Competence Inhibition by the XrpA Peptide Encoded Within the comX Gene of Streptococcus mutans
Streptococcus mutans displays complex regulation of natural genetic competence. Competence development in S. mutans is controlled by a peptide derived from ComS (XIP); which along with the cytosolic regulator ComR controls the expression of the alternative sigma factor comX, the master regulator of competence development. Recently, a gene embedded within the coding region of comX was discovered and designated xrpA (comX regulatory peptide A). XrpA was found to be an antagonist of ComX, but the mechanism was not established. In this study, we reveal through both genomic and proteomic techniques that XrpA is the first describe negative regulator of ComRS systems in streptococci. Transcriptomic and promoter activity assays in the {Delta}xrpA strain revealed an up-regulation of genes controlled by both the ComR- and ComX-regulons. An in vivo protein crosslinking and in vitro fluorescent polarization assays confirmed that the N-terminal region of XrpA were found to be sufficient in inhibiting ComR-XIP complex binding to ECom-box located within the comX promoter. This inhibitory activity was sufficient for decreases in PcomX activity, transformability and ComX accumulation. XrpA serving as a modulator of ComRS activity ultimately results in changes to subpopulation behaviors and cell fate during competence activation.\n\nABBREVIATED SUMMARYStreptococcus mutans displays complex regulation of natural genetic competence, highlighted by a novel gene, xrpA, embedded within the coding region for the master regulator ComX. We show that XrpA modulates ComRS-dependent activation of comX expression, resulting in changes to sub-population behaviors, including cell lysis. XrpA is the first described inhibitor of a ComRS system and, because it is unique to S. mutans it may be targetable to prevent diseases caused by this pathogen.