The s subunit of RNA polymerase (encoded by the rpoS gene) is a master regulator in a complex regulatory network that governs the expression of many stationary-phase-induced and osmotically regulated genes in Escherichia coli. rpoS expression is itself osmotically regulated by a mechanism that operates at the posttranscriptional level. Cells growing at high osmolarity already exhibit increased levels of s during the exponential phase of growth. Osmotic induction of rpoS can be triggered by addition of NaCl or sucrose and is alleviated by glycine betaine. Stimulation of rpoS translation and a change in the half-life of s from 3 to 50 min both contribute to osmotic induction. Experiments with lacZ fusions inserted at different positions within the rpoS gene indicate that an element required for s degradation is encoded between nucleotides 379 and 742 of the rpoS coding sequence.Like most single-cell organisms, bacteria must be able to cope with extreme fluctuations in the composition and physical parameters of their environments. One of these parameters is the osmolarity of the surrounding medium. When Escherichia coli cells experience a shift to high osmolarity, influx of potassium ions and synthesis of glutamate are strongly stimulated. This rapid response is followed by uptake from the medium and/or synthesis of compatible solutes and osmoprotectants like glycine betaine, proline, or trehalose (for a review, see reference 5). In parallel, the induction of numerous proteins can be observed by two-dimensional O'Farrell gel electrophoresis (4, 14). Several corresponding genes have been identified, for instance, by isolating hyperosmotically inducible lacZ or phoA gene fusions (1,3,6,(8)(9)(10)35).With respect to the regulatory mechanisms involved, two systems have been studied in detail. One is the proU operon, which encodes a glycine betaine uptake system (for a recent review, see reference 24), whereas the other is the ompF/ompC porin system, which is controlled by a typical two-component regulatory system consisting of the membrane-bound sensory histidine kinase EnvZ and the response regulatory OmpR (15,17,26,27). However, the regulatory mechanisms involved do not seem to play a general role in the osmotic regulation of many genes, and efforts to identify a global osmotic regulator have failed.By contrast, several other hyperosmotically induced genes (otsBA, treA, osmB, osmY, and bolA) are under the control of s , a sigma subunit of RNA polymerase in E. coli that is encoded by the rpoS gene (13,14,19,34). This seems to implicate s as a global regulator in the osmotic control of gene expression. In fact, this would be a second global regulatory role for s , which is usually regarded as a stationary-phasespecific sigma factor since the genes mentioned above, as well as many other s -dependent genes, are induced during entry into stationary phase (for recent reviews, see references 11, 12, and 23). Besides being growth phase regulated at the levels of transcription, translation, and s protein stability, rpoS has a...

The hfq-encoded RNA-binding protein HF-I has long been known as a host factor for phage Q␤ RNA replication and has recently been shown to be essential for translation of rpoS, which encodes the S subunit of RNA polymerase. Here we demonstrate that an hfq null mutant does not synthesize glycogen, is starvation and multiple stress sensitive, and exhibits strongly reduced expression of representative S -regulated genes. These phenotypes are consistent with strongly reduced S levels in the hfq mutant. However, the analysis of global protein synthesis patterns on two-dimensional O'Farrell gels indicates that approximately 40% of the more than 30 proteins whose syntheses are altered in the hfq null mutant are not affected by an rpoS mutation. We conclude that HF-I is a global regulator involved in the regulation of expression of S and S -independent genes.The rpoS-encoded S subunit of RNA polymerase is a master regulator for many stationary-phase-inducible and generally stress-regulated genes in Escherichia coli. At present, more than 40 genes or operons are known to be under S control. Accordingly, rpoS null mutants exhibit a very pleiotropic phenotype including starvation and multiple-stress sensitivity, the inabilities to synthesize glycogen and trehalose, and a persistence of rod-shaped cells in stationary phase (15).S itself is induced during transition into stationary phase and in response to a variety of other stress conditions. This control is exerted by several mechanisms that act at the levels of transcription as well as translation and protein turnover (13,20