Characterization of a dam mutant of Serratia marcescens and nucleotide sequence of the dam region

The DNA of Serratia marcescens has N-6-adenine methylation in GATC sequence s. Among 2-aminopurine-sensitive mutants isolated from S. marcescens Sr41, one was identified which lacked GATC methylation. The mutant showed up to 3 0-fold increased spontaneous mutability and enhanced mutability after treat ment with 2-aminopurine, ethyl methanesulfonate, or UV light. The gene (dam ) coding for the adenine methyltransferase (Dam enzyme) of S. marcescens wa s identified on a gene bank plasmid which alleviated the 2-aminopurine sens itivity and the higher mutability of a dam-13::Tn9 mutant of Escherichia co li. Nucleotide sequencing revealed that the deduced amino acid sequence of Dam (270 amino acids; molecular mass, 31.3 kDa) has 72% identity to the Dam enzyme off. coli. The dam gene is located between flanking genes which are similar to those found to the sides of the E. coli dam gene. The results o f complementation studies indicated that like Dam of E. coli and unlike Dam of Vibrio cholerae, the Dam enzyme of S. marcescens plays an important rol e in mutation avoidance by allowing the mismatch repair enzymes to discrimi nate between the parental and newly synthesized strands during correction o f replication errors.