title: Spontaneous mutations occur near dam recognition sites in a dam- Escherichia coli host.
abstract: The mismatch repair system of Escherichia coli K12 removes mispaired bases from DNA. Mismatch repair can occur on either strand of DNA if it lacks N6-methyladenines within 5'-GATC-3' sequences. In hemimethylated heteroduplexes, repair occurs preferentially on the unmethylated strand. If both strands are fully methylated, repair is inhibited. Mutant (dam-) strains of E. coli defective in the adenine methylase that recognizes 5'-GATC-3' sequences (Dam), and therefore defective in mismatch repair, show increased spontaneous mutation rates compared to otherwise isogenic dam+ hosts. We have isolated and characterized 91 independent mutations that arise as a consequence of the Dam- defect in a plasmid-borne phage P22 repressor gene, mnt. The majority of these mutations are A:T----G:C transitions that occur within six base pairs of the two 5'-GATC-3' sequences in the mnt gene. In contrast, the spectrum of mnt- mutations in a dam+ host is comprised of a majority of insertions of IS elements and deletions that do not cluster near Dam recognition sites. These results show that Dam-directed post-replicative mismatch repair plays a significant role in the rectification of potential transition mutations in vivo, and suggest that sequences associated with Dam recognition sites are particularly prone to replication or repair errors.
authors.forename: M
authors.initials: M
authors.lastname: Carraway
authors.forename: P
authors.initials: P
authors.lastname: Youderian
authors.forename: M G
authors.initials: MG
authors.lastname: Marinus
mesh_terms: D002872:Chromosome Deletion; D004260:DNA Repair; D004926:Escherichia coli / Q000235:genetics*; D005798:Genes, Bacterial; D008769:Methylnitronitrosoguanidine; D008780:Methyltransferases / Q000235:genetics*; D009154:Mutation; D015265:Site-Specific DNA-Methyltransferase (Adenine-Specific)
chemical_list: D008769:Methylnitronitrosoguanidine; D008780:Methyltransferases; D015265:Site-Specific DNA-Methyltransferase (Adenine-Specific)