Isolation and characterization of Escherichia coli K-12 mutants unable to induce the adaptive response to simple alkylating agents

Abstract

When E. coli cells are exposed to a low level of simple alkylating agents, they induce the adaptive response which renders them more resistant to the killing and the mutagenic effects of the same or other alkylating agents. The isolation of 1 strain that was deficient in mutagenic adaptation and 5 that were deficient in both mutagenic and killing adaptation is reported, confirming previous suggestions that killing and mutagenic adaptation are, at least to some extent, separable. These 6 strains were called Ada mutants. They were more sensitive to the killing and mutagenic effects of N-methyl-N''-nitro-N-nitrosoguanidine (MNNG) than the unadapted Ada+ parent. Thus, the adaptation pathway is responsible for circumventing some alkylation-induced damage even in cells that are not preinduced. The increase in mutation frequency seen in Ada cells treated with MNNG was the same whether the cells were lexA+ or lexA, showing that the extra mutations found in Ada- strains do not depend on the SOS pathway. Ada strains accumulated more O6-methyl guanine lesions than the Ada+ parent on prolonged exposure to MNNG, and this supports the idea that O6-methyl guanine is the most important lesion for MNNG-induced mutagenesis. The ada mutations mapped in the 47-53 min region of the E. coli chromosome.