Carbohydrate and PepO control bimodality in competence development by Streptococcus mutans

In Streptococcus mutans, the alternative sigma factor ComX controls entry into genetic competence. Competence signaling peptide (CSP) induces bimodal expression of comX, with only a fraction of cells in the population becoming transformable. Curiously, bimodal comX activation in response to CSP is affected by peptides in the growth medium and by carbohydrate source. CSP elicits bimodal expression of comX in media rich in small peptides, but in defined media lacking small peptides CSP induces no response in comX. In addition, growth on certain sugars other than glucose increases the proportion of the population that activates comX in response to CSP, relative to growth on glucose. By investigating the connection between media and bimodal comX expression, we find evidence for two mechanisms that modulate transcriptional positive feedback in the ComRS system, which is the origin of comX bimodality. We find that the endopeptidase PepO suppresses the ComRS feedback loop, most likely by degrading the intracellular XIP/ComS signal. Deletion of pepO eliminates bimodality in comX, leading to a unimodal comX response to CSP in defined and complex media. We also find that CSP upregulates comR in a carbohydrate source-dependent fashion, providing an additional stimulus to the ComRS feedback system. Our data provide mechanistic insight into how CSP regulates the bistable competence circuit and explain the puzzle of growth medium-dependence in S. mutans competence regulation.