Molecular mechanisms of signal transduction by the Campylobacter jejuni BumSR two-component system - Project Summary Campylobacter jejuni is both a pathogen that is the leading cause of bacterial diarrheal disease in humans in the US and throughout the world and a commensal of the intestinal tract of many animals and avian species. As such, sporadic cases of diarrheal disease in humans are most often attributed to handling or consuming contaminated poultry meat. For both the susceptible human host and the natural avian host, C. jejuni must identify lower intestinal niches that support growth for infection. We observed that C. jejuni senses and responds to specific metabolites generated by the intestinal microbiota of both avian and human hosts. We discovered that the C. jejuni BumSR two-component signal transduction system (TCS) is required to sense and respond to exogenous butyrate (a short-chain fatty acid; SCFA) and specific branched short-chain fatty acids (BSCFAs). Sensing these metabolites is important for host interactions as the BumSR TCS is required for C. jejuni to infect humans for diarrheal disease and promote optimal commensal colonization of chickens. We discovered that upon sensing butyrate and specific BSCFAs, the BumSR TCS mediates an unusual mode of signal transduction to alter expression of specific C. jejuni genes required for in vivo growth. We observed that BumS functions as a sensor phosphatase, rather than a sensor kinase, whose activity is altered upon sensing its specific cues. Instead of contributing to phosphorylation of a response regulator like most other bacterial TCS sensors, BumS dephosphorylates its cognate BumR response regulator to control its activity in altering gene transcription. Specific BSCFAs are direct cues sensed by BumS to inhibit its dephosphorylation of BumR. However, butyrate did not inhibit BumS phosphatase activity, suggesting that butyrate is indirectly sensed by BumS. Because BumS does not function as a kinase, BumR must use a non-cognate phosphodonor in C. jejuni to form P-BumR. Consequently, the design of this system in employing a sensor that exclusively functions as a phosphatase rather than a kinase requires the BumSR TCS to integrate multiple input cues to properly control transcription of its regulon. Molecular mechanisms for understanding how this unusual TCS senses intestinal metabolites and mediates sensor phosphatase-driven signal transduction remain to be discovered. In Aim 1, we will determine how BumS senses specific BSCFAs as direct cues and functions as a sensor phosphatase in a non-canonical mechanism of signal transduction for a bacterial TCS. In Aim 2, we will examine leading endogenous phosphodonor candidates for BumR and understand how these additional inputs inform the bacterium about conditions in intestinal niches. Completion of goals will establish new molecular mechanisms for how bacterial TCSs execute signal transduction by exclusively employing a sensor phosphatase and reveal how intestinal bacteria sense SCFA or BSCFA metabolites.
