ABSTRACT
In El Tor biotype strains of toxigenic Vibrio cholerae, the CTXÏ prophage often resides adjacent to a chromosomally integrated satellite phage genome, RS1, which produces RS1Ï particles by using CTX prophage-encoded morphogenesis proteins. RS1 encodes RstC, an antirepressor against the CTXÏ repressor RstR, which cooperates with the host-encoded LexA protein to maintain CTXÏ lysogeny. We found that superinfection of toxigenic El Tor strains with RS1Ï, followed by inoculation of the transductants into the adult rabbit intestine, caused elimination of the resident CTX prophage-producing nontoxigenic derivatives at a high frequency. Further studies using recA deletion mutants and a cloned rstC gene showed that the excision event was recA dependent and that introduction of additional copies of the cloned rstC gene instead of infection with RS1Ï was sufficient to enhance CTXÏ elimination. Our data suggest that once it is excised from the chromosome, the elimination of CTX prophage from host cells is driven by the inability to reestablish CTXÏ lysogeny while RstC is overexpressed. However, with eventual loss of the additional copies of rstC, the nontoxigenic derivatives can act as precursors of new toxigenic strains by acquiring the CTX prophage either through reinfection with CTXÏ or by chitin-induced transformation. These results provide new insights into the role of RS1Ï in V. cholerae evolution and the emergence of highly pathogenic clones, such as the variant strains associated with recent devastating epidemics of cholera in Asia, sub-Saharan Africa, and Haiti.
