Signal Transduction and Regulatory Mechanisms Involved in Control of the σ^S (RpoS) Subunit of RNA Polymerase

SUMMARY

The σ^S (RpoS) subunit of RNA polymerase is the master regulator of the general stress response in Escherichia coli and related bacteria. While rapidly growing cells contain very little σ^S, exposure to many different stress conditions results in rapid and strong σ^S induction. Consequently, transcription of numerous σ^S-dependent genes is activated, many of which encode gene products with stress-protective functions. Multiple signal integration in the control of the cellular σ^S level is achieved by rpoS transcriptional and translational control as well as by regulated σ^S proteolysis, with various stress conditions differentially affecting these levels of σ^S control. Thus, a reduced growth rate results in increased rpoS transcription whereas high osmolarity, low temperature, acidic pH, and some late-log-phase signals stimulate the translation of already present rpoS mRNA. In addition, carbon starvation, high osmolarity, acidic pH, and high temperature result in stabilization of σ^S, which, under nonstress conditions, is degraded with a half-life of one to several minutes. Important cis-regulatory determinants as well as trans-acting regulatory factors involved at all levels of σ^S regulation have been identified. rpoS translation is controlled by several proteins (Hfq and HU) and small regulatory RNAs that probably affect the secondary structure of rpoS mRNA. For σ^S proteolysis, the response regulator RssB is essential. RssB is a specific direct σ^S recognition factor, whose affinity for σ^S is modulated by phosphorylation of its receiver domain. RssB delivers σ^S to the ClpXP protease, where σ^S is unfolded and completely degraded. This review summarizes our current knowledge about the molecular functions and interactions of these components and tries to establish a framework for further research on the mode of multiple signal input into this complex regulatory system.