Base substitution mutants of the lac operator: in vivo and in vitro affinities for lac repressor.

16 single-site mutations and a 1-bp deletion in the lac operator have been cloned and examined with regard to repressor binding. A 13-bp, central 'core' operator sequence, bp 5-17 of the natural operator, was also synthesized and cloned. Repressor affinity was assessed in vivo by quantitating the level of beta-galactosidase activity resulting from chromosomal operon derepression and in vitro by measuring the stability of repressor-operator complexes. Our results support the general conclusion that the repressor-operator interaction is asymmetric, particularly across the center of the operator sequence, with little or no specific contact at position 12. Some sequence changes in the right side of the operator markedly reduced repressor affinity, indicating that although binding to this half of the sequence has been suggested to be less important than the left half, it still significantly contributes to the binding affinity.