Thermodynamic origins of specificity in the lac repressor-operator interaction. Adaptability in the recognition of mutant operator sites.

A system has been developed for facile generation and characterization of mutant lac operator sites, free of competing pseudo operator sequences. The interaction of lac repressor with these sites has been investigated by the nitrocellulose filter binding assay. The equilibrium binding affinity for each of three single-site changes was reduced by more than three orders of magnitude relative to the wild-type operator under standard assay conditions. The free-energy changes associated with single base-pair substitutions are not additive. We propose that adaptations in the recognition surface of the repressor involving significant trade-offs between electrostatic versus non-electrostatic interactions and between enthalpic versus entropic contributions to the binding free energy occur, in order to achieve the most stable complex with a given DNA sequence.