, the stationary phase sigma factor of Escherichia coli and Salmonella, is regulated at multiple levels. The s S protein is unstable during exponential growth and is stabilized during stationary phase and after various stress treatments. Degradation requires both the ClpXP protease and the adaptor RssB. The small antiadaptor protein IraP is made in response to phosphate starvation and interacts with RssB, causing s S stabilization under this stress condition. IraP is essential for s S stabilization in some but not all starvation conditions, suggesting the existence of other anti-adaptor proteins. We report here the identification of new regulators of s S stability, important under other stress conditions. IraM (inhibitor of RssB activity during Magnesium starvation) and IraD (inhibitor of RssB activity after DNA damage) inhibit s S proteolysis both in vivo and in vitro. Our results reveal that multiple anti-adaptor proteins allow the regulation of s S stability through the regulation of RssB activity under a variety of stress conditions.
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