Studies of error-prone DNA repair in Escherichia coli K-12 and Ames Salmonella typhimurium strains using a model alkylating agent.

4-Acetoxy-3-acetoxymethyl acetophenone (AAMAP) is mutagenic in Ames Salmonella typhimurium tester strains TA100 and TA98, which carry plasmid pKM101, but not in the isogenic plasmid-less strains TA1535 and TA1538. Similarly, no AAMAP-induced reversion of the his-4 allele is detectable in Escherichia coli K-12 umuC strains in the absence of the plasmid, even when the strains are treated with ethylene-diaminetetraacetate to increase permeability, or when the uvrB allele is introduced to increase error-prone DNA repair. AAMAP is, however, mutagenic in umuC+ strains or in umuC strains in which plasmid pKM101 has been introduced, suggesting that the plasmid-encoded MucAB or the chromosomally determined UmuDC proteins are required for mutagenesis. Mutation frequencies are higher in E. coli umuC (pKM101) strains, which resemble Ames tester strains of S. typhimurium, than in E. coli umuC+ or even umuC+ (pKM101) strains. Therefore, providing that the recommended pKM101-containing tester strains are used, the apparent absence of Umu-like protein activity in S. typhimurium may actually increase the sensitivity of the Ames test for the detection of mutagens that require error-prone DNA repair for activity.