A specificity determinant for phosphorylation in a response regulator prevents in vivo cross-talk and modification by XXXX .

@entity3793 @entity21809 @entity7097 @entity23062 @entity5363
Bacterial two-component systems (TCSs) sense stimuli and transduce signals intracellularly through phosphotransfer between cognate @entity3793 kinases (HKs) and response regulators (RRs) to alter gene expression or behavioral responses. Without high phosphotransfer specificity between cognate HKs and RRs, cross-phosphorylation or cross-talk between different TCSs may occur and diminish responses to appropriate stimuli. Some mechanisms to reduce cross-talk involve HKs controlling levels of cognate RR phosphorylation. Conceivably, some RRs may have evolved HK-independent strategies to insulate themselves from cross-talk with @entity21809 ( @entity23062 ) or other small phosphodonor metabolites. Initial steps in flagellar biosynthesis in @entity5363 stimulate phosphotransfer from the FlgS HK to the FlgR RR to promote   (54)-dependent flagellar gene expression. We discovered that the FlgR C-terminal domain (CTD), which commonly functions as a DNA-binding domain in the NtrC RR family, is a specificity determinant to limit in vivo cross-talk from @entity23062 . FlgR lacking the CTD (FlgR(/\CTD)) used FlgS or @entity23062 as an in vivo phosphodonor and could be reprogrammed in /\flgS mutants to respond to cellular nutritional status via @entity23062 levels. Even though exclusive @entity23062 -mediated activation of FlgR(/\CTD) promoted @entity7097 flagellar gene expression, proper flagellar biosynthesis was impaired. We propose that the FlgR CTD prevents phosphotransfer from @entity23062 so that FlgR is solely responsive to FlgS to promote proper flagellar gene expression and flagellation. In addition to mechanisms limiting cross-talk between noncognate HKs and RRs, our work suggests that RRs can possess domains that prevent in vivo cross-talk between RRs and the endogenous metabolite @entity23062 to ensure signaling specificity.
