Molecular Characterization of Genetic Competence in Streptococcus mitis

Objectives:
Streptococcus mitis is a predominant commensal of the oral cavity, and a primary colonizer of multispecies biofilms. In streptococci of the mitis group, competence for natural transformation is a transient physiological state triggered by competence stimulating peptides (CSPs) that allows incorporation of naked extracellular DNA into the bacterial chromosome. Although low transformation yields and the absence of a widespread functional competence system have been reported for S. mitis, recent studies revealed that, at least for some strains, high efficiencies can be achieved following optimization steps. Here, we used the defined optimal conditions to gain a detailed insight into regulatory pathways of competence development in S. mitis.
Methods: RNA sequencing mapped the CSP response of strains SK321 and NCTC12261^T. RNA samples were collected from S. mitis grown in presence or absence of CSP in C+Y_YB medium. Samples were enriched for mRNA, followed by library preparation using with a dUTP-based method to obtain information on the strand orientation of the transcripts.
Results: All genes induced by ComE in other mitis streptococci, including sigX, were upregulated in the two strains. Likewise, all sets SigX core genes involved in extracellular DNA uptake, recombination, and fratricide were upregulated. We confirmed previous predictions of SigX-recognized boxes in the promoter sites of core genes. Five operons unique to S. mitis with a SigX promoter element were identified, two specific to SK321, and one specific to NCTC12261^T. Two of the strain-specific operons coded for different bacteriocin peptides.
Conclusions: Comparison of the global transcriptional in response to CSP showed the conservation of the ComE and SigX-core regulons in two strains of S. mitis. Strain-specific genes were identified as part of the SigX-regulon, including two different bacteriocin operons. Understanding mechanisms used by S. mitis to orchestrate proficient DNA uptake may bring new insights on transfer of bacterial genetic information in the oral cavity.