hdrM regulates competence and mutacin production in Streptococcus mutans

Objectives: In our previous study, we characterized the hdrRM operon and found that it was induced >10-fold in a high cell density environment. The hdrM gene was also found to be a potent negative regulator of competence. In our present study, we investigated the mechanism by which hdrM negatively regulates genetic competence genes. In addition, since previous studies also reported that both mutacin Ⅳ production and competence development are regulated by the same comCDE quorum-sensing system, we also tested mutacin Ⅳ production in the hdrM mutant. Methods: We performed transformation assays comparing wild-type strain UA140 and competence gene mutant strains in the hdrM background. In addition, we measured transcription of competence genes in the hdrM mutant by Real-Time RT-PCR. Mutacin Ⅳ production was assessed by the deferred antagonism assay. Results: Transformation assays revealed that the double mutants of hdrM and comC or comE had transformation efficiencies >10-fold above wild-type levels, where as both comX and comY mutants remained untransformable. Real-Time RT-PCR data demonstrated that the hdrM mutant had no effect upon comC and comE expression. In contrast, comX and comY gene expression in the hdrM background was greatly increased throughout the growth curve as compared to the wild-type. In the plate assay for mutacin Ⅳ production, the hdrM mutant produced more mutacin Ⅳ than the wild-type, and the hdrM mutant rescued the mutacin Ⅳ^－phenotype of the comE mutant. Conclusions: Results from this study indicated that hdrM comprises part of a second pathway responsible for regulatory control over genetic competence and mutacin Ⅳ production. Interestingly, this pathway appears to be independent of the comCDE quorum-sensing system.

This work was supported by a NCRR P20-RR018741-05 COBRE grant and NIDCR DE018725-01 grant to J. M.