Acquisition and dissemination mechanisms of CTXΦ in Vibrio cholerae: New paradigm for dif residents

Vibrio cholerae (V. cholerae) genome is equipped with a number of integrative mobile genetic element (IMGE) like prophages, plasmids, transposons or genomic islands, which provides fitness factors that help the pathogen to survive in changing environmental conditions. Metagenomic analyses of clinical and environmental V. cholerae isolates revealed that dimer resolution sites (dif) harbor several structurally and functionally distinct IMGEs. All IMGEs present in the dif region exploit chromosomally encoded tyrosine recombinases, XerC and XerD, for integration. Integration takes place due to site-specific recombination between two specific DNA sequences; chromosomal sequence is called attB and IMGEs sequence is called attP. Different IMGEs present in the attP region have different attP structure but all of them are recognized by XerC and XerD enzymes and mediate either reversible or irreversible integration. Cholera toxin phage (CTXΦ), a lysogenic filamentous phage carrying the cholera toxin genes ctxAB, deserves special attention because it provides V. cholerae the crucial toxin and is always present in the dif region of all epidemic cholera isolates. Therefore, understanding the mechanisms of integration and dissemination of CTXΦ, genetic and ecological factors which support CTXΦ integration as well as production of virion from chromosomally integrated phage genome and interactions of CTXΦ with other genetic elements present in the genomes of V. cholerae is important for learning more about the biology of cholera pathogen.