Abstract
Entry into genetic competence in streptococci is controlled by ComX, an alternative sigma factor for genes that enable the import of exogenous DNA. In Streptococcus mutans, the immediate activator of comX is the ComRS quorum system. ComS is the precursor of XIP, a seven-residue peptide that is imported into the cell and interacts with the cytosolic receptor ComR to form a transcriptional activator for both comX and comS Although intercellular quorum signaling by ComRS has been demonstrated, observations of bimodal expression of comX suggest that comRS may also function as an intracellular feedback loop, activating comX without export or detection of extracellular XIP. Here we used microfluidic and single-cell methods to test whether ComRS induction of comX requires extracellular XIP or ComS. We found that individual comS-overexpressing cells activate their own comX, independently of the rate at which their growth medium is replaced. However, in the absence of lysis they do not activate comS-deficient mutants growing in coculture. We also found that induction of comR and comS genes introduced into Escherichia coli cells leads to activation of a comX reporter. Therefore, ComRS control of comX does not require either the import or extracellular accumulation of ComS or XIP or specific processing of ComS to XIP. We also found that endogenously and exogenously produced ComS and XIP have inequivalent effects on comX activation. These data are fully consistent with identification of intracellular positive feedback in comS transcription as the origin of bimodal comX expression in S. mutansIMPORTANCE The ComRS system can function as a quorum sensing trigger for genetic competence in S. mutans The signal peptide XIP, which is derived from the precursor ComS, enters the cell and interacts with the Rgg-type cytosolic receptor ComR to activate comX, which encodes the alternative sigma factor for the late competence genes. Previous studies have demonstrated intercellular signaling via ComRS, although release of the ComS or XIP peptide to the extracellular medium appears to require lysis of the producing cells. Here we tested the complementary hypothesis that ComRS can drive comX through a purely intracellular mechanism that does not depend on extracellular accumulation or import of ComS or XIP. By combining single-cell, coculture, and microfluidic approaches, we demonstrated that endogenously produced ComS can enable ComRS to activate comX without requiring processing, export, or import. These data provide insight into intracellular mechanisms that generate noise and heterogeneity in S. mutans competence.