CTX prophages in classical biotype Vibrio cholerae: Functional phage genesbut dysfunctional phage genomes

CTX phi is a filamentous, lysogenic bacteriophage whose genome encodes chol era toxin, the primary virulence factor produced by Vibrio cholerae, CTX pr ophages in O1 El Tor and O139 strains of V. cholerae are found within array s of genetically related elements integrated at a single locus within the V . cholerae large chromosome. The prophages of O1 El Tor and O139 strains ge nerally yield infectious CTX phi. In contrast, O1 classical strains of V, c holerae do not produce CTX phi, although they produce cholera toxin and the y contain CTX prophages integrated at two sites. We have identified the sec ond site of CTX prophage integration in O1 classical strains and characteri zed the classical prophage arrays genetically and functionally. The genes o f classical prophages encode functional forms of all of the proteins needed for production of CTX phi. Classical CTX prophages are present either as s olitary prophages or as arrays of two truncated, fused prophages, RSI, a ge netic element that is closely related to CTX phi and is often interspersed with CTX prophages in El Tor strains, was not detected in classical V,chole rae, Our model for CTX phi, production predicts that the CTX prophage arran gements in classical strains will not yield extrachromosomal CTX DNA and th us will not yield virions, and our experimental results confirm this predic tion. Thus, failure of O1 classical strains of V, cholerae to produce CTX p hi is due to overall deficiencies in the structures of the arrays of classi cal prophages, rather than to mutations affecting individual CTX prophage g enes.