Methyl directed DNA mismatch repair in Vibrio cholerae.

ABSTRACT

Mismatches in DNA occur either due to replication error or during recombination 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 in the newly synthesized strand of the DNA molecule, otherwise the error will be fixed permanently. Three distinct mechanisms have been proposed 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 cholerae, a highly pathogenic gram negative bacterium and the causative agent of the diarrhoeal disease cholera. The DNA adenine methyltransferase encoding gene (dam) of this organism which is involved in strand discrimination during the repair process has been cloned and the complete nucleotide sequence has been determined. Vibrio cholerae dam gene codes for a 21·5 kDa protein and can substitute for the Escherichia coli enzyme. Overproduction of Vibrio cholerae Dam protein is neither hypermutable nor lethal both in Escherichia coli and Vibrio cholerae. While Escherichia coli dam mutants are sensitive to 2-aminopurine, Vibrio cholerae 2-aminopurine sensitive mutants have been isolated with intact GATC methylation activity. The mutator genes mutS and mutL involved in the recognition of mismatch have been cloned, nucleotide sequence determined 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 which is a DNA helicase II, codes for a 70 kDa protein. The deduced amino acid sequence of the mutU gene hs all the consensus helicase motifs. The DNA cytosine methyltransferase encoding gene (dam) of Vibrio cholerae 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 of DNA mismatches. The vsr gene which is directly involved in VSP repair process codes for a 23 kDa protein. Using these information, the status of DNA mismatch repair in Vibrio cholerae will be discussed.