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BIOLOGICAL SCIENCES / MICROBIOLOGY
The PhoP/PhoQ two-component system stabilizes the alternative sigma factor RpoS in Salmonella enterica
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Department of Molecular Microbiology and Howard Hughes Medical Institute, Washington University School of Medicine, St. Louis, MO 63110; and Laboratory of Molecular Biology, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892
Contributed by Susan Gottesman, July 19, 2006
The sigma factor RpoS regulates the expression of many stress response genes and is required for virulence in several bacterial species. We now report that RpoS accumulates when Salmonella enterica serovar Typhimurium is growing logarithmically in media with low Mg2+ concentrations. This process requires the two-component regulatory system PhoP/PhoQ, which is specifically activated in low Mg2+. We show that PhoP controls RpoS protein turnover by serving as a transcriptional activator of the iraP (yaiB) gene, which encodes a product that enhances RpoS stability by interacting with RssB, the protein that normally delivers RpoS to the ClpXP protease for degradation. Mutation of the phoP gene rendered Salmonella as sensitive to hydrogen peroxide as an rpoS mutant after growth in low Mg2+. In Escherichia coli, low Mg2+ leads to only modest RpoS stabilization, and iraP is not regulated by PhoP/PhoQ. These findings add the sigma factor RpoS to the regulatory proteins and two-component systems that are elevated in a PhoP/PhoQ-dependent fashion when Salmonella face low Mg2+ environments. Our data also exemplify the critical differences in regulatory circuits that exist between the closely related enteric bacteria Salmonella and E. coli.
IraP | magnesium | Escherichia coli
Author contributions: X.T., A.B., S.G., and E.A.G. designed research; X.T., T.L., and A.B. performed research; X.T., A.B., S.G., and E.A.G. analyzed data; and X.T., A.B., S.G., and E.A.G. wrote the paper.
Conflict of interest statement: No conflicts declared.
¶To whom correspondence may be addressed. E-mail: groisman{at}borcim.wustl.edu or susang{at}helix.nih.gov
© 2006 by The National Academy of Sciences of the USA
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