REGULATION OF RSSB-DEPENDENT PROTEOLYSIS IN ESCHERICHIA-COLI - A ROLEFOR ACETYL PHOSPHATE IN A RESPONSE REGULATOR-CONTROLLED PROCESS

sigma(s) (RpoS) is a highly unstable global regulatory protein in Esch erichia coil whose degradation is inhibited by various stress signals, such as carbon starvation, high osmolarity and heat shock, As a conse quence, these stresses result in the induction of sigma(s)-regulated s tress-protective proteins, The two-component-type response regulator, RssB, is essential for the rapid proteolysis of sigma(s) and is probab ly involved in the transduction of some of these stress signals, Acety l phosphate can be used as a phosphodonor for the phosphorylation of v arious response regulators in vitro and, in the absence of the cognate sensor kinases, acetyl phosphate can also modulate the activities of several response regulators in vivo, Here, we demonstrate increased in vivo half-lives of sigma(s) and the RpoS742::LacZ hybrid protein (als o a substrate for RssB-dependent proteolysis) in acetyl phosphate-free (pta-ackA) deletion mutants, even though no sensor kinase was elimina ted, The in vivo data indicate that acetyl phosphate acts through the response regulator, RssB. In vitro, efficient phosphotransfer from rad iolabelled acetyl phosphate to the Asp-58 residue of RssB (the expecte d site of phosphorylation in the RssB receiver domain) was observed, V ia such phosphorylation, acetyl phosphate may thus modulate RssB activ ity even in an otherwise wild-type background, While acetyl phosphate is not essential for the transduction of specific environmental stress signals, it could play the role of a modulator of RssB-dependent prot eolysis that responds to the metabolic status of the cells reflected i n the highly variable cellular acetyl phosphate concentration.