Identification and characterization of the irvA repressor in Streptococcus mutans

Objectives: Previous studies identified irvA as a highly inducible transcription regulator capable of repressing mutacin I gene expression in Streptococcus mutans. Since irvA itself appeared to be strongly regulated at the transcriptional level, we aimed to identify and characterize its regulator(s). An uncharacterized ORF (SMU.1398) located directly adjacent to irvA and annotated as a putative transcription repressor was identified as a likely candidate. Methods: Double crossover was employed to create the mutants and real-time RT-PCR was used to study gene expression. Genetic competence was determined by transformation efficiency assay with the E. coli-S. mutans shuttle vector pDL278. Results: The expression of irvA was highly increased in the SMU.1398 background. Mutation of SMU.1398 (“irvR”) also completely abolished genetic competence and reduced the expression of the late competence genes comEA (SMU.625), comY (SMU.1987c - SMU.1980c), and dprA (SMU.1001). Each of these irvR mutant phenotypes could be rescued with a double mutation of irvA. Surprisingly, no effect was seen upon the expression of the late competence gene regulators comC,D,E, or X. Conclusions: The repression of irvA is dependent upon irvR. This repression is critical for the development of genetic competence and the proper expression of the late competence genes. Furthermore, the irvA-dependent pathway appears to circumvent the upstream components of the competence cascade to affect the late competence genes directly.

This work was supported by an NCRR P20-RR018741-05 COBRE grant to J.M.