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The isolation and characterization of altered repressors of the lac operon which have an increased affinity for an operator should give useful clues about the molecular basis for the very tight and specific interaction between repressor and operator. A selection system has been devised which allows the isolation of such repressor mutants. This system selects for mutant repressors which can overcome lac operator-constitutive (Oc) mutations. By using in vivo assays, 24 candidates were obtained which, compared with wild type, have an increased trans effect of their repressor on one or several Oc operators. Three of these candidates have been investigated in vitro; the affinity of their repressor for inducer was unchanged, whereas the affinity for wild-type operator was increased 15-, 86-, and 262-fold, respectively.

Tight-binding repressors of the lac operon: selection system and in vitro analysis