The RssB response regulator directly targets sigma(S) for degradation by ClpXP

The sigma (s) subunit of Escherichia coli RNA polymerase regulates the expr ession of stationary phase and stress response genes. Control over sigma (s ) activity is exercised in part by regulated degradation of sigma (s). In v ivo, degradation requires the ClpXP protease together with RssB, a protein homologous to response regulator proteins. Using purified components, we re constructed the degradation of sigma (s) in vitro and demonstrate a direct role for RssB in delivering sigma (s) to ClpXP. RssB greatly stimulates sig ma (s) degradation by ClpXP. Acetyl phosphate, which phosphorylates RssB, i s required. RssB participates in multiple rounds of sigma (s) degradation, demonstrating its catalytic role. RssB promotes sigma (s) degradation speci fically; it does not affect degradation of other ClpXP substrates or other proteins not normally degraded by ClpXP. sigma (s) and RssB form a stable c omplex in the presence of acetyl phosphate, and together they form a ternar y complex with ClpX that is stabilized by ATP[gamma -S]. Alone, neither sig ma (s) nor RssB binds ClpX with high affinity. When ClpP is present, a larg er sigma (s)-RssB-ClpXP complex forms. The complex degrades sigma (s) and r eleases RssB from ClpXP in an ATP-dependent reaction. Our results illuminat e an important mechanism for regulated protein turnover in which a unique t argeting protein, whose own activity is regulated through specific signalin g pathways, catalyzes the delivery of a specific substrate to a specific pr otease.