Escherichia coli mutants deficient in 3-methyladenine-DNA glycosylase

Two Escherichia coli K12 mutants defective in 3-methyladenine-DNA glycosylase have been isolated following mutagenesis by N-methyl-N-nitro-N-nitrosoguanidine. The mutants, which are of independent origin and have been designated tag-1 and tag-2, contain greatly reduced amounts of 3-methyladenine-DNA glycosylase activity in cell-free extracts. The defect in the tag-1 strain is observed at 43 °C but not at 30 °C, and a partially purified enzyme from this strain is unusually heat-labile, indicating that the defect in the tag-1 strain is due to a mutation in the structural gene for 3-methyladenine-DNA glycosylase.We have shown that 3-methyladenine-DNA glycosylase is responsible for the rapid removal of 3-methyladenine from the DNA of E. coli cells treated with monofunctional alkylating agents. The active release of this base is greatly impaired in the mutant strains. Both tag mutant strains are abnormally sensitive to killing by monofunctional alkylating agents and are defective in the host cell reactivation of methyl methanesulphonate-treated bacteriophage A. The tag mutation does not confer an increased sensitivity to ultraviolet or X-irradiation, and host cell reactivation of irradiated λ is normal in these strains. Further, there was no increase in the rate of spontaneous mutation in a tag strain.Three-factor transductional crosses with nalA and nrdA have shown that the tag-2 mutation is located at 47.2 minutes on the map of the E. coli K12 chromosome. In the mapping experiments, the tag-1 mutation behaved differently and appeared to be located at 43 to 46 minutes, in a closely situated but non-adjacent gene. Possible implications of the non-identity of the tag-1 and tag-2 mutations are discussed.