Conserved and divergent functions of RcrRPQ in Streptococcus gordonii and S. mutans

In the dental caries pathogen Streptococcus mutans, an MarR-like transcriptional regulator (RcrR), two ABC efflux pumps (RcrPQ) and two effector peptides encoded in the rcrRPQ operon provide molecular connections between stress tolerance, (p)ppGpp metabolism and genetic competence. Here, we examined the role of RcrRPQ in the oral commensal S. gordonii. Unlike in S. mutans, introduction of polar or non-polar rcrR mutations into S. gordonii elicited no significant changes in transformation efficiency. However, S. gordonii rcrR mutants were markedly impaired in their ability to grow in the presence of hydrogen peroxide, paraquat, low pH or elevated temperature. Sensitivity to paraquat could also be conferred by mutation of cysteine residues that are present in the RcrR protein of S. gordonii, but not in S. mutans RcrR. Thus, stress tolerance is a conserved function of RcrRPQ in a commensal and pathogenic streptococcus, but the study reveals additional differences in regulation of genetic competence development between S. mutans and S. gordonii.