Fig. 2.

Cartoon of the lac repressor (in gray with black contour line) bound simultaneously to the main (O1) and one auxiliary operator (Oid, with the sequence of the ideal operator), which are represented by white rectangles. Binding of the repressor to O1 represses the lac Z gene in the experiments (7). The simultaneous binding to O1 and Oid with free energies ΔG_O1 and ΔG_Oid, respectively, competes with the unfavorable process of forming a loop (ΔG_l). By using this type of lac constructs, Muller et al. (7) measured the in vivo repression of the lac Z gene as a function of the distance between the centers of the O1 and Oid operators.