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Tactic Responses to Oxygen in the Phototrophic Bacterium Rhodobacter sphaeroides WS8N. Simona Romagnoli, 2002.The temporal and spatial behavior of a number of mutants of the photosynthetic, facultative anaerobe Rhodobacter sphaeroides to both step changes and to gradients of oxygen was analyzed . Wild-type cells, grown under a range of conditions, showed microaerophilic behavior, accumulating in a 1.3-mm band about 1.3 mm from the meniscus of capillaries . Evidence suggests this is the result of two signaling pathways . The strength of any response depended on the growth and incubation conditions . Deletion of either the complete chemosensory operons 1 and 2 plus the response regulator genes cheY4 and cheY5 or cheA2 alone led to the loss of all aerotactic responses, although the cells still swam normally . The Prr system of R . sphaeroides responds to electron flow through the alternative high-affinity cytochrome oxidase, cbb3, controlling expression of a wide range of metabolic pathways . Mutants with deletions of either the complete Prr operon or the histidine kinase, PrrB, accumulated up to the meniscus but still formed a thick band 1.3 mm from the aerobic interface . This indicates that the negative aerotactic response to high oxygen levels depends on PrrB, but the mutant cells still retain the positive response . Tethered PrrB- cells also showed no response to a step-down in oxygen concentration, although those with deletions of the whole operon showed some response . In gradients of oxygen where the concentration was reduced at 0.4 µM/s, tethered wild-type cells showed two different phases of response, with an increase in stopping frequency when the oxygen concentration fell from 80 to 50% dissolved oxygen and a decrease in stopping at 50 to 20% dissolved oxygen, with cells returning to their normal stopping frequency in 0% oxygen . PrrB and CheA2 mutants showed no response, while PrrCBA mutants still showed some response . Distinct Roles of PII-Like Signal Transmitter Proteins and amtB in Regulation of nif Gene Expression, Nitrogenase Activity, and Posttranslational Modification of NifH in Azoarcus sp . Strain BH72. Dietmar E. Martin, 2002.PII-like signal transmitter proteins, found in Bacteria, Archaea, and plants, are known to mediate control of carbon and nitrogen assimilation . They indirectly regulate the activity of key metabolic enzymes and transcription factors by protein-protein interactions with signal transduction proteins . Many Proteobacteria harbor two paralogous PII-like proteins, GlnB and GlnK, whereas a novel third PII paralogue (GlnY) was recently identified in Azoarcus sp . strain BH72, a diazotrophic endophyte of grasses . In the present study, evidence was obtained that the PII-like proteins have distinct roles in mediating nitrogen and oxygen control of nif gene transcription and nitrogenase activity . Full repression of nif gene transcription in the presence of a combined nitrogen source or high oxygen concentrations was observed in wild-type and glnB and glnK knockout mutants, revealing that GlnB and GlnK can complement each other in mediating the repression . In contrast, in a glnBK double mutant strain in the presence of only GlnY, nif gene transcription was still detectable, albeit at a lower level, on nitrate or 20% oxygen . As another level of control, nitrogenase activity was regulated by at least three types of mechanisms in strain BH72: covalent modification of dinitrogenase reductase (NifH), probably by ADP-ribosylation, and two other, unknown means . Functional inactivation upon ammonium addition (switch-off) required the putative high-affinity ammonium transporter AmtB and GlnK, but not GlnB or GlnY . Functional inactivation in response to anaerobiosis did not depend on AmtB, GlnK, or GlnB . In contrast, covalent modification of NifH required both GlnB and GlnK and AmtB as response to ammonium addition, whereas it required either GlnB or GlnK and not AmtB when cells were shifted to anaerobiosis . In a glnBK double mutant expressing only GlnY, NifH modification was completely abolished, further revealing functional differences between the three PII paralogues . Characterization of the Two Mycobacterium tuberculosis recA Promoters. Krishna K. Gopaul, 2003.The recA gene of Mycobacterium tuberculosis is unusual in that it is expressed from two promoters, one of which, P1, is DNA damage inducible independently of LexA and RecA, while the other, P2, is regulated by LexA in the classical way (E . O . Davis, B . Springer, K . K . Gopaul, K . G . Papavinasasundaram, P . Sander, and E . C . Böttger, Mol . Microbiol . 46:791-800, 2002) . In this study we characterized these two promoters in more detail . Firstly, we localized the promoter elements for each of the promoters, and in so doing we identified a mutation in each promoter which eliminates promoter activity . Interestingly, a motif with similarity to Escherichia coli  70 -35 elements but located much closer to the -10 element is important for optimal expression of P1, whereas the sequence at the -35 location is not . Secondly, we found that the sequences flanking the promoters can have a profound effect on the expression level directed by each of the promoters . Finally, we examined the contribution of each of the promoters to recA expression and compared their kinetics of induction following DNA damage .
Secretory Antibodies Do Not Affect the Composition of the Bacterial Microbiota in the Terminal Ileum of 10-Week-Old Mice. Leanne Sait, 2003.Terminal restriction fragment length polymorphism (T-RFLP) analysis was conducted on the 16S rRNA genes of the bacterial communities colonizing the epithelial surfaces of the terminal ilea of open conventionally housed mice in an institutional small-animal facility . Polymeric-immunoglobulin-receptor-deficient (pIgR-/-) mice that were unable to secrete antibodies across mucosal surfaces were cohoused with normal and otherwise genetically identical wild-type (C57BL/6) mice for 4 weeks . If secretory antibodies played a role in modeling the gastrointestinal microbiota, C57BL/6 mice would have had a more distinct and uniform microbiota than their pIgR-/- cage mates . The T-RFLP profiles of the bacterial communities were compared by using Sorensen's pairwise similarity coefficient, a newly developed weighted pairwise similarity coefficient, and on the basis of Shannon's and Simpson's diversity indices . No systematic differences were observed between the dominant components of the mucosa-associated bacterial communities of the terminal ileal walls of the two types of mice, indicating that secretory antibodies do not control the composition of this microbiota . Similar analyses of experiments conducted at two different times, between which the bacterial community composition of the mouse colony in the small-animal facility appeared to have changed, showed that differences could have been detected, had they existed .
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