Fig. 1.

Schematic representation of the five possible states for lac repressor binding to two operators for the lac constructs used in the analyzed experiments from Muller et al. (7). The lac Z gene, downstream the main operator, is repressed when the lac repressor (in gray with black contour line) is bound to the main operator (states iii, iv, and v); and unrepressed when the main operator is unoccupied (states i and ii). The thick black line represents the DNA, and the two lac operators are shown as white (auxiliary operator) and highlighted (main operator) boxes. The thick dotted line indicates the different lengths of the spacer DNA between operators.