MUTATION FREQUENCY DECLINE IN MMS-TREATED ESCHERICHIA-COLI K-12 MUTS STRAINS

It has been shown that the decline in mutant frequency (MFD) (argE3((o chre))-->Arg(+)) which occurs in MMS-treated and then transiently star ved AB1157 Escherichia coli K-12 cells concerns revertants which arose by slept suppressor formation in a process which is umuDC dependent. Here we have examined whether MMS-induced Arg(+) revertants are suscep tible to decline when bacteria are deficient in mismatch repair. We sh ow that there is an absence of MFD in MMS-treated M1 (mutS) and in EC2 416 (mutS Delta umuDC) cells defective in mismatch repair which is ass ociated with a change in the spectrum of MMS-induced mutations formed. In contrast to AB1157, transformation of M1 (mutS) bacteria with plas mids harbouring various combinations of umuD(D')C genes does not enhan ce the level of MMS-induced mutations but may influence the proportion of supL mutations. These supL mutations show MFD. Repair processes un der MFD conditions were confirmed by analysis of plasmid DNA isolated from MMS-treated bacteria at different stages of their starvation and digestion with Fpg protein.