Mismatch correction at O⁶-methylguanine residues in E. coli DNA

Abstract

Escherichia coli has a correction system which removes mismatched bases from DNA¹. Mutants (dam) which lack the major DNA adenine methylase² are hypersensitive to the effects of base analogue mutagens such as 2-aminopurine³ and appear to be defective in mismatch correction. Phenotypic revertants of dam mutants to base analogue resistance include second site mutations in mutL or mutS genes⁴, which are also part of the correction system. We report here that E. coli dam mutants are also sensitive to the DNA methylating agent N-methyl-N′-nitro-N-nitrosoguanidine (MNNG), which introduces O⁶-methylguanine (m⁶G) into the DNA. This sensitivity, however, was not observed using methylating agents which generate only low amounts of this alkylated base. Furthermore, the introduction of either a mutL or a mutS mutation into dam strains abolished the sensitivity to MNNG. These results suggest that mismatch correction occurs at m⁶G residues in DNA. These lesions miscode in DNA polymerase I-mediated DNA synthesis in vitro⁵ and are known to be mutagenic in vivo⁶. Nevertheless, it appears that mismatch correction at m⁶G residues in DNA does not lead to reduced induction of mutation by MNNG.