THE FREQUENCY OF MMS-INDUCED, UMUDC-DEPENDENT, MUTATIONS DECLINES DURING STARVATION IN ESCHERICHIA-COLI

It has been found that the level of methyl methanesulfonate (MMS)-indu ced mutation in Escherichia coli is dependent on the level of UmuD(D') C proteins. The frequency of argE(ochre)-->Arg(+) mutations (which occ ur predominantly by AT-->TA transversions) and Rif(S)-->Rif(R) mutatio ns is much higher when UmuDC or UmuD'C are overproduced in the cell. W hen MMS-treated bacteria were starved for progressively longer times a nd hence the expression of mutations delayed, the level of mutations o bserved progressively declined. This same treatment had no effect on t he degree of SOS induction. Examination of plasmid DNAs, isolated from MMS-treated cells, for their sensitivity to the specific endonuclease s Fpg and Nth revealed that MMS causes formation of abasic sites, whic h are repaired during cell starvation. It is assumed that, in non-divi ding cells, apurinic sites are mostly repaired by RecA-mediated recomb inational repair. This pathway, which is error-free, is compared with the processing pathway in metabolically active cells, where translesio n synthesis by the UmuD'C-2-RecA-DNA polymerase III holoenzyme complex occurs; this latter pathway is error-prone.