Isolation of pseudorevertants of lac oc mutants: selection system for superoperator mutations.

A selection procedure and rapid screening test are described that allow the isolation of rarely occurring pseudorevertants of lac oc mutants. These techniques were developed with the aim of isolating superoperator (os) mutants in which a secondary mutation in the operator would increase the affinity of the repressor of superoperator, thereby overcoming the effect of the oc mutation. The occurrence of superoperator mutants is predicted on the basis of a probable twofold symmetry in the lac repressor-operator recognition. Over 2,300 oc pseudorevertants were isolated and screened. In vitro measurements of the affinity of lac repressor for the operator region indicate that, among these, one oc pseudorevertant probably results from an oc mutation. The selection procedure also yielded other types of regulatory mutants that behave as promoter mutants and as i gene mutants in which the repressor is capable of overcoming an oc mutation.