The carbon catabolite repressor CcpA mediates optimal competence development in Streptococcus oligofermentans through post-transcriptional regulation.

Natural transformation increases the genetic diversity of bacteria, but is costly and must be strictly controlled. We previously found that deletion of ccpA, a key regulator of carbon catabolite repression (CCR), reduced transformation efficiency of Streptococcus oligofermentans, the current work further investigated the regulatory mechanisms of CcpA. The competence operon comCDE is subjected to basal and autoregulatory transcription. A luciferase reporter detected a transcriptional readthrough (TRT) from the upstream tRNA into the comCDE operon, which was induced by -arginine. Insertion of the Escherichia coli T1T2 terminator downstream of tRNA abolished TRT, and reduced the basal comCDE transcription by 77% and also the transformation efficiency. Deletion of ccpA increased tRNA TRT and tRNA -comCDE polycistronic transcript by twofold. An in vitro transcription assay determined that CcpA promoted the transcription termination of tRNA TRT, and RNA EMSA and SPR assays detected equal binding affinity of CcpA to both the RNA and DNA of tRNA . These results indicate that CcpA controls the basal comCDE transcription by post-transcriptional actions. Overexpression of comDE or its phospho-mimicking mutant comDE reduced transformation efficiency, indicating that excessive ComE impairs competence development. CCR-regulated competence was further confirmed by higher tRNA TRT but lower transformation efficiency in galactose than in glucose.