Measurements of unwinding of lac operator by repressor

Abstract

WE report here measurements of the rates of association and dissociation of the lac repressor and covalently closed λplac DNAs of various degrees of superhelicity, from which we deduce that the binding of repressor unwinds the lac operator by 40° to 90°. This small unwinding angle is inconsistent with binding mechanisms involving considerable disruption of DNA base-pairs and, in particular, with the model first proposed by Gierer¹ for specific repressor–operator interactions. Our measurements were prompted by the existence of a twofold symmetry in the base sequence of the lac operator, which is a prerequisite of the Gierer model. The minimal base sequence of the lac operator, determined by Gilbert and Maxam², is depicted in Fig. 1 as a regular double helix (Fig. 1a), and as a Gierer-type structure (Fig. 1b). According to Gierer's model, the lac repressor interacts specifically with the hairpinned structure shown in Fig. 1b.