Satellite phage TLCφ enables toxigenic conversion by CTX phage through dif site alteration

Bacterial chromosomes often carry integrated genetic elements (for example plasmids, transposons, prophages and islands) whose precise function and contribution to the evolutionary fitness of the host bacterium are unknown. The CTX phi prophage, which encodes cholera toxin in Vibrio cholerae(1), is known to be adjacent to a chromosomally integrated element of unknown function termed the toxin-linked cryptic (TLC)(2). Here we report the characterization of a TLC-related element that corresponds to the genome of a satellite filamentous phage (TLC-Kn phi 1), which uses the morphogenesis genes of another filamentous phage (fs2 phi) to form infectious TLC-Kn phi 1 phage particles. The TLC-Kn phi 1 phage genome carries a sequence similar to the dif recombination sequence, which functions in chromosome dimer resolution using XerC and XerD recombinases(3). The dif sequence is also exploited by lysogenic filamentous phages (for example CTX phi) for chromosomal integration of their genomes. Bacterial cells defective in the dimer resolution often show an aberrant filamentous cell morphology(3,4). We found that acquisition and chromosomal integration of the TLC-Kn phi 1 genome restored a perfect dif site and normal morphology to V. cholerae wild-type and mutant strains with dif(-) filamentation phenotypes. Furthermore, lysogeny of a dif(-) non-toxigenic V. cholerae with TLC-Kn phi 1 promoted its subsequent toxigenic conversion through integration of CTX phi into the restored dif site. These results reveal a remarkable level of cooperative interactions between multiple filamentous phages in the emergence of the bacterial pathogen that causes cholera.