MUTATION FREQUENCY DECLINE IN ESCHERICHIA-COLI .1. EFFECTS OF DEFECTSIN MISMATCH REPAIR

Mutation frequency decline (MFD) in Escherichia coli was examined for effects associated with genetic defects in mismatch repair. The kineti cs of MFD are slower when the B/r strain WU3610 carries the mutation m utS201::Tn5 or mutL::Tn10, both of which affect mismatch repair. Simil ar slow kinetics are produced by mutH34 but not by mutH471::Tn5; the l atter has no apparent effect. Strain WU3610-45 (mfd-1) produces the sl ower kinetics if transcription is inhibited during the post-UV incubat ion, although it produces no decline in normal circumstances. The slow er kinetics are therefore attributed to bulk excision repair that rema ins when rapid transcription-coupled repair (TCR) is eliminated by cer tain defects in mismatch repair. A model is proposed wherein mismatch repair defects are thought to slow the activity of TCR but, unlike an mfd defect, not to impede dissociation of stalled transcription comple xes at lesions in the transcribed DNA strand.