Multiple pathways for regulation of σS (RpoS) stability in Escherichia coli via the action of multiple anti-adaptors

sigma(S), the stationary phase sigma factor of Escherichia coli and Salmonella, is regulated at multiple levels. The sigma(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 sigma(S) stabilization under this stress condition. IraP is essential for sigma(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 sigma(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 sigma(S) proteolysis both in vivo and in vitro. Our results reveal that multiple anti-adaptor proteins allow the regulation of sigma(S) stability through the regulation of RssB activity under a variety of stress conditions.