Figure 1.

(A) Phase diagram illustrating how noise coupled with positive feedback can lead to spontaneous state transitions. The black dots denote stable fixed points and the blue arrows indicate the direction of flow when X is perturbed away from the fixed points. The red wiggling arrow denotes possible fluctuations away from one of the stable fixed points. Note that any fluctuation that goes past the unstable fixed point (white dot) can spontaneously drive the system toward the other state (in the case of the excitable competence induction network studied by Suel et al, fluctuations drive the system away from the one and only stable fixed point before eventually returning to the same fixed point). (B) A simplified version of the B. subtilis competence induction network. Note that comG is illustrated here to aid the description of the experiments; it is one of many proteins activated by comK, the master transcription factor in the process.