In γ-proteobacteria such asEscherichia coli, the general stress response is mediated by σs, the stationary phase dissociable promoter specificity subunit of RNA polymerase. σsis degraded by ClpXP during active growth in a process dependent on the RssB adaptor, which acts catalytically and is thought to be stimulated by phosphorylation of a conserved aspartate in its N-terminal receiver domain. Here we present the crystal structure of full-length RssB bound to a beryllofluoride phosphomimic. Compared to the inhibited IraD anti-adaptor-bound RssB structure, our study reveals movements and coil-to-helix transitions in the C-terminal region of the RssB receiver domain and in the inter-domain segmented helical linker, accompanied by packing of the C-terminal effector domain onto the [alpha]4-β5-α5 (4-5-5) signaling face of the RssB receiver domain. This face is often the locus of protein-protein interactions in unphosphorylated receiver domains, but its masking is unusual in their phosphorylated forms. Our structure emphasizes the remarkable plasticity that underpins regulatory strategies within the large family of response regulators.

In the model organism Escherichia coli and related species, the general stress response relies on tight regulation of the intracellular levels of the promoter specificity subunit RpoS. RpoS turnover is exclusively dependent on RssB, a two‐domain response regulator that functions as an adaptor that delivers RpoS to ClpXP for proteolysis. Here, we report crystal structures of the receiver domain of RssB both in its unphosphorylated form and bound to the phosphomimic BeF3−. Surprisingly, we find only modest differences between these two structures, suggesting that truncating RssB may partially activate the receiver domain to a “meta‐active” state. Our structural and sequence analysis points to RssB proteins not conforming to either the Y–T coupling scheme for signaling seen in prototypical response regulators, such as CheY, or to the signaling model of the less understood FATGUY proteins.

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