Regulation of RpoS proteolysis in Escherichia coli: The response regulatorRssB is a recognition factor that interacts with the turnover element in RpoS

The degradation of the RpoS (sigma(S)) subunit of RNA polymerase in Escheri chia coli is a prime example of regulated proteolysis in prokaryotes. RpoS turnover depends on ClpXP protease, the response regulator RssB, and a hith erto uncharacterized "turnover element" within RpoS itself. Here we localiz e the turnover element to a small element (around the crucial amino acid ly sine-173) directly downstream of the promoter-recognizing region 2.4 in Rpo S. Its sequence as well as its location identify the turnover element as a unique proteolysis-promoting motif. This element is shown to be a site of i nteraction with RssB. Thus, RssB is functionally unique among response regu lators as a direct recognition factor in ClpXP-dependent RpoS proteolysis. Binding of RssB to RpoS is stimulated by phosphorylation of the RssB receiv er domain, suggesting that environmental stress affects RpoS proteolysis by modulating RssB affinity for RpoS. Initial evidence indicates that lysine- 173 in RpoS, besides being essential of RpoS proteolysis, may play a role i n promoter recognition. Thus the same region in RpoS is crucial for proteol ysis as well as for activity as a transcription factor.