Phosphorylation at the D53 but Not the T65 Residue of CovR Determines the Repression of rgg and speB Transcription in emm1- and emm49-Type Group A Streptococci.

CovR/CovS is a two-component regulatory system in group A Streptococcus and primarily acts as a transcriptional repressor. The D53 residue of CovR (CovRD53) is phosphorylated by the sensor kinase CovS, and the phosphorylated CovRD53 protein binds to the intergenic region of rgg-speB to inhibit speB transcription. Nonetheless, the transcription of rgg and speB is suppressed in covS mutants. The T65 residue of CovR is phosphorylated in a CovS-independent manner, and phosphorylation at the D53 and T65 residues of CovR is mutually exclusive. Therefore, how phosphorylation at the D53 and T65 residues of CovR contributes to the regulation of rgg and speB expression was elucidated. The transcription of rgg and speB was suppressed in the strain that cannot phosphorylate the D53 residue of CovR (CovRD53A mutant) but restored to levels similar to those of the wild-type strain in the CovRT65A mutant. Nonetheless, inactivation of the T65 residue phosphorylation in the CovRD53A mutant cannot derepress the rgg and speB transcription, indicating that phosphorylation at the T65 residue of CovR is not required for repressing rgg and speB transcription. Furthermore, trans complementation of the CovRD53A protein in the strain that expresses the phosphorylated CovRD53 resulted in the repression of rgg and speB transcription. Unlike the direct binding of the phosphorylated CovRD53 protein and its inhibition of speB transcription demonstrated previously, the present study showed that inactivation of phosphorylation at the D53 residue of CovR contributes dominantly in suppressing rgg and speB transcription.IMPORTANCE CovR/CovS is a two-component regulatory system in group A Streptococcus (GAS). The D53 residue of CovR is phosphorylated by CovS, and the phosphorylated CovRD53 binds to the rgg-speB intergenic region and acts as the transcriptional repressor. Nonetheless, the transcription of rgg and Rgg-controlled speB is upregulated in the covR mutant but inhibited in the covS mutant. The present study showed that nonphosphorylated CovRD53 protein inhibits rgg and speB transcription in the presence of the phosphorylated CovRD53 in vivo, indicating that nonphosphorylated CovRD53 has a dominant role in suppressing rgg transcription. These results reveal the roles of nonphosphorylated CovRD53 in regulating rgg transcription, which could contribute significantly to invasive phenotypes of covS mutants.