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Microbiology and Molecular Biology Reviews, September 2002, p. 373-395, Vol. 66, No. 3
1092-2172/02/$04.00+0     DOI: 10.1128/MMBR.66.3.373-395.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

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

Institut für Biologie, Mikrobiologie, Freie Universität Berlin, 14195 Berlin, Germany

The ^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 ^S, exposure to many different stress conditions results in rapid and strong ^S induction. Consequently, transcription of numerous ^S-dependent genes is activated, many of which encode gene products with stress-protective functions. Multiple signal integration in the control of the cellular ^S level is achieved by rpoS transcriptional and translational control as well as by regulated ^S proteolysis, with various stress conditions differentially affecting these levels of ^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 ^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 ^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 ^S proteolysis, the response regulator RssB is essential. RssB is a specific direct ^S recognition factor, whose affinity for ^S is modulated by phosphorylation of its receiver domain. RssB delivers ^S to the ClpXP protease, where ^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.