ALT Four green panels show different states of the LexA regulon with either high (darker green left panels) or low (lighter green right panels) concentration of LexA, depicted as green circles, and a target promoter either bound by the LexA repressor (top panels) or unbound and active (bottom panels). The panels depicting active promoters contain large red blobs representing RNA polymerases. The figure also indicates, with black arrows, the typical rates of transitions between the four different states. While transitions from bound to unbound always occur at a rate of one per minute, rebinding is very fast when LexA is high (around one second) and very slow when LexA is low (around one hour). Switches to the low LexA state happen roughly once an hour when DNA damage occurs, but only last on the order of one minute because DNA is repaired quickly. Expression bursts occur stochastically during short periods when LexA levels are low, as shown in the lower right panel.