Abstract
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.
Highlights
Survival in a hostile environment requires gene products that protect an organism from the particular stresses present in that environment
A phosphorelay consists of sensor proteins that respond to specific signals by activating a cognate regulatory protein that alters gene expression
We report that the regulator RcsB is activated by both cognate and non-cognate phosphorelay proteins in the bacterium Salmonella enterica serovar Typhimurium
Summary
Survival in a hostile environment requires gene products that protect an organism from the particular stresses present in that environment. Phosphorelay systems allow bacteria to respond to environmental insults by activating a regulatory protein that alters the expression of a subset of genes [1, 2]. To ensure a specific response to a given stress condition, phosphorelay systems usually restrict activation to their respective cognate regulatory proteins [3,4,5]. We report that a regulatory protein is activated both by cognate and non-cognate phosphorelay proteins depending on the stress experienced by the bacterium Salmonella enterica serovar Typhimurium. Bacteria and cell wall-containing eukaryotes utilize two-component systems to alter behavior in response to changes in environmental or cellular conditions [6, 7]. The prototypical two-component system consists of a sensor that responds to a specific signal(s) by modifying the activity of a cognate regulator through phosphorylation. Phosphorelays are versions of two-component systems in which there is a three-step phosphotransfer, whereby the phosphoryl group is shuttled from the sensor kinase to the response regulator via an intermediary phosphotransferase protein or domain, as opposed to the single phosphotransfer event from a sensor to a regulator in classical two-component systems [8, 9]
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