VGJφ integration and excision mechanisms contribute to the genetic diversity of Vibrio cholerae epidemic strains

Most strains of Vibrio cholerae are not pathogenic or cause only local outbreaks of gastroenteritis. Acquisition of the capacity to produce the cholera toxin results from a lysogenic conversion event due to a filamentous bacteriophage, CTX phi. Two V. cholerae tyrosine recombinases that normally serve to resolve chromosome dimers, XerC and XerD, promote CTX phi integration by directly recombining the ssDNA genome of the phage with the dimer resolution site of either or both V. cholerae chromosomes. This smart mechanism renders the process irreversible. Many other filamentous vibriophages seem to attach to chromosome dimer resolution sites and participate in the rapid and continuous evolution of toxigenic V. cholerae strains. We analyzed the molecular mechanism of integration of VGJ phi, a representative of the largest family of these phages. We found that XerC and XerD promote the integration of VGJ phi. into a specific chromosome dimer resolution site, and that the dsDNA replicative form of the phage is recombined. We show that XerC and XerD can promote excision of the integrated prophage, and that this participates in the production of new extrachromosomal copies of the phage genome. We further show how hybrid molecules harboring the concatenated genomes of CTX phi and VGJ phi can be produced efficiently. Finally, we discuss how the integration and excision mechanisms of VGJ phi can explain the origin of recent epidemic V. cholerae strains.