Identification and characterization of naturally occurring DSF ‐family quorum sensing signal turnover system in the phytopathogen X anthomonas

Summary Molecules of the diffusible signal factor ( DSF )‐family are a class of quorum sensing ( QS ) signals used by the phytopathogens X anthomonas . Studies during the last decade have outlined how X anthomonas cells enter the QS phase. However, information on the mechanism underlying its exit from the QS phase is limited. RpfB has recently been reported as a fatty acyl‐ CoA ligase ( FCL ) that activates a wide range of fatty acids to their CoA esters in vitro . Here, we establish an improved quantification assay for DSF ‐family signals using liquid chromatography‐mass spectrometry in X . campestris pv. campestris ( X cc ). We first demonstrated that RpfB represents a naturally occurring DSF ‐family signal turnover system .   RpfB effectively turns over DSF ‐family signals DSF and BDSF   in vivo . RpfB FCL enzymatic activity is required for DSF and BDSF turnover. Deletion of rpf B slightly increased X cc virulence in the C hinese radish and overexpression of rpf B significantly decreased virulence. We further showed that the expression of rpf B is growth phase‐dependent, and its expression is significantly enhanced when X cc cells enter the stationary phase. DSF regulates rpf B expression in a concentration‐dependent manner. rpf B expression is also negatively regulated by the DSF signalling components RpfC , RpfG and C lp. The global transcription factor C lp directly binds to the AATGC ‐tgctgc‐ GCATC motif in the promoter region of rpf B to repress its expression. Finally, RpfB ‐dependent signal turnover system was detected in a wide range of bacterial species, suggesting that it is a conserved mechanism.