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σS (RpoS) is a highly unstable global regulatory protein in Escherichia coli, whose degradation is inhibited by various stress signals, such as carbon starvation, high osmolarity and heat shock. As a consequence, these stresses result in the induction of σS-regulated stress-protective proteins. The two-component-type response regulator, RssB, is essential for the rapid proteolysis of σS and is probably involved in the transduction of some of these stress signals. Acetyl phosphate can be used as a phosphodonor for the phosphorylation of various 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 σS and the RpoS742::LacZ hybrid protein (also a substrate for RssB-dependent proteolysis) in acetyl phosphate-free (ptaackA) deletion mutants, even though no sensor kinase was eliminated. The in vivo data indicate that acetyl phosphate acts through the response regulator, RssB. In vitro, efficient phosphotransfer from radiolabelled acetyl phosphate to the Asp-58 residue of RssB (the expected site of phosphorylation in the RssB receiver domain) was observed. Via such phosphorylation, acetyl phosphate may thus modulate RssB activity 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 proteolysis that responds to the metabolic status of the cells reflected in the highly variable cellular acetyl phosphate concentration.