CTX phi contains a hybrid genome derived from tandemly integrated elements

CTX phi is a filamentous. temperate bacteriophage whose genome includes ctx AB, the genes that encode cholera toxin. In toxigenic isolates of Vibrio ch olerae. tandem arrays of prophage DNA. usually interspersed with the relate d genetic element RS1, are integrated site-specifically within the chromoso me. We have discovered that these arrays routinely yield hybrid virions, co mposed of DNA from two adjacent prophages or from a prophage and a downstre am RS1. Coding sequences are always derived from the 5' prophage whereas mo st of an intergenic sequence, intergenic region 1, is always derived from t he 3' element. The presence of tandem elements is required for production o f virions: V. cholerae strains that contain a solitary prophage rarely yiel d CTX virions. and the few virions detected result from imprecise excision of prophage DNA. Thus, generation of the replicative form of CTX phi, pCTX, a step that precedes production of virions, does not depend on reversal of the process for site-specific integration of CTX phi DNA into the V. chole rae chromosome. Production of pCTX also does not depend on RecA-mediated ho mologous recombination between adjacent prophages. We hypothesize that the CTX phi-specific proteins required for replication of pCTX can also functio n on a chromosomal substrate, and that, unlike the processes used by other integrating phages, production of pCTX and CTX phi does not require excisio n of the prophage from the chromosome. Use of this replication strategy max imizes vertical transmission of prophage DNA while still enabling dissemina tion of CTX phi to new hosts.