Fig. 7.
Elimination of σB control of W expression results in ALL or NONE bistability in the σB network. Stable steady state solutions are shown by dashed (OFF) and dash-dotted (ON) lines whereas an unstable steady state between the two stable states is shown by the dotted line. The OFF state corresponds to a low level of σB, as would result from expression of its structural gene when the σB–dependent promoter of the sigB operon is not activated. The ON state corresponds to a high level of σB, as would result when the σB–dependent promoter of the sigB operon is fully activated by stress. (a) For low W expression (1.2×10−4 μMs−1) the network displays two stable steady states only at low dephosphorylation rates; at high rates only the ON state exists. An irreversible transition from OFF to ON can be induced by an increasing dephosphorylation rate, as indicated by the up arrow. (b) For intermediate W expression (5×10−4μMs−1) the network displays two stable steady states over the entire range of dephosphorylation rates. (c) For high W expression (3×10−3μMs−1) the network displays two stable steady states only at high dephosphorylation rates; at low rates only the OFF state exists. An irreversible transition from ON to OFF can be induced by a decreasing dephosphorylation rate, as indicated by the down arrow. See text for details.