Mutation frequency decline in MMS-treated Escherichia coli K-12 mutS strains.

It has been shown that the decline in mutant frequency (MFD) (argE3(ochre)-->Arg+) which occurs in MMS-treated and then transiently starved AB1157 Escherichia coli K-12 cells concerns revertants which arose by supL suppressor formation in a process which is umuDC dependent. Here we have examined whether MMS-induced Arg+ revertants are susceptible to decline when bacteria are deficient in mismatch repair. We show that there is an absence of MFD in MMS-treated M1 (mutS) and in EC2416 (mutS delta umuDC) cells defective in mismatch repair which is associated with a change in the spectrum of MMS-induced mutations formed. In contrast to AB1157, transformation of M1 (mutS) bacteria with plasmids harbouring various combinations of umuD(D')C genes does not enhance the level of MMS-induced mutations but may influence the proportion of supL mutations. These supL mutations show MFD. Repair processes under 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.