Search

A-G mispairs are subject to correction by two distinct pathways in cell-free extracts of Escherichia coli [Su, S.-S., Lahue, R.S., Au, K.G. & Modrich, P. (1988) J. Biol. Chem. 263, 6829-6835; Lu, A.-L. & Chang, D.Y. (1988) Genetics 118, 593-600]. One is the mutHLS-dependent, methyl-directed pathway that recognizes a variety of mismatches and repairs the unmethylated strand of DNA heteroduplexes that are hemimethylated at d(GATC) sequences. The other pathway appears to be specific for A-G mispairs, yields C.G base pairs exclusively, and is independent of the presence of d(GATC) sites. Analyses of cell-free extracts prepared from E. coli mutY strains and isogenic parents have demonstrated that the mutY gene product is involved in the methyl-independent pathway, which converts A-G mispairs to C.G pairs. The specificity of this activity is consistent with the mutator phenotype associated with the mutY locus, which generates G.C----T.A transversions [Nghiem, Y., Cabrera, M., Cupples, C.G. & Miller, J.H. (1988) Proc. Natl. Acad. Sci. USA 85, 2709-2713]. We propose that the mutY product functions at a late stage of a pathway that excludes A-G mispairs during chromosome replication and that involves the function of the mutT gene product. This model suggests that the mutT function acts at an early stage of this pathway to exclude A-G mismatches where the adenine resides on the template DNA strand. A-G mispairs that persist after passage of the replication fork would contain guanine on the template strand and thus be processed to C.G base pairs by the mutY-dependent repair system.