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

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.