Stress response in pathogenic bacteria.

Bacterial pathogens survive under two entirely different conditions, namely, their natural environment and in their hosts. Response of these pathogens to stresses encountered during transition from the natural environment to human hosts has been described. The virulence determinants of pathogenic bacteria are under the control of transcriptional activators which respond to fluctuations in growth temperature, osmolarity, metal ion concentration and oxygen tension of the environment. The regulation of stress induced genes may occur at the level of transcription or translation or by post-translational modifications. Under certain stress conditions local changes in the superhelicity of DNA induce or repress genes. In addition to their role in survival of bacteria under stressful situations, the stress induced proteins are also implicated in the manifestation of pathogenicity of bacterial pathogens in vivo.

Construction of shuttle vectors for cloning in Vibrio cholerae and Escherichia coli.

Starting from a naturally occurring cryptic plasmid pVC540 of Vibrio cholerae non-OI. strain 1095, a number of plasmid vectors have been constructed for cloning genes in Vibrio cholerae by introducing antibiotic resistance markers containing a set of unique cloning sites. The constructs pVC810 and pVE920 have the origins of both Vibrio cholerae and Escherichia coli replicons and are stable in both organisms in the absence of selective pressure. These plasmids can serve as shuttle vectors between Escherichia coli and Vibrio cholerae. The plasmid vectors reported here along with the demonstration of transformation in Vibrio cholerae by plasmid DNA will facilitate genetic analysis of this important human pathogen.

Methyl directed DNA mismatch repair in Vibrio cholerae.

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.