IraM Remodels the RssB Segmented Helical Linker to Stabilize σsagainst Degradation by ClpXP

ABSTRACT Upon Mg 2+ starvation, a condition often associated with virulence, enterobacteria inhibit the ClpXP-dependent proteolysis of the master transcriptional regulator, σ s , via IraM, a poorly understood anti-adaptor that prevents RssB-dependent loading of σ s onto ClpXP. This inhibition results in σ s accumulation, and expression of stress resistance genes. Here we report on the structural analysis of RssB bound to IraM, which reveals that IraM induces two folding transitions within RssB, which are amplified via a segmental helical linker. This work highlights the remarkable structural plasticity of RssB and reveals how a stress-specific RssB antagonist modulates a core stress response pathway that could be leveraged to control biofilm formation, virulence, and the development of antibiotic resistance.