A new link between two Streptococcus mutans pheromones

Objectives: Streptococcus mutans, a bacterium associated with dental caries, has two different pheromones responsible for induction of competence for natural transformation. The most studied is the competence stimulating peptide (CSP) and the other is the recently discovered sigX inducing peptide (XIP). Competence is a physiological state that allows bacteria to take up and incorporate extracellular DNA, such as antibiotic resistance and virulence genes. XIP induces a robust competence response that is independent of the CSP system in chemically defined media (CDM). The aim of this study was to investigate the XIP effect on S. mutans global transcriptome profile.
Methods: Whole RNA sequencing was conducted for a comprehensive assessment of changes in gene expression. S. mutans cultures were grown in CDM with or without XIP. RNA-seq reads were aligned to the S. mutans UA159 genome and visualized using JBrowser. DESeq was used for comparative analysis. Deletion mutants were constructed by PCR-ligation mutagenesis.
Results: Among the upregulated transcripts were four distinct loci involved in bacteriocin production, as well as the CSP receptor gene, comD, and its cognate regulator comE. Induction of comED was a late response, and depended on SigX, the alternative sigma factor that activates the genes for DNA uptake and recombination. Analysis of the comED upstream region revealed a putative SigX-box. Inversion of the SigX-box sequence abolished the XIP inducing effect on comED expression, but had no effect on sigX expression or on transformation efficiency. Thus, the results suggest that SigX may be the proximal regulator of comED.
Conclusions: In defined medium XIP acted upstream of the CSP-signaling pathway, indicating a hitherto unknown link between the two S. mutans pheromone systems. Understanding cell-to-cell communication among oral streptococci, the most predominant bacteria in dental biofilms, may lead to new strategies to fight oral diseases.