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Structure and Electrophysiological Properties of the YscC Secretin from the Type III Secretion System of Yersinia enterocolitica. Peter Burghout, 2004.YscC is the integral outer membrane component of the type III protein secretion machinery of Yersinia enterocolitica and belongs to the family of secretins . This group of proteins forms stable ring-like oligomers in the outer membrane, which are thought to function as transport channels for macromolecules . The YscC oligomer was purified after solubilization from the membrane with a nonionic detergent . Sodium dodecyl sulfate did not dissociate the oligomer, but it caused a change in electrophoretic mobility and an increase in protease susceptibility, indicating partial denaturation of the subunits within the oligomer . The mass of the homo-oligomer, as determined by scanning transmission electron microscopy, was approximately 1 MDa . Analysis of the angular power spectrum from averaged top views of negatively stained YscC oligomers revealed a 13-fold angular order, suggesting that the oligomer consists of 13 subunits . Reconstituted in planar lipid bilayers, the YscC oligomer displayed a constant voltage-independent conductance of approximately 3 nS, thus forming a stable pore . However, in vivo, the expression of YscC did not lead to an increased permeability of the outer membrane . Electron microscopy revealed that the YscC oligomer is composed of three domains, two stacked rings attached to a conical domain . This structure is consistent with the notion that the secretin forms the upper part of the basal body of the needle structure of the type III secreton . mig-14 Is a Salmonella Gene That Plays a Role in Bacterial Resistance to Antimicrobial Peptides. Igor E. Brodsky, 2002.It was previously demonstrated that the mig-14 gene of Salmonella enterica serovar Typhimurium is necessary for bacterial proliferation in the liver and spleen of mice following intragastric inoculation and that mig-14 expression, which is induced within macrophages, is under the control of the global regulator PhoP . Here we demonstrate that the mig-14 promoter is induced by growth in minimal medium containing low magnesium or acidic pH, consistent with regulation by PhoP . In addition, mig-14 is strongly induced by polymyxin B, protamine, and the mammalian antimicrobial peptide protegrin-1 . While phoP is necessary for the induction of mig-14 in response to protamine and protegrin, mig-14 is still induced by polymyxin B in a phoP background . We also demonstrate that mig-14 is necessary for resistance of S . enterica serovar Typhimurium to both polymyxin B and protegrin-1 . Gram-negative resistance to a variety of antimicrobial peptides has been correlated with modifications of lipopolysaccharide structure . However, we show that mig-14 is not required for one of these modifications, the addition of 4-aminoarabinose to lipid A . Additionally, sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of wild-type and mig-14 lipopolysaccharide also shows no detectable differences between the two strains . Therefore, mig-14 contributes to Salmonella resistance to antimicrobial peptides by a mechanism that is not yet fully understood . Transcription of the Staphylococcus aureus cid and lrg Murein Hydrolase Regulators Is Affected by Sigma Factor B. Kelly C. Rice, 2004.The Staphylococcus aureus lrg and cid loci are homologous operons that have been shown to regulate murein hydrolase activity and affect sensitivity to penicillin . Although the mode of action of these operons has not been demonstrated, a model based on the similarities of the lrgA and cidA gene products to the bacteriophage holin family of proteins has been proposed . In this study, the transcription organization and regulation of these operons were examined by Northern blot analyses . Unexpectedly, cidB and a gene located immediately downstream, designated cidC, were found to be cotranscribed on a 2.7-kb transcript . Maximal cidBC transcription occurred during early exponential growth, and high-level transcription of cidBC was dependent on the rsbU-mediated activation of the alternative sigma factor B (  B) . In contrast, lrgAB transcription in stationary phase was negatively regulated by
B . Although cidABC transcription was not detected by Northern blot analysis, reverse transcriptase PCR revealed that these genes are also cotranscribed as a single RNA message in early exponential growth . Primer extension analysis revealed the presence of two cidBC transcription start sites, but no apparent
B-dependent promoter consensus sequence was identified in these regions . The rsbU gene was also shown to have a positive impact on murein hydrolase activity but a negligible effect on sensitivity to penicillin-induced killing . These results suggest that the lrgAB and cidBC genes may be part of the S . aureus
B-controlled stress regulon .
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