FIGURE 4.

(A) Response of comX to CSP (blue) and XIP (red) is acutely sensitive to local extracellular pH. In microfluidic studies the median comX activity in a population of S. mutans is maximal near pH 7 and declines sharply away from pH neutral conditions. Dashed lines show spline fit.

(B)–(C) Mechanisms downstream of comX introduce additional heterogeneity. When exogenous XIP is supplied to a dual reporter strain carrying PcomX-gfp and PcomY-rfp, all cells activate comX (as indicated by the green reporter fluorescence) (B), but only a subset also activates comY (indicated by the red fluorescence) (C) and other late competence genes. Thus an additional noisy switch lies between comX and comY.