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SoxRS-Regulated Expression and Genetic Analysis of the yggX Gene of Escherichia coli. Pablo J. Pomposiello, 2003.Genomic studies with bacteria have identified redox-responsive genes without known roles in counteracting oxidative damage . Previous transcriptional profiling showed that expression of one such gene, yggX, was activated by superoxide stress in Escherichia coli . Here we show that this activation could be mimicked by artificial expression of the regulatory protein SoxS . Northern analysis confirmed the transcriptional activation of yggX by oxidative stress or SoxS expression but not in response to the related MarA or Rob proteins . Northern analysis showed that mltC, which codes for a peptidoglycan hydrolase and is positioned immediately downstream of yggX, was also regulated by oxidative stress or ectopic expression of SoxS . Purified SoxS protein bound to the predicted yggX promoter region, between positions 223 and 163 upstream from the yggX translational start site . Within this region, a 20-bp sequence was found to be necessary for oxidative stress-mediated activation of yggX transcription . A yggX deletion strain was hypersensitive to the redox-cycling agent paraquat, and a plasmid expressing YggX complemented the sensitivity of the deletion strain . Under exposure to paraquat, the yggX deletion strain showed a deficiency in aconitase activity compared to the isogenic wild-type strain, while expression of YggX from a multicopy plasmid increased the aconitase levels above those of the wild-type strain . These results demonstrate the direct regulation of the yggX gene by the redox-sensing SoxRS system and provide further evidence for the involvement of yggX in protection of iron-sulfur proteins against oxidative damage . Horizontal Transfer of CS1 Pilin Genes of Enterotoxigenic Escherichia coli. Barbara Froehlich, 2004.CS1 is one of a limited number of serologically distinct pili found in enterotoxigenic Escherichia coli (ETEC) strains associated with disease in people . The genes for the CS1 pilus are on a large plasmid, pCoo . We show that pCoo is not self-transmissible, although our sequence determination for part of pCoo shows regions almost identical to those in the conjugative drug resistance plasmid R64 . When we introduced R64 into a strain containing pCoo, we found that pCoo was transferred to a recipient strain in mating . Most of the transconjugant pCoo plasmids result from recombination with R64, leading to acquisition of functional copies of all of the R64 transfer genes . Temporary coresidence of the drug resistance plasmid R64 with pCoo leads to a permanent change in pCoo so that it is now self-transmissible . We conclude that when R64-like plasmids are transmitted to an ETEC strain containing pCoo, their recombination may allow for spread of the pCoo plasmid to other enteric bacteria . Colonization of Flax Roots and Early Physiological Responses of Flax Cells Inoculated with Pathogenic and Nonpathogenic Strains of Fusarium oxysporum. Chantal Olivain, 2003.Fusarium oxysporum includes nonpathogenic strains and pathogenic strains that can induce necrosis or tracheomycosis in plants . The objective of this study was to compare the abilities of a pathogenic strain (Foln3) and a nonpathogenic strain (Fo47) to colonize flax roots and to induce early physiological responses in flax cell culture suspensions . Both strains colonized the outer cortex of the root; however, plant defense reactions, i.e., the presence of wall appositions, osmiophilic material, and collapsed cells, were less frequent and less intense in a root colonized by Foln3 than by Fo47 . Early physiological responses were measured in flax cell suspensions confronted with germinated microconidia of both strains . Both pathogenic (Foln3) and nonpathogenic strains (Fo47) triggered transient H2O2 production in the first few minutes of the interaction, but the nonpathogenic strain also induced a second burst 3 h postinoculation . Ca2+ influx was more intense in cells inoculated with Fo47 than in cells inoculated with Foln3 . Similarly, alkalinization of the extracellular medium was higher with Fo47 than with Foln3 . Inoculation of the fungi into flax cell suspensions induced cell death 10 to 20 h postinoculation, with a higher percentage of dead cells observed with Fo47 than with Foln3 beginning at 14 h . This is the first report showing that early physiological responses of flax cells can be used to distinguish pathogenic and nonpathogenic strains of the soil-borne fungus F . oxysporum .
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