The response regulator RssB controls stability of the σ^S subunit of RNA polymerase in Escherichia coli

The rpoS-encoded σ^S subunit of RNA polymerase is a central regulator in a regulatory network that governs the expression of many stationary phase-induced and osmotically regulated genes in Escherichia coli. σ^S is itself induced under these conditions due to an increase in rpoS transcription (only in rich media) and rpoS translation as well as a stabilization of σ^S protein which in growing cells is subject to rapid turnover. We demonstrate here that a response regulator, RssB, plays a crucial role in the control of the cellular σ^S content. rssB null mutants exhibit nearly constitutively high levels of σ^S and are impaired in the post-transcriptional growth phase-related and osmotic regulation of σ^S. Whereas rpoS translational control is not affected, σ^S is stable in rssB mutants, indicating that RssB is essential for σ^S turnover. RssB contains a unique C-terminal output domain and is the first known response regulator involved in the control of protein turnover.