Signaling Specificity in a Campylobacter jejuni Two-Component System to Mediate Proper Flagellar Gene Transcription
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Campylobacter jejuni is a worldwide leading cause of bacterial gastroenteritis. While infection of humans leads to diarrheal disease, C. jejuni asymptomatically colonizes the intestinal tract of many agriculturally-significant animals, especially poultry. Flagellar motility is essential for Campylobacter jejuni to promote commensal colonization of avian species and for infection of humans to result in disease. Expression of flagellar genes is regulated by alternative σ factors, whose activities are controlled by a regulatory cascade. Previous genetic screens discovered the flagellar type III secretion system (T3SS), the FlgSR two-component regulatory system (TCS), and the FlhF GTPase as requirements to positively regulate expression of σ54-dependent genes that encode flagellar rod and hook proteins. Our laboratory previously proposed that signal transduction through the FlgSR TCS initiates with FlgS detecting formation of the flagellar T3SS and culminates in phosphorylation of the FlgR response regulator and expression of flagellar genes. I investigated this model by determining if any other flagellar components are required for activation of FlgSR and expression of σ54-dependent flagellar genes. I found that mutants lacking the MS ring (FliF), the rotor component of the C ring (FliG), and the rod proteins (FliE, FlgB, FlgC and FlgF) expressed reduced levels of σ54-dependent flagellar genes. These findings suggest a more complex flagellar structure rather than solely the flagellar T3SS is required to activate σ54-dependent gene expression. Due to data generated by additional experimentation, I propose a revised model in which the C. jejuni flagellar T3SS facilitates polymerization of the MS ring and rotor component of the C ring, which together form a cytoplasmic domain that is likely the direct signal sensed by FlgS to activate signal transduction required for flagellar gene expression in C. jejuni. Previous analysis discovered that activation of σ54-dependent flagellar gene expression in C. jejuni is dependent on phosphotransfer through the FlgSR two-component system. Whereas this signaling mechanism results in specific activation of FlgR via its cognate FlgS sensor kinase, I identified a domain of FlgR that possesses an unusual activity in specifically preventing in vivo crosstalk with small phosphodonor metabolites. Through genetic and biochemical analysis, I demonstrated that the metabolite acetyl-phosphate (AcP) serves as an efficient phosphodonor for FlgR lacking its C-terminal domain but not for wild-type FlgR. Additionally, I could reprogram FlgR-dependent flagellar gene expression to respond to the metabolic state of the cell and restore wild-type levels of flagellar gene expression in the absence of FlgS. However, flagellar biosynthesis was not restored to wild-type levels when AcP was the sole phosphodonor for FlgR. This study illustrates how signaling specificity in a TCS ensures a correct output response and highlights the importance of controlling proper signaling between cognate histidine kinase and response regulator pairs in bacterial TCS systems.