The phosphorelay BarA/SirA activates the non-cognate regulator RcsB in Salmonella enterica

To survive an environmental stress, organisms must detect the stress and mount an appropriate response. One way that bacteria do so is by phosphorelay systems that respond to a stress by activating a regulator that modifies gene expression. To ensure an appropriate response, a given regulator is typically activated solely by its cognate phosphorelay protein(s). However, we now report that the regulator RcsB is activated by both cognate and non-cognate phosphorelay proteins, depending on the condition experienced by the bacterium Salmonella enterica serovar Typhimurium. The RcsC and RcsD proteins form a phosphorelay that activates their cognate regulator RcsB in response to outer membrane stress and cell wall perturbations, conditions Salmonella experiences during infection. Surprisingly, the non-cognate phosphorelay protein BarA activates RcsB during logarithmic growth in Luria-Bertani medium in three ways. That is, BarA's cognate regulator SirA promotes transcription of the rcsDB operon; the SirA-dependent regulatory RNAs CsrB and CsrC further increase RcsB-activated gene transcription; and BarA activates RcsB independently of the RcsC, RcsD, and SirA proteins. Activation of a regulator by multiple sensors broadens the spectrum of environments in which a set of genes is expressed without evolving binding sites for different regulators at each of these genes.