Figure 4.

Complete dissociation of the tetrameric repressor triggers induction. (A) A high concentration of intracellular inducer can force dissociation of the repressor from its operators, as described by Jacob and Monod (6). (B) At low or intermediate concentrations of intracellular inducer, partial dissociation from one operator by the tetrameric LacI repressor is followed by a fast rebinding. Consequently, no more than one transcript is generated during such a brief dissociation event. However, the tetrameric repressor can dissociate from both operators spontaneously and stochastically, then sequestered by inducer such that it cannot rebind, leading to a large burst of expression. (C) A time-lapse sequence captures a phenotype switching event. In the presence of 50 µM TMG, one daughter cell of a dividing cell switches phenotype to express many LacY-YFP molecules (yellow fluorescence overlay) while the other daughter cell does not (see Movie S1).