expressed only in the white and not in the gray cell-type, and overexpression of Efg1p in the gray form causes a rapid conversion to the white form.

So far there are few data that says that dimorphism and phenotypic switching use common molecular components. However, it is not inconceivable that phenotypic switching may occur in response to some change in the environment as well as being a spontaneous event. How SIR2 itself is regulated in S. cerevisiae may yet provide clues as to the switching mechanisms of Candida albicans.

One of the most interesting features of the Candida albicans genome is the occurrence of numeric and structural chromosomal rearrangements as means of generating genetic diversity, named chromosome length polymorphisms (contraction/expansion of repeats), reciprocal translocations, chromosome deletions and trisomy of individual chromosomes. These karyotypic alterations lead to changes in the phenotype, which is an adaptation strategy of this fungus. These mechanisms will be better understood with the complete analysis of the Candida albicans genome.

The Candida albicans genome is being sequenced at the Stanford DNA Sequencing and Technology Center. Funding for this project is provided by National Institute of Dental and Craniofacial Research and Burroughs-Wellcome Fund. A pilot sequencing program is also being carried on by The Sanger Center. Funding for this project is provided by Beowulf Genomics.

Saccharomyces cerevisiae is a species of budding yeast. It is perhaps the most relevant yeast for mankind, both for its use since ancient times in baking and brewing, and for being one of the most intensively studied eukaryotic model organisms in molecular and cell biology.

Saccharomyces cerevisiae was the first eukaryote to have its genome sequenced (published in 1996). The genome is composed of about 13,000,000 base pairs and 6,275 genes. It is estimated that yeast shares about 23% of its genome with humans.

Schizosaccharomyces pombe, also called "fission yeast," is a species of yeast. It is used as a model organism in molecular and cell biology. It is a unicellular eukaryote, whose cells are rod-shaped. These cells maintain their shape by growing exclusively through the cell tips and divide by medial fission to produce two daughter cells of equal sizes.

Fission yeast was isolated in 1893 by Lindner from East African beer. The species name is derived from the Swahili word for beer (Pombe).

The sequence of the S. pombe genome was published in 2002.

The fission yeast researcher Paul Nurse won the 2001 Nobel Prize in Physiology or Medicine, together with Lee Hartwell and Tim Hunt, for work on the cell cycle.

Yeast, any of a number of microscopic, one-celled fungi important for their ability to ferment carbohydrates in various substances. Yeasts in general are widespread in nature, occurring in the soil and on plants. Most cultivated yeasts belong to the genus Saccharomyces; those known as brewer's yeasts are strains of Saccharomyces cerevisiae. For a discussion of the physiology and reproductive behaviour of yeasts, see Fungi: Classification: Ascomycota.

Yeasts have been used since prehistoric times in the making of bread and wine, but their cultivation and use in large quantities was put on a scientific basis by the work of the French microbiologist Louis Pasteur in the 19th century. Today they are used industrially in a wide range of fermentation processes; medicinally, as a source of B-complex vitamins and thiamine and as a stage in the production of various antibiotics and steroid hormones; and as animal feed and foodstuff for humans.

Pure yeast cultures are grown in a medium of sugars, nitrogen sources, minerals, and water. The final product may take the form of dried yeast cells, or the yeast may be pressed into cakes with some starchy material. When a batch of yeast for baking, medicinal, or food purposes is completed, the medium in which the yeast was grown is discarded. In the making of wines, beers, spirits, and industrial alcohol, however, the fermented medium is the desired product, and the yeast itself is discarded or used to make animal feed. See also Brewing.

A baker's yeast is a yeast culture which has been bred specifically for the purpose of leavening breads.

Members of the Division Ascomycota are known as the Sac Fungi and are fungi that produce spores in a distinctive type of microscopic sporangium called an ascus (Greek for a "bag" or "wineskin"). This monophyletic grouping was formerly known as the Ascomycetae and is an extremely significant and successful group of organisms (12,000 species in 1950), accounting for some 75% of all described fungi. Included are most of the fungi that combine with algae to form lichens. The majority of fungi that lack morphological evidence of sexual reproduction are placed here. Better known examples of sac fungi are yeasts, morels, truffles, and Penicillium. The majority of plant-pathogenic fungi belong to this group, or the related Deuteromycota. Species of ascomycetes are also popular in the laboratory. Sordaria fimicola, Neurospora crassa and several species of yeasts are used in many genetics and cell biology experiments.

An ascomycete produces great numbers of asci at any one time, and these may be contained in a structure called an ascocarp. Each ascus contains eight (or a multiple of 8) ascospores, the result of one round of mitosis following meiosis. The resulting haploid nuclei are surrounded by membranes (from the plasma membrane in Euascomycetes; from the nuclear membrane in Hemiascomycetes) and eventually a spore wall.

An exception to the structure described above are the yeasts, which are secondarily unicellular.

An ascospore is a spore contained in an ascus or that was produced inside an ascus. This kind of spore is specific to ascomycetes.

Yeast are small single cell organisms. It is everywhere you can imagine. When making beer the yeast we are referring to is Saccharomyces cerevisiae for ales and Saccharomyces uvarum for lagers. There are many strains of this yeast to obtain different flavor profiles. There are other types of yeast and bacteria that one can use in brewing. Lambic styles call for some different bacteria that wouldn't normally be used in making beer, thus lending the sour taste that lambics are known for. Yeast can be purchased in dry or liquid suspension forms. The liquid yeast gives the brewer more control over the flavor profile. You can also culture your own yeast from starters or obtained from other beers.

Ale yeast, which is top fermenting, tends to make a soft or sometimes fruity beer, while lager yeast, which is bottom fermenting, can make it dry and crisp. Ales ferment at higher temperatures (66 - 74° F) than do lagers (45 - 65° F). The lower temperature prevents the fruity flavors from being absorbed.

You may find some discrepancies in the yeast names. In some places the name Saccharomyces carlsbergensis is used. This is just an older name for Saccharomyces uvarum. In other places, you may not see either the calsbergenesis or uvarum names. Recently the powers that be did some regrouping of the names and put them together under cerevisiae.

As yeast feeds on the sugars in the wort they reproduce. After fermentation starts a layer of foam will be present in the fermenting vessel. This will soon turn into a rocky, dirty looking head that can vary in size. A lot of carbon dioxide is being produced and if the fermenter is not able to expel this it can explode. A blow off tube may be a good idea if you have high gravity wort.

Yeasts are single-celled fungi. As fungi, they are related to the other fungi that people are more familiar with. These include edible mushrooms available at the supermarket, common baker’s yeast used to leaven bread, molds that ripen blue cheese and the molds that produce antibiotics for medical and veterinary use. Many consider edible yeast and fungi to be as natural as fruits and vegetables.

Over 600 different species of yeast are known and they are widely distributed in nature. They are found in association with other microorganisms as part of the normal inhabitants of soil, vegetation, marine and other aqueous environments. Some yeast species are also natural inhabitants of man and animals. While some species are highly specialized and found only in certain habitats at certain times of the year, other species are generalists and can be isolated from many different sources.

Baker’s yeast is used to leaven bread throughout the world and it is the type of yeast that people are most familiar with. Baker’s yeast is produced from the genus and species of yeast called Saccharomyces cerevisiae. The scientific name of the genus of baker’s yeast, Saccharomyces, refers to “saccharo” meaning sugar and “myces” meaning fungus. The species name, cerevisiae, is derived from the name Ceres, the Roman goddess of agriculture. Baker’s yeast products are made from strains of this yeast selected for their special qualities relating to the needs of the baking industry.

The typical yeast cell is approximately equal in size to a human red blood cell and is spherical to ellipsoidal in shape. Because of its small size, it takes about seven billion yeast cells to make up to one gram of compressed baker’s yeast. Yeast reproduce vegetatively by budding, a process during which a new bud grows from the side of the existing cell wall. This bud eventually breaks away from the mother cell to form a separate daughter cell. Each yeast cell, on average, undergoes this budding process 12 to 15 times before it is no longer capable of reproducing. During commercial production, yeast is grown under carefully controlled conditions on a sugar containing media typically composed of beet and cane molasses. Under ideal growth conditions a yeast cell reproduces every two to three hours.

Yeast is the essential ingredient in many bakery products. It is responsible for leavening the dough and imparting a delicious yeast fermentation flavor to the product. It is used in rather small amounts in most bakery products, but having good yeast and using the yeast properly often makes the difference between success and something less than success in a bakery operation.

Candida albicans is a dimorphic fungus that grows at 37oC. Its normal habitat is the mucosal membranes of humans and other warm-blooded animals, where it grows as a yeast (Fig. C) and causes little or no damage. In fact, it can be isolated from the mucosa of up to 50% of humans - from the mouth, the gut, the vagina or, less often, from the surface of the skin.

In some circumstances, however, the same strains of C. albicans that grow as harmless commensals can become pathogenic, invading the mucosa and causing significant damage. This usually happens when a variety of predisposing factors cause the yeast population to multiply, escaping the normal competition from resident bacteria which keep the yeast population in check. Then the yeast cells sprout a hyphal outgrowth (Fig. D) which locally penetrates the mucosal membrane, causing irritation and shedding of the tissues.

One of the best examples of this is the disease termed thrush - a white speckling of the tongue and the back of the throat, resembling the speckling on the bird's chest. This is common in newborn babies, perhaps resulting from passage through an infected birth canal. It is also common in AIDS patients and people who have had a prolonged course of antibacterial therapy, reducing the normal resident bacterial population.

C. albicans also causes vaginitis - inflammation and invasion of the vaginal mucosa, especially during the third trimester of pregnancy and in women who take the pill. The predisposing factors seem to be hormonal, associated with changes in the balance of cell types in the lining epithelium of the vagina. A similar condition termed stomatitis is common in people who wear dentures. Candida can adhere to denture resin, and high sugar levels in the diet can also increase the adhesion by enhancing the production of a mannoprotein adhesive on the yeast cell surface. Systemic candidosis is a more serious condition, when yeast cells proliferate in the circulatory system. This can occur after invasive surgical techniques, including the insertion of intravenous catheters to which the yeast cells adhere, providing a base from which the cells can bud and be disseminated.

All these examples illustrate that C. albicans is a classic opportunistic pathogen, normally kept in check but capable of flaring up in specific, predisposing conditions. It can be identified quite readily from clinical specimens by its ability to sprout hyphae when yeast cells at 37oC are transferred to tubes of horse serum and incubated for 3-5 hours (Fig. D). Only C. albicans and a few related pathogenic Candida species do this. But the fungus has a strong tendency to revert to the yeast phase after only a short period of hyphal growth. Figures E and F show this for horse serum incubated for 24 hours - the hyphae themselves have a beaded appearance, and they give rise to budding yeast cells at the sites where the hyphae of other fungi would form branches.

Cryptococcus albidus is another budding yeast, shown here by phase-contrast microscopy but also with negative staining (the cells are suspended in India ink). Various stages of bud development are seen. The cells are surrounded by a rigid polysaccharide capsule, typical of the genus Cryptococcus, and seen as distinct haloes where the India ink particles have been excluded.

Cryptococcus species are common on leaf surfaces. But the most important species from the human standpoint is C. neoformans, a significant pathogen of immunocompromised people, causing the disease termed cryptococcosis. This disease occurs in about 7-8% of AIDS patients in the USA, and a slightly smaller percentage (3-6%) in western Europe. The capsule is a significant virulence determinant of C. neoformans because it helps to prevent the cells from being recognised and engulfed by white blood cells. Additionally, C. neoformans is unique among Cryptococcus species in producing a phenoloxidase. This enzyme acts on phenolic compounds to produce melanin, which might help to protect the cells against the antimicrobial effects of oxidants in host tissues.

C. neoformans grows commonly on old "weathered" bird droppings in cities, but does not compete well with bacteria in wet droppings. It infects through the lungs, where it causes a mild or chronic, persistent pneumonia, depending on the person's degree of immunity. Random testing of people for skin reactions to C. neoformans antigens in Britain, Australia and the USA indicates that many people have unknowingly been exposed to the fungus with no serious effect. However, in a small proportion of the population the fungus can disseminate "silently" in the central nervous system, causing fatality.

For many years it was assumed that yeast cells inhaled in dried, powdered bird droppings were the source of lung infection. But a sexual stage of the fungus has now been discovered in laboratory culture; it is typical of the fungal group Basidiomycota (which includes the mushroom fungi) but is microscopic, and it leads to the release of small (about 3 micrometre) airborne basidiospores. These are the ideal size for deposition in the lungs (see Airborne Microbes). They are thought to be the main means of infection, but their environmental source is unknown - perhaps a yeast stage growing on vegetation.

Saccharomyces cerevisiae is the budding yeast used for bread-making, where the carbon dioxide produced by growth in the dough causes the bread to rise. Essentially similar yeasts, but now given different species names, are used for production of beers, wines and other alcoholic drinks. This phase-contrast micrograph shows cells in various stages of budding. The buds are small at first, but enlarge progressively and eventually separate from the mother cell by formation of a septum (cross wall).

Few of the cellular organelles can be seen by light microscopy, unless they are stained specifically. The only conspicuous organelle seen in Fig. A is the large central vacuole which contributes to cell expansion. S. cerevisiae is a member of the fungal group Ascomycota (the ascus-forming fungi).

Yeasts are fungi that grow as single cells, producing daughter cells either by budding (the budding yeasts) or by binary fission (the fission yeasts). They differ from most fungi, which grow as thread-like hyphae. But this distinction is not a fundamental one, because some fungi can alternate between a yeast phase and a hyphal phase, depending on environmental conditions. Such fungi are termed dimorphic (with two shapes) and they include several that cause disease of humans.

Yeasts grow typically in moist environments where there is a plentiful supply of simple, soluble nutrients such as sugars and amino acids. For this reason they are common on leaf and fruit surfaces, on roots and in various types of food. With few exceptions, they are unable to degrade polymers, such as starch and cellulose which are used by many hyphal fungi.

Yeast are unicellular fungi. The precise classification is a field that uses the characteristics of the cell, ascospore and colony. Physiological characteristics are also used to identify species. One of the more well known characteristics is the ability to ferment sugars for the production of ethanol. Budding yeasts are true fungi of the phylum Ascomycetes, class Hemiascomycetes. The true yeasts are separated into one main order Saccharomycetales.

Yeasts are characterized by a wide dispersion of natural habitats. Common on plant leaves and flowers, soil and salt water. Yeasts are also found on the skin surfaces and in the intestinal tracts of warm-blooded animals, where they may live symbiotically or as parasites. The common "yeast infection" is typically Candidiasis is caused by the yeast-like fungus Candida albicans. In addition to being the causative agent in vaginal yeast infections Candida is also a cause of diaper rash and thrush of the mouth and throat.

Yeasts multiply as single cells that divide by budding (eg Saccharomyces) or direct division (fission, eg. Schizosaccharomyces), or they may grow as simple irregular filaments (mycelium). In sexual reproduction most yeasts form asci, which contain up to eight haploid ascospores. These ascospores may fuse with adjoining nuclei and multiply through vegetative division or, as with certain yeasts, fuse with other ascospores.

The awsome power of yeast genetics is partially due to the ability to quickly map a phenotype producing gene to a region of the S. cerevisiae genome. For the past two decades S. cerevisiae has been the model system for much of molecular genetic research because the basic cellular mechanics of replication, recombination, cell division and metabolism are generally conserved between yeast and larger eukaryotes, including mammals.

The most well-known and commercially significant yeasts are the related species and strains of Saccharomyces cerevisiae. These organisms have long been utilized to ferment the sugars of rice, wheat, barley, and corn to produce alcoholic beverages and in the baking industry to expand, or raise, dough. Saccharomyces cerevisiae is commonly used as baker's yeast and for some types of fermentation. Yeast is often taken as a vitamin supplement because it is 50 percent protein and is a rich source of B vitamins, niacin, and folic acid.

In brewing, Saccharomyces carlsbergensis, named after the Carlsberg Brewery in Copenhagen, where it was first isolated in pure culture by Dr. Emil Christian Hansen (1842-1909) in 1883, is used in the production of several types of beers including lagers. S. carlsbergensis is used for bottom fermentation. S. cerevisiae used for the production of ales and conducts top fermentation, in which the yeast rise to the surface of the brewing vessel. In modern brewing many of the original top fermentation strains have been modified to be bottom fermenters. Currently the S. carlsbergensis designation is not used, the S. cerevisiae classification is used instead.

The yeast's function in baking is to ferment sugars present in the flour or added to the dough. This fermentation gives off carbon dioxide and ethanol. The carbon dioxide is trapped within tiny bubbles and results in the dough expanding, or rising. Sourdough bread, is not produced with baker's yeast, rather a combination of wild yeast (often Candida milleri) and an acid-generating bacteria (Lactobacillus sanfrancisco sp. nov). It has been reported that the ratio of wild yeast to bacteria in San Francisco sourdough cultures is about 1:100. The C. milleri strengthens the gluten and the L. sanfrancisco ferments the maltose. For more information about sourdough see rec.food.sourdough FAQ.

The fermentation of wine is initiated by naturally occurring yeasts present in the vineyards. Many wineries still use nature strains, however many use modern methods of strain maintenance and isolation. The bubbles in sparkling wines is trapped carbon dioxide, the result of yeast fermenting sugars in the grape juice. One yeast cell can ferment approximately its own weight of glucose per hour. Under optimal conditions S. cerevisiae can produce up to 18 percent, by volume, ethanol with 15 to 16 percent being the norm. The sulfur dioxide present in commercially produced wine is actually added just after the grapes are crushed to kill the naturally present bacteria, molds, and yeasts.

The yeastlike fungus, Candida albicans, is commonly found in the mouth, vagina, and intestinal tract. Candida is a normal inhabitant of humans and normally causes no ill effects. However, among infants and individuals with other illness a variety of conditions can occur. Candidiasis of the mucous membranes of the mouth is known as thrush. Candidiasis of the vagina is called vaginitis. Candida also causes severe disease in persons with AIDS and chemotherapy patients.

Budding yeasts are true fungi of the phylum Ascomycetes, class Hemiascomycetes. The true yeasts are separated one main order Saccharomycetales, which includes at least ten families. Yeasts are heterotrophic, lack chlorophyll, and are characterized by a wide dispersion of natural habitats. Common on plant leaves and flowers, yeasts are also found on the skin surfaces and in the intestinal tracts of warm-blooded animals, where they may live symbiotically or as parasites. k, g, d, b, l. In women, who are pregnant or taking antibiotics, an infection of the vagina and vulva caused by a yeastlike fungus Candida albicans, is common. Yeasts are also found in soil and saltwater, where they contribute to the decomposition of plant and algal matter.

Are yeasts Fungi?

Yes, the 700 known yeasts really are fungi, although their colonies often look more like those of bacteria.

Most fungi explore their surroundings by producing miles of fine, branching filaments called hyphae, but most yeasts have become more or less unicellular, with rounded cells. This is often an adaptation to living in a liquid medium of high osmotic pressure. This usually means media with a high sugar content, such as is found in the nectaries of flowers or on the surface of fruits, where if they present the least possible surface area (as close to spherical as possible), it makes it easier for them to control the movement of dissolved substances in and out of their cells.

The cells of most yeasts can be regarded as asexual propagules, and they produce more of the same by a variety of methods similar to those found in moulds.

Some yeasts are related to ascomycetes, others to basidiomycetes, and even zygomycetes sometimes take on a yeast-like appearance.

Some yeasts make hyphae as well as unicells, and some are even exclusively hyphal, being recognizable as yeasts only by biochemical characters.

Yeasts are, of course, among the most important fungi, because they raise bread, ferment sugars to make beer, wine, and spirits, and represent a concentrated food and a source of B vitamins.

A few yeasts cause serious diseases of people.

Yeast name applied specifically to a certain group of microscopic fungi and to commercial products consisting of masses of dried yeast cells or of yeast mixed with a starchy material and pressed into yeast cakes. Although a number of fungi are sometimes called yeasts, the true yeasts are unicellular, consist of oval or round cells, and reproduce chiefly by budding. Under certain conditions some yeast cells secrete a thickened wall, and the cytoplasm of the single cell within divides to form four or eight cells, or spores, known as ascospores, which emerge when the wall ruptures. In a few species two cells fuse before undergoing spore formation. There are about 500 species in all. 1 Yeasts, especially those of the genus Saccharomyces, have long been of commercial importance because they are the chief agents in alcoholic fermentation. Because of this they are essential to the making of beer, wine, and other alcoholic beverages and industrial alcohol. Wild yeasts, those found in nature and probably carried by insects from the soil to fruits, are frequently active in the fermentation process. In breadmaking the yeasts act upon the carbohydrates in the dough, forming carbon dioxide and ethyl alcohol, which are driven off in the baking process; the escaping carbon dioxide causes the bread to rise. Since early times yeast has been used in treating various ailments; brewer’s yeast has a high content of thiamine and other vitamins of the B-complex group. Yeasts are classified in the kingdom Fungi, phyla (divisions) Ascomycota and Basidiomycota.

Yeast is a living, microscopic, single-cell organism that, as it grows, converts its food (through a process known as fermentation) into alcohol and carbon dioxide. This trait is what endears yeast to winemakers, brewmasters and breadbakers. In the making of wine and beer, the yeast's manufacture of alcohol is desired and necessary for the final product; and carbon dioxide is what makes BEER and CHAMPAGNE effervescent. The art of breadmaking needs the carbon dioxide produced by yeast in order for certain doughs to rise. To multiply and grow, all yeast needs is the right environment, which includes moisture, food (in the form of sugar or starch) and a warm, nurturing temperature (70° to 85°F is best). Wild yeast spores are constantly floating in the air and landing on uncovered foods and liquids. No one's sure when these wild spores first interacted with foods but it's known that the Egyptians used yeast as a LEAVENING agent more than 5,000 years ago. Wine and other fermented beverages were made for millennia before that. Today, scientists have been able to isolate and identify the various yeasts that are best for winemaking, beermaking and baking. The two types commercially available are baker's yeast and brewer's yeast. Baker's yeast, as the name implies, is used as a leavener. It's catagorized into three basic types-active dry yeast, compressed fresh yeast and YEAST STARTERS. Active dry yeast is in the form of tiny, dehydrated granules. The yeast cells are alive but dormant because of the lack of moisture. When mixed with a warmliquid (105° to 115°F), the cells once again become active. Active dry yeast is available in two forms, regular and quick-rising, of which the latter takes about half as long to leaven bread. They may be used interchangeably (with adjustments in rising time) and both are available in 1/4-ounce envelopes. Regular active dry yeast may also be purchased in 4-ounce jars or in bulk in some health-food stores. It should be stored in a cool, dry place, but can also be refrigerated or frozen. It should always be at room temperature before being dissolved in liquid. Properly stored, it's reliable when used by the expiration date, which should be stamped on the envelope or jar label. One package of dry yeast is equal to 1 scant tablespoon dry yeast or 1 cake of compressed fresh yeast. Compressed fresh yeast, which comes in tiny (0.06-ounce), square cakes, is moist and extremely perishable. It must be refrigerated and used within a week or two, or by the date indicated on the package. It can be frozen, but should be defrosted at room temperature and used immediately. One cake of fresh yeast can be substituted for one envelope of dry yeast. The use of compressed fresh yeast has been primarily replaced by the more convenient active dry yeast. All baker's yeast should be given a test called proofing to make sure it's still alive. To proof yeast, dissolve it in warm water and add a pinch of sugar. Set the mixture aside in a warm place for 5 to 10 minutes. If it begins to swell and foam, the yeast is alive, active and capable of leavening bread. Brewer's yeasts are special non-leavening yeasts used in beermaking. Because it's a rich source of B vitamins, brewer's yeast is also used as a food supplement. It's available in health-food stores. Brewer's yeasts are also marketed in specialty beermaking equipment shops, with different strains used for different beers.

Prior to the evolution of commercially available baking powders and yeasts during the 19th century, yeast starters were the LEAVENERS used in breadmaking. Such starters are a simple mixture of flour, water, sugar and YEAST. (At one time, airborne yeast was the only source used, but today convenient commercially packaged baker's yeast is more common.) This batter is set aside in a warm place until the yeast ferments and the mixture is foamy. A portion of the starter-usually about 2 cups-is removed and used as the base and leavener for some bread recipes. Once fermented, yeast starters-the most famous of which is sourdough starter-can be kept going in the right environment for years simply by adding equal parts flour and water. Herman starter is a colloquialism (of unknown origin) for a honey- or sugar-sweetened starter used primarily for sweet breads. Starter should be refrigerated and can be stored this way indefinitely as long as it's replenished every 2 weeks. Before using or replenishing, it should be brought to room temperature. If a starter turns orange or pink and develops an unpleasantly acrid odor, undesirable bacteria have invaded it and the mixture must be discarded. Two cups of the foamy starter mixture can be substituted for each package of yeast called for in a recipe.

yeast, name applied specifically to a certain group of microscopic fungi and to commercial products consisting of masses of dried yeast cells or of yeast mixed with a starchy material and pressed into yeast cakes. Although a number of fungi are sometimes called yeasts, the true yeasts are unicellular, consist of oval or round cells, and reproduce chiefly by budding. Under certain conditions some yeast cells secrete a thickened wall, and the cytoplasm of the single cell within divides to form four or eight cells, or spores, known as ascospores, which emerge when the wall ruptures. In a few species two cells fuse before undergoing spore formation. There are about 500 species in all.

Yeasts multiply as single cells that divide by budding or direct division (fission), or they may grow as simple irregular filaments (mycelium). In sexual reproduction most yeasts form asci, which contain up to eight haploid ascospores. These ascospores may fuse with adjoining nuclei and multiply through vegetative division or, as with certain yeasts, fuse with other ascospores. The awsome power of yeast genetics is partially due to the ability to quickly map a phenotype producing gene to a region of the S. cerevisiae genome. For the past two decades S. cerevisiae has been the model system for much of molecular genetic research because the basic cellular mechanics of replication, recombination, cell division and metabolism are generally conserved between yeast and larger eukaryotes, including mammals. The most well-known and commercially significant yeasts are the related species and strains of Saccharomyces cerevisiae. k, b, b, c, k. These organisms have long been utilized to ferment the sugars of rice, wheat, barley, and corn to produce alcoholic beverages and in the baking industry to expand, or raise, dough. Saccharomyces cerevisiae is commonly used as baker's yeast and for some types of fermentation. Yeast is often taken as a vitamin supplement because it is 50 percent protein and is a rich source of B vitamins, niacin, and folic acid.

Yeasts, especially those of the genus Saccharomyces, have long been of commercial importance because they are the chief agents in alcoholic fermentation. Because of this they are essential to the making of beer, wine, and other alcoholic beverages and industrial alcohol. Wild yeasts, those found in nature and probably carried by insects from the soil to fruits, are frequently active in the fermentation process. In breadmaking the yeasts act upon the carbohydrates in the dough, forming carbon dioxide and ethyl alcohol, which are driven off in the baking process; the escaping carbon dioxide causes the bread to rise. Since early times yeast has been used in treating various ailments; brewer's yeast has a high content of thiamine and other vitamins of the B-complex group. Yeasts are classified in the kingdom Fungi, phyla (divisions) Ascomycota and Basidiomycota.

Yeast is a living, microscopic, single-cell organism that, as it grows, converts its food (through a process known as fermentation) into alcohol and carbon dioxide. This trait is what endears yeast to winemakers, brewmasters and breadbakers. In the making of wine and beer, the yeast's manufacture of alcohol is desired and necessary for the final product; and carbon dioxide is what makes beer and champagne effervescent. The art of breadmaking needs the carbon dioxide produced by yeast in order for certain doughs to rise. To multiply and grow, all yeast needs is the right environment, which includes moisture, food (in the form of sugar or starch) and a warm, nurturing temperature (70 degrees to 85 degrees F is best). Wild yeast spores are constantly floating in the air and landing on uncovered foods and liquids. No one's sure when these wild spores first interacted with foods but it's known that the Egyptians used yeast as a leavening agent more than 5,000 years ago. Wine and other fermented beverages were made for millennia before that. Today, scientists have been able to isolate and identify the various yeasts that are best for winemaking, beermaking and baking. The two types commercially available are baker's yeast and brewer's yeast. Baker's yeast, as the name implies, is used as a leavener. It's catagorized into three basic types--active dry yeast, compressed fresh yeast and yeast starters. Active dry yeast is in the form of tiny, dehydrated granules. The yeast cells are alive but dormant because of the lack of moisture. When mixed with a warm liquid (105 degrees to 115 degrees), the cells once again become active. Active dry yeast is available in two forms, regular and quick-rising, of which the latter takes about half as long to leaven bread. They may be used interchangeably (with adjustments in rising time) and both are available in 1/4-ounce envelopes. Regular active dry yeast may also be purchased in 4-ounce jars or in bulk in some health-food stores. It should be stored in a cool, dry place, but can also be refrigerated or frozen. It should always be at room temperature before being dissolved in liquid. Properly stored, it's reliable when used by the expiration date, which should be stamped on the envelope or jar label. One package of dry yeast is equal to 1 scant tablespoon dry yeast or 1 cake of compressed fresh yeast. Compressed fresh yeast, which comes in tiny (0.6-ounce), square cakes, is moist and extremely perishable. It must be refrigerated and used within a week or two, or by the date indicated on the package. It can be frozen, but should be defrosted at room temperature and used immediately. One cake of fresh yeast can be substituted for one envelope of dry yeast. The use of compressed fresh yeast has been primarily replaced by the more convenient active dry yeast. All baker's yeast should be given a test called proofing to make sure it's still alive. To proof yeast, dissolve it in warm water and add a pinch of sugar. Set the mixture aside in a warm place for 5 to 10 minutes. If it begins to swell and foam, the yeast is alive, active and capable of leavening bread. Brewer's yeasts are special non-leavening yeasts used in beermaking. Because it's a rich source of B vitamins, brewer's yeast is also used as a food supplement. It's available in health-food stores. Brewer's yeasts are also marketed in specialty beermaking equipment shops, with different strains used for different beers.

Budding yeasts are true fungi of the phylum Ascomycetes, class Hemiascomycetes. The true yeasts comprise the family Saccharomycetes, which has but one genus Saccharomyces, but includes at least ten species. The classification of yeasts is a specialized field using cell, ascospore, and colony characteristics for distinguishing genera, and physiological characteristics - particularly the ability to ferment individual sugars - to identify species. Yeasts are heterotrophic, lack chlorophyll, and are characterized by a wide dispersion of natural habitats. Common on plant leaves and flowers, yeasts are also found on the skin surfaces and in the intestinal tracts of warm-blooded animals, where they may live symbiotically or as parasites. In women, who are pregnant or taking antibiotics, an infection of the vagina and vulva caused by a yeast like fungus Candida albicans, is common. Yeasts are also found in soil and saltwater, where they contribute to the decomposition of plant and algal matter. Yeasts multiply as single cells that divide by budding or direct division, or they may grow as simple irregular filaments. In sexual reproduction most yeasts form asci, which contain up to eight haploid ascospores. These ascospores may fuse with adjoining nuclei and multiply through vegetative division or, as with certain yeasts, fuse with other ascospores. The most well-known and commercially significant yeasts are the related species and strains of Saccharomyces cerevisiae. These organisms have long been utilized to ferment the sugars of rice, wheat, barley, and corn to produce alcoholic beverages and in the baking industry to expand, or raise, dough. Saccharomyces cerevisiae is commonly used as baker's yeast and for some types of fermentation. Yeast is often taken as a vitamin supplement because it is 50 percent protein and is a rich source of B vitamins, niacin, and folic acid. The yeast's function in baking is to ferment sugars present in the flour or added to the dough. This fermentation gives off carbon dioxide and ethanol. The carbon dioxide is trapped within tiny bubbles and results in the dough expanding, or rising.

Brewer’s yeast is the dried, pulverized cells of Saccharomyces cerevisiae, a type of fungus. It is a rich source of B-complex vitamins, protein (providing all essential amino acids), and minerals, including a biologically active form of chromium known as glucose tolerance factor (GTF). Brewer’s yeast should not be confused with nutritional yeast or torula yeast, which are low in chromium.

Brewer’s yeast, perhaps by changing the microbial flora in the large intestine, might be helpful in the treatment of some cases of infectious diarrhea.

Where is it found? Brewer’s yeast, which has a very bitter taste, is recovered after being used in the beer-brewing process. Brewer’s yeast can also be grown specifically for harvest as a nutritional supplement. “De-bittered” yeast is also available, though most yeast sold in health food stores that does not taste bitter is not real brewer’s yeast.

Brewer’s yeast is not an essential nutrient, but it can be used as a source of B-complex vitamins and protein. It is by far the best source of chromium, both in terms of quantity and bio-availability.

How much is usually taken? Brewer’s yeast is often taken as a powder, or as tablets or capsules. High-quality brewer’s yeast powder or flakes contain as much as 60 mcg of chromium per tablespoon (15 grams). When doctors recommend brewer’s yeast, they will often suggest 1–2 tablespoons (15–30 grams) of this high-potency bulk product per day. Remember, if it is not bitter, it is not likely to be real brewer’s yeast and therefore will not contain biologically active chromium. In addition, “primary grown” yeast (i.e., that grown specifically for harvest, as opposed to that recovered in the brewing process) may not contain GTF.

Are there any side effects or interactions? Side effects have not been reported from the use of brewer’s yeast, although allergies to it exist in some people. It is not related to Candida albicans fungus, which causes yeast infection.

Because it contains a highly biologically active form of chromium, supplementation with brewer’s yeast could potentially enhance the effects of drugs for diabetes (e.g., insulin or other blood sugar-lowering agents) and possibly lead to hypoglycemia. Therefore, people with diabetes taking these medications should supplement with chromium or brewer’s yeast only under the supervision of a doctor.

Saccharomyces boulardii is registered in Europe under the name Saccharomyces cerevisiae, though the manufacturer states that S. boulardii is not the same as brewer’s yeast (S. cerevisiae). There is a case report of a person with severely impaired immune function who, after receiving treatment with S. boulardii, developed an invasive fungal infection identified as S. cerevisiae.1 People with severe impairment of the immune system should therefore not take brewer’s yeast or S. boulardii unless supervised by a doctor

Molds can grow almost everywhere. In a review of fungus allergens, the various hab- itats, living requirements and where to find the molds have been extensively described (8). The particular species and concentrations represented in the air at any given time depend on temperature, degree of precipitation, prevailing winds, seasonal climatic factors, circadian patterns of sunlight and darkness, and atmospheric moisture. The ability of molds to quickly occupy and inhabit suitable ecological niches indoors and outdoors and to form and disperse such masses of spores, perhaps explains their evolutionary success in nature and their role as allergen sources. The fungi grow, die and disintegrate and people are almost always exposed to varying concentrations of mold particles at home, at work, or outdoors, which may result in the sensitization of atopic individuals.

During any season however, the number of mold particles a patient inhales on any given day is purely conjectural. A certain type of activity is sometimes connected with mold exposure, such as raking leaves or being in the barn, but these instances are not daily occurrences for most patients. Indoor spores are present throughout the year.

In a 1985 survey in 6 countries in Europe, the occurrence of fungal spores was as follows: 1. Cladosporium, 2. Ascomycetes, 3. Sporobolomyces, 4. Basidiomycetes, 5. Aspergillus and Penicillium 6. Yeasts, 7. Ustilago (rusts), 8. Alternaria. In a compilation of studies carried out mostly in the United States but also Thailand, India and Australia the ranking was as follows: 1. Cladosporium, 2. Penicillium, 3. Yeasts, 4. Alternaria, 5. Aspergillus, 6. Aureobasidium, 7. Helminthosporium, S. Fusarium, 9. Epicoccum (5). Bush & Yunginger (9) refer to Cladosporium, Alternaria, Fusarium, Drechslera (Helminthosporium) and several yeasts as universal dominants outdoors. Local variations can be large and the conclusions drawn from the studies may not always be applicable to all parts of these or other areas.

Yeast are unicellular fungi. The precise classification is a field that uses the characteristics of the cell, ascospore and colony. Physiological characteristics are also used to identify species. One of the more well known characteristics is the ability to ferment sugars for the production of ethanol. Budding yeasts are true fungi of the phylum Ascomycetes, class Hemiascomycetes. The true yeasts are separated into one main order Saccharomycetales. Yeasts are characterized by a wide dispersion of natural habitats. Common on plant leaves and flowers, soil and salt water. Yeasts are also found on the skin surfaces and in the intestinal tracts of warm-blooded animals, where they may live symbiotically or as parasites. j, d, k, a, d. The common "yeast infection" is typically Candidiasis is caused by the yeast-like fungus Candida albicans. In addition to being the causative agent in vaginal yeast infections Candida is also a cause of diaper rash and thrush of the mouth and throat.

Genetics of Chromosome Structure and Segregation.

The condensation of chromatin into chromosomes prior to mitosis achieves several important goals. 1) It ensures that the DNA is not broken during segregation at mitosis. 2) It leads to the assembly of the kinetochore which interacts with the spindle microtubules and moves the sister chromatids to the poles. How higher order chromosome structure is achieved is poorly understood. Mitotic mutants and mutations that affect the fidelity of chromosome transmission are likely to alter chromosomal constituents that help assemble the chromosome. The bimD6 mutation affects the ability of the chromosomes to interact with spindle microtubules. We have identified four extragenic suppressor genes to the bimD6 mutation. One of these genes, sudA, is a member of the SMC family of proteins. SMC proteins are widely distributed and are known to function in chromosome condensation. A second gene, sudD, codes for another evolutionarily conserved protein. We have cloned the budding yeast and human homologues of sudD and begun working with these to determine this genes function. Curiously, the SUDD protein is cytoplasmic and not nuclear as would be predicted for a suppressor of a mitotic mutant. Data will be presented that suggests sudD and possibly sudB function not as chromosomal proteins but upstream in the chromosome condensation pathway.

Regulation of sexual development in basidiomycete fungi.

In a number of basidiomycete fungi the mating type loci encoding pheromones and receptors have been identified. While the complex structural organization of these loci is now established the question how the pheromone signals are used to control diverse intracellular processes are just beginning to be understood.

Our work concentrates on the phytopathogenic fungus Ustilago maydis. This fungus exhibits a dimorphic life style. Haploid sporidia grow yeast-like by budding and are non-pathogenic while the dikaryon grows filamentous and is able to induce tumors in maize plants. We will review the known contributions of the mating type loci to this morphological transition and then focus on an analysis of the pheromone signalling cascade. In particular we will describe the characterisation of a G alpha subunit gene whose product plays an essential role in transmission of the pheromone signal. The G alpha subunit required for pheromone signaling appears to be essential for pathogenic development as well, althought this process does not require pheromone stimulation. Interestingly, when constitutively active, this G alpha protein affects tumor morphology. This may provide a handle to isolate molecules involved in the communication of U. maydis with its host plant.

Vegetative incompatibility: A question of identity.

Vegetative incompatibility is responsible for intraspecific self/nonself recognition in filamentous fungi during the assimilative phase of growth. In Neurospora crassa, vegetative incompatibility can be caused by heteroallelism at any of at least 11 het loci, including the mating type locus. The mating-type locus encodes putative transcriptional regulators that affect expression of various target genes. A suppressor of mating-type associated incompatibility, tol has been cloned and encodes a putative 922 aa polypeptide; RIP mutants are fully compatible with both A and a strains, but have no other observable phenotype. We have also cloned and characterized two additional het loci, het-c and het-6. The het-cor (for Oak Ridge) allele encodes a 966 aa coiled-coil, glycine-rich polypeptide that contains a signal sequence, suggesting that it is targeted to the fungal cell wall. RIP mutants in het-c are fully compatible, but have no other observable phenotype. Analysis of N. crassa strains indicates that het-c specificity is mediated by three alleles. Chimeric constructs localized the region determining het-c specificity to a highly variable domain of approximately 30 aa. Analysis of N. crassa strains, other Neurospora species and related genera show that the three allelic motifs that determine het-c specificity region are highly conserved.

Hexokinase activity in S. cerevisiae. 1. The role of mitochondrial deficiency, glucose and ethanol concentrations.

The genetic regulation of hexokinase enzyme in Saccharomyces cerevsiae was investigated through the effect of the deficiency in mitochondrial function and regulatory effect of glucose and ethanol concentration as well as aeration. Petite mutants were induced by nicotine mutagen and identified in two genetically different haploid yeast strains. Petite mutants showed no chromosomal reverted genes, different patterns of sugar utilization, when tested on defferent carbon sources , and a variable increase in enzyme activity than that in its parental strains . Different types of diploids (normal mitochondrial function, partially or totally nonmitochondral function diploids) showed differences in their sporulation time where the petite diploid "non-functional mitochondrial diploid" never sporulated. Diploids exerted variable hybrid vigour in their enzyme production . Tetrad analysis of different diploids resulted in variable segregation patterns in the enzyme activity . Parental, petite mutants, diploids and tetrad segregants strains tolerated the increase in ethanol concentration up to 10% , glucose concentration up to 40% and aerobic or anaerobic conditions to different degrees which was reflected on their enzyme activity as a function of their chromosomal and cytoplasmic genes.

 Characterization of the fluffy gene of Neurospora crassa.

The fluffy (fl) mutant of Neurospora crassa blocks the development of macroconidia. Genetic mapping places fl between mus-23 and trp-3 on the right arm of chromosome III. A cosmid was identified that complemented both mus-23 and fl. Fl was subcloned to a 4.5 kbp fragment and sequence analysis of this fragment revealed an open reading frame (ORF) containing a zinc finger DNA binding domain and two putative introns. The ORF has limited sequence similarity to Nit-4 and NirA, regulators of nitrogen assimilation in N. crassa and Aspergillus nidulans respectively. We have sequenced four alleles of fl, and these data demonstrate that the predicted ORF is required for proper gene function. RIP inactivation of the 4.5 kbp fragment produced progeny with the fluffy phenotype. The sequence of the RIP mutant contained 74 mutations due to base pair changes including one changing the translational start codon and one destroying the zinc finger domain. Expression of fl appears to peak six hours after induction of development, and it is induced by nitrogen starvation.

Yeasts multiply as single cells that divide by budding (eg Saccharomyces) or direct division (fission, eg. Schizosaccharomyces), or they may grow as simple irregular filaments (mycelium). In sexual reproduction most yeasts form asci, which contain up to eight haploid ascospores. These ascospores may fuse with adjoining nuclei and multiply through vegetative division or, as with certain yeasts, fuse with other ascospores. The awsome power of yeast genetics is partially due to the ability to quickly map a phenotype producing gene to a region of the S. cerevisiae genome. h, e, j, k, c. For the past two decades S. cerevisiae has been the model system for much of molecular genetic research because the basic cellular mechanics of replication, recombination, cell division and metabolism are generally conserved between yeast and larger eukaryotes, including mammals.

A prion-like protein in the filamentous fungus Podospora anserina.

The het-s locus of the fungus Podospora anserina is involved in vegetative incompatibility. Coexpression of the two alternate alleles het-s and het-S is lethal leading to incompatibility between strains. Strains containing the het-s allele can exhibit two different phenotypes: either they are incompatible with strains containing the antagonistic het-S allele ([s] phenotype) or they are neutral in incompatibility ([s*] phenotype). This later phenotype can be propagated vegetatively and is maternally inherited through meiosis. However a [s*][s] switch can occur either spontaneously at a very low rate or at a high frequency after a contact with a strain which exhibits the [s] phenotype. Once induced, the [s*][s] transition spreads as an infectious process within the mycelium. We found that the HET-s protein is expressed at the same level in strains that display these alternate phenotypes leading to the conclusion that a post-translational modification of the protein is responsible for the difference between [s] and [s*] strains. These different results suggested that the HET-s protein should behave as the prion protein PrP: the protein present in [s*] and [s] strains should display different conformations and the [s*] molecule could be converted to the [s] form by a physical interaction between the molecules. Different results confirmed that the HET-s protein display properties common with the prion protein. i) the HET-s protein strongly aggregates in vitro and can form homodimers in vivo as demonstrated using the yeast two hybrid system. ii) the proteins present in [s*] and [s] strains display different sensitivity to proteolytic enzymes. iii) overexpression of the HET-s protein strongly enhances the frequency of the [s*][s] switch.

Cdc42-signaling during cytokinesis in Ustilago maydis .

The phytopathogenic fungus Ustilago maydis exhibits a dimorphic life style. Haplold sporidia grow yeast-like by budding and are non-pathogenic. The dikaryon grows filamentous and is able to induce tumors in maize plants. To identify genes that are involved in this morphogenetic switch we have isolated a number of mutants with aberrant morphology. Among these we could identify two mutants that are affected in cytokinesis. Both the donl and don3 mutants show normal nuclear division but the mutant cells fail to separate after bud formation. Cells remain connected through a septum that can be stained by calcofluor. The corresponding genes have been isolated by complementation. Molecular analysis of these genes revealed that the final step of cytokinesis seems to be regulated by a member of the Rho/Rac/Cdc42 family of small GTP binding proteins. The Donl protein shows high similarity to guanine exchange factors specific for these small Ras-like proteins, the don3 gene codes for a kinase that is homologous to the yeast STE20 and the mammalian pak/p65 serine/threonine kinases. Using the yeast two-hybrid-system we could demonstrate that the U.maydis Cdc42 homologue interacts with both Donl and the Don3. We propose that cell separation is triggered by a signal that is transmitted via Cdc42.

Mutations, pumps, antibodies, and polar growth.

Concanamycin A (CCA) is a potent specific inhibitor of vacuolar ATPases. Mutants were selected for growth on medium containing 1.0 uM CCA. Suprisingly, 65 of 66 mutations mapped in the region of the pma1 locus, which encodes the plasma membrane H+-ATPase. Plasma membrane H+-ATPase activity in isolated plasma membranes from the mutants was 17-84% of the level seen in the wild type. The most interesting change in the plasma membrane H+ATPase was in kinetic behavior. The wild-type enzyme showed sigmoid dependence on MgATP concentration with a Hill number of 2.0, while 7 of the 8 mutants tested exhibited hyperbolic kinetics with a Hill number of 1.0. One interpretation of these data was that the enzyme had changed from a functional dimer to a functional monomer.

Mutation of the plasma membrane H+-ATPase did not confer resistance by preventing uptake of CCA. In the presence of CCA both wild type and mutant strains were unable to accumulate arginine, failed to concentrate chloroquine in acidic vesicles, and exhibited gross alterations in hyphal morphology, indicating that the CCA had entered the cells and inactivated the vacuolar ATPase. Instead, we hypothesize that the mutations conferred resistance because the altered plasma membrane H+-ATPase could more efficiently rid the cell of toxic levels of Ca++ or protons or other ions accumulated in the cytoplasm following inactivation of the vacuolar ATPase by CCA.

Overproduction of aflatoxin precursors is associated with altered sclerotial development in Aspergillus parasiticus.

A genetic relationship between aflatoxin biosynthesis and sclerotial development was tested in A. parasiticus SRRC 2043, an O-methylsterigmatocystin accumulating strain, by transforming it with the aflatoxin pathway genes, aflR and/or aflJ. Elevated production of aflatoxin precursors, norsolorinic acid, averantin, versicolorin A and O-methylsterigmatocystin, resulted from introduction of extra copies of either aflR or aflR plus aflJ in transformants, but not by introduction of aflJ alone. Similarly, sclerotial production on PDA increased only in the transformants receiving either aflR or aflR plus aflJ and not in the transformant which received aflJ alone. However, the sclerotial production of the aflR plus aflJ transformant was substantially decreased on CZ plates. Increased production of aflatoxin precursors was associated with changes in sclerotial morphology; sclerotial shape became more elongated in transformants. Scanning electron micrographs showed that the sclerotia of the aflR plus aflJ transformant from PDA was not as compact as that observed for the wild-type strain. These results suggest a regulatory association between sclerotial morphogenesis and aflatoxin biosynthesis.

Regulation of the acetate utilization pathway in the basidiomycete Coprinus cinereus.

Growth on acetate as sole carbon source requires the induction of enzymes of the glyoxylate cycle and the acetate mobilising enzyme, acetyl CoA synthetase. Induction is dependent on the function of a regulatory gene, acu-1. acu-1 was isolated by functional complementation in C. cinereus. DNA sequence analysis identifies a Zinc finger DNA-binding domain in its protein that has homology to the finger in corresponding acetate regulatory proteins of Aspergillus nidulans (facB) and Saccharomyces cerevisiae (cat8). Surprisingly, there is little other sequence conservation in the three proteins despite the obvious functional homology. We will present our sequence analysis of this gene and its predicted protein and describe experiments that confirm its regulatory role with respect to two of the key enzymes induced by acetate, acetyl CoA synthetase and isocitrate lyase. Elements within the promoters of the acs-1 and acu-7 genes have been identified as possible acu-1 binding sites and these are also present within the promoter of acu-1 itself indicating autogenous regulation. Other interesting aspects of acu-1 regulatory function will be presented.

Protein-protein interactions between arginine biosynthetic enzymes of Neurospora.

N-acetyl glutamate synthase (AGS) and N-acetyl glutamate kinase (AGK) are the first two enzymes in the arginine biosynthetic pathway in Neurospora crassa. AGS and AGK are encoded by two independent genes, arg-14 and arg-6, respectively. Early biochemical and genetic evidence indicated that mutations in AGK affected both AGK and AGS activities, and that a wild-type AGK gene could restore AGS activity in arg-6 mutants. In order to investigate the nature of this activation, we tested the possibility of protein-protein interaction between AGS and AGK proteins using the yeast Two-Hybrid System. We report the first molecular evidence for direct AGS and AGK interaction. AGS and AGK interaction is strong and visualization through a -galactosidase assay is rapid: blue colonies were observed in less than 30 minutes. The interaction domains of AGS and AGK have been identified using various deletion constructs. The interaction domain of AGS resides at the N-terminus and the interaction domain of AGK is at the C-terminus.

Arginine feedback resistant mutation in Neurospora.

Mutations linked to the complex arg-6 locus (su(pro-3) ) are feedback resistant and suppress proline auxotrophic mutations (pro-3). N-acetyl glutamate kinase (AGK) is one of the proteins encoded by the arg-6 gene and is inhibited by arginine. We postulated that the su(pro-3) mutation could be located at the arginine feedback site of AGK. In order to identify the feedback resistant site in AGK and the nature of su(pro-3) mutations, we cloned AGK from several su(pro-3) mutant stains. Genomic DNA from several su(pro-3) strains was isolated and PCR was performed. The PCR products were introduced into a N. crassa double mutant (arg-6, pro-3) and transformants were identified by arginine prototrophy. Transformants that contained the su(pro-3) mutant gene suppressed the proline requirement and were able to grow in the absence of proline. The su(pro-3) mutations were mapped to the N-terminus of AGK, and all su(pro-3) mutant genes have a single amino acid change from phenylalanine (F) to leucine (L) at amino acid 81. To confirm that the F to L mutation alone caused the su(pro-3) phenotype, wild-type AGK was mutated using overlap extension PCR mutagenesis. The mutated AGK was introduced into the N. crassa double mutant (arg-6, pro-3) and transformants were identified by arginine prototrophy. A single F to L change in the wild-type AGK gene resulted in the su(pro-3) phenotype: transformants grew in the absence of proline. Southern blot analysis verified the presence of the mutant AGK gene in transformants.

The most well-known and commercially significant yeasts are the related species and strains of Saccharomyces cerevisiae. These organisms have long been utilized to ferment the sugars of rice, wheat, barley, and corn to produce alcoholic beverages and in the baking industry to expand, or raise, dough. Saccharomyces cerevisiae is commonly used as baker's yeast and for some types of fermentation. Yeast is often taken as a vitamin supplement because it is 50 percent protein and is a rich source of B vitamins, niacin, and folic acid. In brewing, Saccharomyces carlsbergensis, named after the Carlsberg Brewery in Copenhagen, where it was first isolated in pure culture by Dr. Emil Christian Hansen (1842-1909) in 1883, is used in the production of several types of beers including lagers. S. carlsbergensis is used for bottom fermentation. S. cerevisiae used for the production of ales and conducts top fermentation, in which the yeast rise to the surface of the brewing vessel. h, h, g, j, g. In modern brewing many of the original top fermentation strains have been modified to be bottom fermenters. Currently the S. carlsbergensis designation is not used, the S. cerevisiae classification is used instead.

Transgene induced gene silencing "quelling" in Neurospora crassa.

Transgene-induced gene silencing of several genes used in transformation experiments in Neurospora crassa has been termed "quelling". In studies using the carotenoid biosynthetic gene albino- 1 as a visual reporter for quelling, silencing was found to be reversible, and reversion was accompanied by loss of exogenous copies. Gene silencing acts at a post-transcriptional level leading to a strong reduction of steady-state mRNA level of the duplicated gene. Silencing was shown to be a dominant trait, operative in heterokaryotic strains, indicating the involvement of diffusable, trans-acting molecules. The production of an unintended transgene derived sense RNA was found to correlate with the gene silencing.These findings are compatible with a model in which RNA-RNA interaction is involved in gene silencing.

The isolation of a transformant strain silenced for the albino-1 gene in which quelling is stable, allowed us to perform UV mutagenesis to isolate, by simple visual screening, mutant strains impaired in gene silencing. Among 120,000 survivors screened after exposure to UV, 25 strains showing the recovery of wild type (orange) phenotype were isolated. Genetic analysis indicates that all the strains, containing recessive mutations, fall into four different complementation groups. The isolation of the corresponding genes will be of fundamental importance in the elucidation of the molecular mechanism of gene silencing in Neurospora as well as in plants.

Clock Mutants and Light Entrainment in Neurospora.

A defining feature of circadian oscillators is their ability to be entrained by environmental stimuli. The most universal of these stimuli are light and temperature. In Neurospora crassa, the effects of light and temperature on the clock can be followed at the phenotypic level by monitoring the circadian conidiation rhythm, and at the molecular level by monitoring the expression of known clock components.

The clock in Neurospora is comprised of a negative feedback loop in which the frequency (frq) gene encodes the FRQ protein which, in turn, either directly or indirectly represses its own expression. With appropriate delays built in, this feedback loop yields the oscillation in the amount of frq transcript and FRQ protein which collectively comprises the Neurospora clock. Light delivered at any point within the circadian cycle acts rapidly to increase the level of frq transcript and thus resets the clock.

In two "photo-blind" strains, white collar-1 (wc-1) and white collar-2 (wc-2), we have found that both light and temperature treatments that normally synchronize clock wild-type cultures do not result in a rhythmic phenotype. Furthermore, light induced accumulation of frq transcript is blocked in wc-1, and the sustained, light-driven increase in frq mRNA that is seen in wild type strains is blocked in wc-2. Both frq transcript and FRQ protein levels are low and arrhythmic in both of these mutants. These results together suggest that in addition to their role in light signal transduction, WC-1 and WC2 may be required for operation of a functional circadian clock.

In addition, we are currently using insertional mutagenesis to identify other genes involved in circadian rhythmicity. By screening for mutants with altered clock output phenotype, we hope to identify mutations which disrupt clock input, clock output and the central oscillator, as well as developmental mutants involved in conidiation. To date, several phenotypically arrhythmic strains have been isolated.

Studies on pectinolytic enzymes produced in submerged culture by mutant exo-1 of Neurospora crassa.

The pectinolytic complex is composed by enzymes which degrade pectic substances and produce oligogalacturonates (hidrolases) or insatured products (lyases). These enzymes play an important role in processing and manufacturing industries as the agents of maceration, fruit juice clarification, and others. The exo-1 synthesized and secreted five to six times more pectic enzymes than the wild type. They are produced by many microorganisms, higher plants and some insects. the best condition to produce these enzymes was obtained in two-stage cultures: (1) pre-cultivation at 30 C for 24 h in Vogel's medium with 2% glucose; (2) transfer of the mycelial mass to fresh media with different carbon sources, and incubation for 72 h. The exo-1 in this condition produced four extracellular pectinases (pectin-lyase, pectate-lyase and two polygalacturonase) those were purified in two peaks using DEAE- and CM-cellulose column and one intracellular polygalacturonase activity purified by DEAE-cellulose column. The optimum of temperature and pH for lyases activity were respectively 50 C and above 9.0, and for polygalacturonases activities, these values were about 40-45 C and 5.0-5.5, respectively. The amino acid composition of intracellular polygalacturonase was analysed and compared with a computer program (PROSEARCH) using as a data the molecular weight (31.565 Da) and pI (7.4) and this composition analysis resembled with a polygalacturonase of Prunus persica (peach).

Characterization of the FluG protein required for asexual sporulation in Aspergiflus nidulans.

We propose that the A. nidulans sporulation gene product FluG directs a low constitutive production of an extracellular diffusible factor with development initiating once a threshold level accumulates. Although FluG levels are relatively constant throughout the life cycle, development in submerged culture is limited by fluG expression levels because overexpression can obviate the need for air in sporulation. Deletion analysis has revealed that the C-terminal half bearing 28% identity with GSI-type prokaryotic glutamine synthetases is sufficient for development and its overexpression can cause spurious development in submerged culture. However, FluG appears to have a function distinct from glutamine biosynthesis because flug mutants are not glutamine auxotrophs. Moreover, the induced expression of the heterologous Anabaena glutamine synthetase in A. nidulans does not initiate sporulation. To identify other components acting downstream of FluG and presumably functioning in the response to the factor, we have isolated a dominant suppressor of a fluG mutation. Suppression results in precocious (conidiophores at the end of germ tubes) and prolific conidiation as well as inhibited hyphal growth on solid medium and in submerged culture. This indicates a bypass of FluG function as well as developmental signals acting upstream of FluG.

Processing and Function of a Polyprotein Precursor of Two Mitochondrial Enzymes in Neurospora crassa.

In Neurospora crassa, the mitochondrial proteins N-acetylglutamate kinase (AGK) and N-acetyl--glutamyl-phosphate reductase (AGPR) are processed from a 96-kDa cytosolic precursor encoded by the ARG6 gene. The proximal kinase and distal reductase domains are separated by a short connector region. Processing was analyzed in vitro by import assays with isolated mitochondria and in vivo by immunoblot and N-terminal sequencing of processed products. Substitutions of arginines at positions -2 and -3 upstream of the N-terminus of the AGPR domain or replacement of threonine at position +3 in the mature AGPR domain revealed a second processing site. Substitution of arginine at position -22, in combination with changes at -2 and -3, prevented cleavage of the precursor. These results indicate a two-step proteolytic cleavage of the connector region at the sequences Arg-Gly Tyr-Leu-Thr at the N-terminus of the AGPR domain and at Arg-Gly-Tyr Ser-Thr located 20 residues upstream. Proteolytic cleavage at both sites was prevented by inhibitors of the mitochondrial processing peptidase. Enzyme assays and complementation studies have shown that misprocessed and unprocessed precursors yield catalytically active enzyme(s). The AGPR specific activity of strains expressing the unprocessed precursor is much higher than that of a wild type strain, suggesting that the function of the polyprotein precursor may be to protect the AGPR domain from proteolysis before reaching the mitochondrial matrix.

Coordinate induction of the genes encoding PKA subunits during sporulation in B. emersonii.

The aquatic fungus B. emersonii presents an interesting developmental cycle, which begins with the zoospore, a motile non growing cell, that is triggered to germinate by the presence of nutrients or the addition of cAMP, dfferentiating into the germling cell. This cell is capable of vegetative growth, which is characterized by intense nuclear proliferation, without cell division, leading to the formation of a multinucleated cell: the sporangium. At any time during growth, shortage of nutrients can induce the sporulation process, which culminates with the intrasporangial formation of the zoospores and their release from the mother cells. A single PKA is present in B. emersonii. Its activity is low in vegetative cells, rising sharply during sporulation, reaching maximum levels in the zoospores. After germination PKA activity decreases back to low basal levels. Work from our laboratory has shown that these variations in activity are due to a coordinate transcriptional control of the genes encoding the regulatory (R) and the catalytic (C) subunits. To investigate sequence elements common to both R and C gene promoters, which could be involved in the coordinate regulation of these genes, their 5' flanking regions were analysed by gel mobility shift and DNA-footprinting assays. It was determined that different DNA-protein complexes are generated when fragments of the R and C gene promoters are incubated with extracts from cells expressing or not expressing both subunits. Furthermore, DNaseI-footprinting experiments have indicated that a 7-nucleotide sequence element present in both promoters, was protected from DNase I digestion only by non-expressing cell extracts, suggesting a negative control for the induction of R and C genes during sporulation.

The yeast's function in baking is to ferment sugars present in the flour or added to the dough. This fermentation gives off carbon dioxide and ethanol. The carbon dioxide is trapped within tiny bubbles and results in the dough expanding, or rising. Sourdough bread, is not produced with baker's yeast, rather a combination of wild yeast (often Candida milleri) and an acid-generating bacteria (Lactobacillus sanfrancisco sp. nov). It has been reported that the ratio of wild yeast to bacteria in San Francisco sourdough cultures is about 1:100. The C. milleri strengthens the gluten and the L. sanfrancisco ferments the maltose. For more information about sourdough see rec.food.sourdough FAQ. The fermentation of wine is initiated by naturally occurring yeasts present in the vineyards. Many wineries still use nature strains, however many use modern methods of strain maintenance and isolation. The bubbles in sparkling wines is trapped carbon dioxide, the result of yeast fermenting sugars in the grape juice. c, d, g, k, a. One yeast cell can ferment approximately its own weight of glucose per hour. Under optimal conditions S. cerevisiae can produce up to 18 percent, by volume, ethanol with 15 to 16 percent being the norm. The sulfur dioxide present in commercially produced wine is actually added just after the grapes are crushed to kill the naturally present bacteria, molds, and yeasts.

Effects of removing the mitochondrial outer membrane protein TOM70 Neurospora crassa.

Recognition, unfolding, insertion, and translocation of preproteins at the mitochondrial outer membrane is achieved by the TOM complex (translocase of the outer membrane of mitochondria). The largest component of this complex, TOM70, is an integral outer membrane protein with a large cytosolic domain thought to serve as a preprotein receptor. To investigate the role of TOM70 in mitochondrial import, a tom7O null mutant was generated using the method of RIP (repeat induced point mutations). In contrast to lethal phenotype of null mutations in TOM20 or TOM22, two other receptor components of the TOM complex, the effects of destroying TOM70 function are relatively mild. The TOM70-deficient mutant is still viable, has a growth rate about 60% that of wild-type, and produces few conidia. Unexpectedly TOM70-deficient mutant was also found to contain enlarged mitochondria with outer membranes more prone to breakage under stressful conditions than those of wild-type mitochondria. The null mutant has been successfully rescued by transformation with a wild-type copy of tom7O demonstrating that the effects of RIP are specific to only the tom7O gene. Preliminary assays of in vitro mitochondrial protein import suggest a significant decrease in the import efficiency of the ADP/ATP carrier protein (ACC).

sreA, a new GATA factor encoding gene of Aspergillus nidulans.

GATA-binding proteins constitute a family of transcription factors that recognize the consensus motif GATA. This group includes a range of major regulatory proteins from organisms as different as fungi, mammals, birds, insects and plants. The different members of this protein family are related by a high degree of amino acid sequence identity within their DNA-binding domains. In Aspergillus nidulans at least three GATA binding activities other than the extensively characterized nitrogen regulatory protein AREA can be distinguished using gel mobility shift assays. As a prerequisite to defining the function and interaction of different GATA factors in A. nidulans, we employed different PCR approaches to isolate additional genes of this family. In the course of this study we identified a gene (sreA) encoding a new GATA factor of A. nidulans, containing two GATA type zinc fingers. The deduced amino acid sequence reveals 62% overall identity to SREP of Penicillium chrysogenum. Furthermore, the 200 amino acid sequence embracing the two zinc fingers and the intervening region displays 50% identity to URBS1 from Ustilago maydis, a repressor of siderophore biosynthesis. Northern blot and cDNA analysis revealed two transcripts, 2.3 kb and 2.8 kb in length, due to two different major transcriptional start sites. If A. nidulans was cultured in low iron media, transcription of sreA was found to be repressed. In synopsis the data suggest a regulatory function of SREA in iron metabolism analogous to URBS1 in U. maydis.

Isolation of two new genes encoding subunits of the V-type ATPase of Neurospora crassa.

The vacuolar H+-ATPase (or V-ATPase) is a large multimeric enzyme composed of several subunits that acidify intracellular compartments in eukaryotic cells and play an important role in a variety of cellular processes.

Here, we report the isolation of two new genes responsible for the expression of putative V-ATPase subunits in Neurospora crassa. The first gene (vma10) encodes a protein with a molecular weight of 13,693 Daltons. The protein (subunit G) contains 113 amino acids and is highly hydrophilic. With similar subunits reported in yeast, bovine and insect it appears this subunit is ubiquitous to all V-ATPases. The second gene (vma9) appears to encode a 19,125 Dalton protein that contains 177 amino acids and although observed in other organisms on SDSPAGE gels has yet to be isolated and sequenced. This report thus represents the first characterization of this subunit, tentatively called subunit H. The role these two subunits play with regard the structure-function of the V-ATPase is unsure. However, both show sequence homology to the b-subunit of the closely related F-ATPase that is thought to be involved in coupling proton conduction and ATP synthesis. We thus propose a similar mechanism.

Serendipitously, we have also isolated a gene that encodes an amino acid transport protein with a molecular weight of 65kDa. The protein shows no sequence identity with any of the known Neurospora crassa amino acid transporters but does have strong homology with several basic amino acid permeases in yeast. We are presently pursuing characterization studies of this protein.

*Both vma genes were originally identified from EST's sequenced by the Neurospora Genome Project, Department of Biology, University of New Mexico.

Molecular and functional analysis of the hymA gene in Aspergillus nidulans.

The filamentous fungus Aspergillus nidulans is able to reproduce asexually with conidia and sexually with ascospores. In this study insertional mutagenesis was used to isolate mutants defective in asexual sporulation. One of these mutants was analyzed at a molecular level. In this strain hyphal growth rate was slightly decreased and branching frequency increased in comparison to wildtype. The conidiophore development was specifically blocked at the metula stage. Metulae rather resembled hyphae, because they were elongate, multinucleate and septate. Thus, the gene was named hymA (hypha-like metula).

A 2.4 kb hymA complementing genomic DNA fragment was isolated which detected a 1.8 kb trancript in hyphae and in conidiophores. Sequence comparision of genomic DNA and corresponding cDNAs revealed 4 Introns between 58 and 65 bp in size. The 384 amino acid deduced HymA protein showed homology to proteins from Saccharomyces cereviseae, Caenorhabditis elegans and Mus musculus. The mouse protein is thought to be a Ca2+-binding protein. The region harbouring the Ca2+-binding site in the mouse protein was highly conserved in HymA. However, a functional role of HYMA in Ca2+-dependent processes remains to be determined.

Catalase is specifically oxidized by singlet oxygen during conidiation.

We have proposed cell differentiation as an avoidance response to a hyperoxidant state. Total protein and specific enzymes were oxidized and degraded at the start of each of the three morphogenetic transitions of the conidiation process in Neurospora crassa. In this work, we asked if catalase was also oxidized at the hyperoxidant states of cell-differentiation.

Cat-l a and Cat-lc activity increased in hyphae at the prestationary growth phase and in the first two morphogenetic transitions of the conidiation process. With formation of conidia, in addition to a Cat-la and Cat-lc increase, Cat-le and Cat-2 appeared transiently. Conidia had over 60 times more catalase activity than exponentially growing hyphae. Increments were biphasic: after an initial rise, a decrease in specific activity coincident in time with the hyperoxidant state followed by catalase induction in the differentiated cell-structures. Accumulation of Cat-1 mRNA followed this biphasic activity pattern.

Purified catalase Cat-la gave rise to Cat-lc and Cat-le. Cat-la was shown to be oxidized through the sequential reaction of the four monomers with singlet oxygen, generating active catalase conformers with more acidic isoelectric points. Modification was brought about specifically by singlet oxygen, singlet oxygen scavengers prevented modification; superoxide and hydroxyl radical had no effect on electrophoretic mobility. Modified Cat-1 monomers had a similar molecular mass than the unmodified ones. Cat-2 from N. crassa and catalases from different organisms were also susceptible to modification by singlet oxygen.

The yeast Saccharomyces cerevisiae is clearly the most ideal eukaryotic microorganism for biological studies. The "awesome power of yeast genetics" has become legendary and is the envy of those who work with higher eukaryotes. The complete sequence of its genome has proved to be extremely useful as a reference towards the sequences of human and other higher eukaryotic genes. Furthermore, the ease of genetic manipulation of yeast allows its use for conveniently analyzing and functionally dissecting gene products from other eukaryotes. Despite being only one-tenth the size of a white blood cell, many of the cellular functions of higher species are present in unicellular yeast. Two species of yeast have been pressed into service as model organisms: S. cerevisiae and its distant cousin S. pombe. l, e, d, g, i. Each one has nearly 200 genes homologous to human genes involved in disease, with 23 for cancer alone. Used since Ancient Egypt, yeast has economic importance in beer and bread making. Catalase is oxidized by singlet oxygen during germination.

Specific modification of Cat-1 by singlet oxygen occurred during the hyperoxidant states of the conidiation process. In this work we asked if a hyperoxidant state and Cat-1 oxidation by singlet oxygen also takes place during germination.

Neurospora crassa conidia did not germinate under an atmosphere of nitrogen or argon. However, they did germinate under N2 or Ar if a pulse of hydrogen peroxide or oxygen saturated water was given to generate a transient hyperoxidant state, measured by chemioluminescence.

Cat-l a stored in conidia was oxidized during germination giving rise to Cat-1c. This could be detected even when germinating in the dark. Under conditions of increasingly higher concentration of singlet oxygen, such as intense light, a source of singlet oxygen, or in carotenoid deficient mutant strains, a higher proportion of Cat-l a was oxidized to Cat-lc and Cat-le. Increased and precocious accumulation of Cat-1 mRNA was observed. Oxidized catalase conformers disappeared rapidly and new Cat-la appeared.

A hyperoxidant state is probably required for breakage of dormancy. Oxidation of Cat-la indicated intracellular generation of singlet oxygen during germination under all conditions tested. Modified Cat-1 conformers probably have a shorted half life than the unmodified enzyme. Carotenoids appeared to protect Cat-1 from in vivo modification by singlet oxygen.

Regulation of a conidiation gene, con-10, of Neurospora by starvation and stresses is unmasked by mutation of a developmental regulator, rco-1.

The Neurospora crassa con-10 gene encodes a small polypeptide that has homology with a general stress-induced gene, gsiB, of Bacillus subtilis. A regulatory gene, rco-1, controls con-10 expression and sporulation in N. crassa. RCO1 appears to function as a repressor of con-10 during mycelial growth. In wild-type strains, prolonged carbon starvation results in development and con-10 expression, however, con-10 is not highly induced by glucose starvation in the absence of development. In the rco-1 mutant strain, con-10 expression is rapidly and highly induced by carbon or nitrogen starvation and heat shock. Thus, in the wild-type, rco-1 functions to repress con-10 expression in mycelia and to block rapid induction of the gene in response to starvation and stresses.

Partial Purification and Biochemical Properties of Trehalose-6-Phosphate Synthase of Neurospora crassa.

Trehalose is synthesized in yeast from glucose-6-phosphate (G6P) and uridine-diphosphoglucose (UDPG) in a two step reaction catalyzed by the trehalose-6-phosphate synthase (Tre6P synthase)/trehalose-6-phosphate phosphatase (Tre6P phosphatase) complex. Previous report of our laboratory showed that the synthesis and breakdown of trehalose in N. crassa conidiospores submitted to temperature shifts. Tre6P synthase exhibited optimum of temperature and pH of 55 C and pH 7.0, respectively. During heatshock this activity increased about 100% simultaneously with accumulation of trehalose, independent of synthesis of protein. This metabolic event may be primarily due to temperature-dependent changes in the kinetic properties of Tre6P synthase. Different sensibility to inhibition by phosphate, F6P, ATP and T6P was verified when the enzyme was assayed either at 30, 40 or 50 C. The increase of temperature diminished inhibition by phosphate compounds. A similar difference in degree of inhibition was also observed between crude extracts of cells submitted or not to heat shock. Tre6P synthase was induced four-fold during germination in glucose, reaching in 6 h its maximum sp. act. In glycerol, the activity was 15-17 fold lower. The enzyme was partially purified by phosphocellulose chromatography and FPLC(BioGel SP5 plus). Tre6P exhibited Mwapp of about 430 kDa, suggesting its presence in a complex. Km and Vmax, were respectively 0.814 mM and 528 nmoles/min/mg prot for UDPG, and 9.5 mM and 77.7 nmoles/min/mg prot for G6P. Next we shall test whether the fundamental regulatory roles attributed to yeast Tre6P also apply to filamentous fungi. Support: FAPESP and CNPq

Yeasts have been of great importance in biotechnology from the earliest days of humanity. They were used in the manufacture of bread, beer, wine and other fermented beverages, alone or in combination with bacteria, usually lactic acid bacteria, for so long that the dates are lost in the mists of antiquity. More recently, yeasts have been used in the food industries, as sources of flavorings and as sources of proteins, enzymes and other chemical products.

Yeasts have also been of great importance in the investigation of "pure" genetics and molecular biology, which has led to a great increase in their application to biotechnological processes. Most recently, the discovery of processes for transformation of yeasts as well as bacteria has led to the development of "genetic engineering" techniques, which are used in the manufacture of such new products as vaccines, hormones and other pharmaceuticals, too numerous to list in detail.

Most recently of all, the sequencing of the yeast genome has made available the knowledge of the detailed structure of the genes. Comparison of the genes of different species of organisms has shown many unsuspected similarities between genetic sequences in yeast (and its 6,000 genes)and the genomes of many other species. When the genes have been sequenced and compared, there should be many improved pharmaceuticals and similar products, in particular new proteinaceous materials, that can be manufactured.

The original "yeast" was undoubtedly a species of Saccharomyces, probably S. cerevisiae, though a fission yeast, Schizosaccharomyces pombe, has been used for at least as long for the manufacture of fermented beverages and fermented foods, in tropical countries.

It is also important, that "yeast" is not merely yeast; there are at least 700 different species now known, and an increasing number of them, have uses in biotechnology. There are species which metabolize starch completely, species which utilize lactose, some which metabolize methanol, (and which can find a great many applications in genetic engineering as hosts for heterologous genes encoding valuable proteins), species such as Yarrowia lipolytica, which can metabolize hydrocarbons (and at the same time, produce citric acid and other organic acids) and other yeasts, which can metabolize aromatic compounds and in the process, carry out the remediation of polluted soils and waters.

The hydrocarbon-metabolizing species, and other yeast species as well, can be used for production of biomass which can be used for human and animal foods.

Many of these species have been described by K. Wolf, in a recent book, "The Non-Conventional Yeasts" but there are many more. For instance, there are the yeasts which produce surfactants (surface-active agents) in high yield; Candida bombicola, Candida bogorensis, Candida apicola and others; some which produce polyhydroxy alcohols, (Zygosaccharomyces rouxii, Debaryomyces hansenii, Pichia farinosa and numerous others) which are useful as sweeteners in products ranging from foods and candies, to toothpastes; yeasts which produce uncommon enzymes (Trigonopsis variabilis) which in addition to having an unusual morphology (triangular or tetrahedral), produces a D-amino acid oxidase; There are also the yeasts which ferment pentoses to ethanol, which may be useful in producing this compound from hemicelluloses. These yeasts can be engineered either by protoplast fusion or transformation, to produce a yeast which will ferment hemicelloses directly. Yeasts which ferment xylose include Candida shehatae, Candida tropicalis, Pachysolen tannophilus, Picha farinosa and Pichia stipitis.

The basidiomycetous genera Cryptococcus and Rhodotorula, which produce both milk-clotting enzymes and carotenoid pigments (including Phaffia rhodozyma; perfect form Xanthophyllomyces dendrorous, which produces astaxanthin,. a pigment used to color the flesh of trout and salmon and egg yolks) The pigments of Rhodotorula spp may be used in food colorings.

There are other basidiomycetous yeast species (Sporobolomyces singularis) which metabolize lactose, but not galactose. and synthesize the tri- and tetrasaccharides, galactosyl-lactose and galactosyl-galactosyl lactose. These oligosaccharides are used as additives to baby foods in Japan.

In the beginning there was yeast, and it raised bread, brewed beer, and made wine. It was named Saccharomyces cerevisae, after many years. Then there was Schizosaccharomyces pombe, which multiplied by fission rather than by budding. In a few more years, there were many more yeasts, and the era of the non-conventional yeasts had begun.

All cells, from bacteria and yeasts to mammalian cells, respond to cues from their environment. A variety of mechanisms exist for the transduction of these external signals to the interior of the cell, resulting in altered patterns of protein activity. Eukaryotic cells commonly transduce external cues via a conserved module composed of three protein kinases, the mitogen-activated protein kinase (MAPK) cascade. This module can then activate substrates, some of which include transcriptional activators. Multiple MAPK signalling pathways coexist in a cell. This review considers different MAPK cascade signalling pathways that govern several aspects of the life cycle of budding and fission yeasts: conjugation and meiosis by the pheromone response pathway, stress response by the high-osmolarity sensing pathway, cell wall biosynthesis in response to activation of the low-osmolarity and heat-sensing pathway, and pseudohyphal growth in response to activation of a subset of the components of the pheromone response pathway. Because the MAPK cascade components are highly conserved, a key question in studies of these pathways is the mechanism by which specificity of response is achieved. Several other issues to be addressed in this review concern the nature of the receptors used to sense the external signals and the mechanism by which the receptors communicate with other components leading to activation of the MAPK cascade. Recently, it has become apparent that MAPK cascades are important in governing the pathogenicity of filamentous fungi.

Malassezia yeasts (lipophilic yeasts), have been classified to include seven species. Although molecular methods such as sequencing of RNA and karyotyping were used to determine the species, traditional techniques are also being explored for their identification. These include studies of morphology and the utilization of individual lipids. Reports now show the predominance of individual species recovered from normal skin and from patients with diseases such as pityriasis versicolor and seborrhoeic dermatitis. The majority of systemic infections reported have been in the bloodstream of premature neonates. Clusters of cases have occurred and molecular techniques employed to study the epidemiology. With the development of discriminatory methods to determine individual species and strains present in disease and in nature, our understanding of the pathogenicity and the epidemiology of this genus can be advanced.

The single celled yeast organisms is either oval or spherical shaped and reproduces by the "budding" system. This is accomplished when the nucleus divides, passing one to a bud forming from the wall of the mother cell. A wall then builds between the bud and the mother cell and separates to become a daughter cell. This is repeated until many smaller daughter cells are produced. These gradually increase in size, producing their own daughter cells. Both yeasts and molds prefer a mildly acidic atmosphere.

Candidiasis is caused by the genus Candida, which is a true yeast and not dimorphic, although chains of yeast cells sometimes link together, forming a structure known as pseudo or false hyphae. When C. albicans lives as a part of the normal body flora (the mucous membranes of the mouth, the vagina, and the intestinal tract), it is typically seen as a yeast, but when it is actively causing disease, it is more likely to appear with mycelia (a mass of threadlike processes). It becomes a problem only when the immune system becomes weakened and unable to fight off the growing numbers. Other agents capable of causing systemic fungal infections are mycelial when free-living, but adopt a yeast form when causing infection.

C. albicans is the usual cause of the oral 'thrush' and diaper rash infections often seen in infants, as well as the vaginal infections that plague women. Candidiasis of the skin may appear in areas where skin is damp (it does not grow well in dry environments) or irritated, but it seems to be more prevalent in persons on broad-spectrum antibiotic therapy. Members of this genus can also produce such serious illnesses as endocarditis, septicemia, protracted urinary tract infections, kidney and lung infections, esophagitis, and other soft tissue infections. Even with treatment at this stage, mortality rates are high.

Even though C. albicans remains the most common cause of serious candidiasis, other Candida species are becoming more and more common. This yeast is not to be confused with baker's yeast or brewer's yeast (Saccharomyces cerevisiae) or the English food Marmite, which uses Candida utilis. It was developed during shortages of WWII for use on toast. Live baker's yeast should be avoided since it depletes the body of B vitamins and other vital nutrients.

Symptoms of candida-related conditions include the following: fatigue or lethargy, poor memory, feeling spacey or unreal, numbness, burning, tingling, insomnia, muscle aches, weakness, abdominal pain, constipation, bloating, intestinal gas, vaginal itching/burning/discharge, prostatitis, PMS, attacks of anxiety or crying, shaking or irritability when hungry, headaches, sinusitis, moodiness, white tongue, tendency to bruise easily, chronic rashes, itching, food sensitivities or intolerance, mucus in stools, rectal itching, hoarseness or loss of voice, and nasal itching.

Ethanol and thermotolerance in the bioconversion of xylose by yeasts

The mechanisms underlying ethanol and heat tolerance are complex. Many different genes are involved, and the exact basis is not fully understood. The integrity of cytoplasmic and mitochondrial membranes is critical to maintain proton gradients for metabolic energy and nutrient uptake. Heat and ethanol stress adversely affect membrane integrity. These factors are particularly detrimental to xylose-fermenting yeasts because they require oxygen for biosynthesis of essential cell membrane and nucleic acid constituents, and they depend on respiration for the generation of ATP. Physiological responses to ethanol and heat shock have been studied most extensively in S. cerevisiae. However, comparative biochemical studies with other organisms suggest that similar mechanisms will be important in xylose-fermenting yeasts. The composition of a cellas membrane lipids shifts with temperature, ethanol concentration, and stage of cultivation. Levels of unsaturated fatty acids and ergosterol increase in response to temperature and ethanol stress. Inositol is involved in phospholipid biosynthesis, and it can increase ethanol tolerance when provided as a supplement. Membrane integrity determines the cellas ability to maintain proton gradients for nutrient uptake. Plasma membrane ATPase generates the proton gradient, and the biochemical characteristics of this enzyme contribute to ethanol tolerance. Organisms with higher ethanol tolerance have ATPase activities with low pH optima and high affinity for ATP. Likewise, organisms with ATPase activities that resist ethanol inhibition also function better at high ethanol concentrations. ATPase consumes a significant fraction of the total cellular ATP, and under stress conditions when membrane gradients are compromised the activity of ATPase is regulated. In xylose-fermenting yeasts, the carbon source used for growth affects both ATPase activity and ethanol tolerance. Cells can adapt to heat and ethanol stress by synthesizing trehalose and heat-shock proteins, which stabilize and repair denatured proteins. The capacity of cells to produce trehalose and induce HSPs correlate with their thermotolerance. Both heat and ethanol increase the frequency of petite mutations and kill cells. This might be attributable to membrane effects, but it could also arise from oxidative damage. Cytoplasmic and mitochondrial superoxide dismutases can destroy oxidative radicals and thereby maintain cell viability. Improved knowledge of the mechanisms underlying ethanol and thermotolerance in S. cerevisiae should enable the genetic engineering of these traits in xylose- fermenting yeasts.

Yeasts have been known and used in food and alcoholic fermentations ever since the Neolithic Age. In more recent times, on the basis of their peculiar features and history, yeasts have become very important experimental models in both microbiological and genetic research, as well as the main characters in many fermentative production processes. In the last 40 years, advances in molecular biology and genetic engineering have made possible not only the genetic selection of organisms, but also the genetic modification of some of them, especially the simplest of them, such as bacteria and yeasts. These discoveries have led to the availability of new yeast strains fit to fulfill requests of industrial production and fermentation. Moreover, genetically modified and transformed yeasts have been constructed that are able to produce large amounts of biologically active proteins and enzymes. Thus, recombinant yeasts make it easier to produce drugs, biologically active products, diagnostics, and vaccines, by inexpensive and relatively simple techniques. Yeasts are going to become more and more important in the "biotechnological revolution" by virtue of both their features and their very long and safe use in human nutrition and industry.

Yeast genetic nomenclature

Yeast genes are given 3 letter names with one or two digits after them, such as CDC33. Classically, yeast gene names were given for the phenotype of the mutant. Thus ste genes such as ste2, ste3, etc., confer a sterile phenotype and his3 mutants require histidine. Genes can also be named after the proteins or RNAs they encode, an example being CMD1, which encodes calmodulin. As you can see, we use the italic (or underscore) to denote genes and upper case to denote wild-type. Loss of function mutants are lower case. Known alleles are given after a hyphen. A dominant mutant is usually upper case with an allele designation, such as DAF1-1. Corresponding proteins are capitalized as proper nouns, such as Kex2. Phenotypes are usually indicated as follows: STE+ and TS+ mean not sterile and not temperature sensitive. ts- and ste- mean temperature sensitive and sterile. The plus means okay for that phenotype and the minus means the particular strain has the phenotype.

What Is Salmonella?, What Is Dna?, What Is MIC?, What Is Functional Genomics?, What Is Biofilm?, a, Microbe, e, Microbiology, c, Bacterium, r, Bacteriology, s, Microorganism, c, Pseudomonas aeruginosa, c, Schizosaccharomyces, e, Enterobacteriacea, s, Microorganism, c, Antimicrobial, n, Staphylococcus, n, Kluyveromyces




 

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Last modified: May 25, 2005