Influence of DNA-repair deficiencies on MMS- and EMS-induced mutagenesis in Escherichia coli K-12.

The mutagenic effects of the alkylating agents, MMS and EMS, were studied by measuring the reversion of an arg (ochre) mutation in various DNA-repair-deficient strains. When compared with the wild-type strain, EMS-induced mutagenesis was reduced in the recA13 strain but not in the lexA3 strain. MMS-induced mutagenesis was reduced to background levels in the recA13 strain and reduced to intermediate levels in the lexA3 strain. The umuC36 strain showed intermediate levels of mutagenesis with both mutagens which suggests that a substantial portion of both MMS- and EMS-induced mutagenesis depends upon this component of the error-prone "SOS" repair pathway. The uvrD101, recL152 and recF143 mutations produced increased levels of MMS-induced reversion but had no effect upon the levels of EMS-induced mutagenesis, suggesting that the pathways affected by these genes may play a role in the error-free repair of MMS but not EMS damage. In contrast, a large increase in the level of mutagenesis was noted in a delta uvrB101 mutant with EMS but not MMS. This hypermutability with EMS was also seen in uvrA6, uvrB5 and uvrC34 mutant strains and suggests a role for excision in the error-free repair of ethylation but not methylation damage to DNA.