Carbohydrate and PepO control bimodality in competence development byStreptococcus mutans

InStreptococcus mutans, the alternative sigma factor ComX controls entry into genetic competence. Competence signaling peptide (CSP) induces bimodal expression ofcomX, with only a fraction of cells in the population becoming transformable. Curiously, bimodalcomXactivation in response to CSP is affected by peptides in the growth medium and by carbohydrate source. CSP elicits bimodal expression ofcomXin media rich in small peptides, but in defined media lacking small peptides CSP induces no response incomX. In addition, growth on certain sugars other than glucose increases the proportion of the population that activatescomXin response to CSP, relative to growth on glucose. By investigating the connection between media and bimodalcomXexpression, we find evidence for two mechanisms that modulate transcriptional positive feedback in the ComRS system, which is the origin ofcomXbimodality. We find that the endopeptidase PepO suppresses the ComRS feedback loop, most likely by degrading the intracellular XIP/ComS signal. Deletion ofpepOeliminates bimodality incomX, leading to a unimodalcomXresponse to CSP in defined and complex media. We also find that CSP upregulatescomRin 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 inS. mutanscompetence regulation.