METHYL DIRECTED DNA MISMATCH REPAIR IN VIBRIO-CHOLERAE

Mismatches in DNA occur either due to replication error or during reco mbination between homologous but non-identical DNA sequences or due to chemical modification of bases. The mismatch in DNA, if not repaired, result in high spontaneous mutation frequency. The repair has to be i n the newly synthesized strand of the DNA molecule, otherwise the erro r will be fixed permanently. Three distinct mechanisms have been propo sed for the repair of mismatches in DNA in prokaryotic cells and gene functions involved in these repair processes have been identified. The methyl-directed DNA mismatch repair has been examined in Vibrio chole rae, a highly pathogenic gram negative bacterium and the causative age nt of the diarrhoeal disease cholera. The DNA adenine methyltransferas e encoding gene (dam) of this organism which is involved in strand dis crimination during the repair process has been cloned and the complete nucleotide sequence has been determined. Vibrio cholerae dam gene cod es for a 21.5 kDa protein and can substitute for the Escherichia coli enzyme. Overproduction of Vibrio cholerae Dam protein is neither hyper mutable nor lethal both in Escherichia coli and Vibrio cholerae. While Escherichia coli dam mutants are sensitive to 2-aminopurine, Vibrio c holerae 2-aminopurine sensitive mutants have been isolated with intact GATC methylation activity. The mutator genes mutS and mutL involved i n the recognition of mismatch have been cloned, nucleotide sequence de termined and their products characterized. Mutants of mutS and mutL of Vibrio cholerae have been isolated and show high rate of spontaneous mutation frequency. The mutU gene of Vibrio cholerae, the product of w hich is a DNA helicase II, codes for a 70 kDa protein. The deduced ami no acid sequence of the mutU gene hs all the consensus helicase motifs . The DNA cytosine methyltransferase encoding gene (darn) of Vibrio ch olerae has also been cloned. The dcm gene codes for a 53 kDa protein. This gene product might be involved in very short patch (VSP) repair o f DNA mismatches. The vsr gene which is directly involved in VSP repai r process codes for a 23 kDa protein. Using these information, the sta tus of DNA mismatch repair in Vibrio cholerae will be discussed.