The single-stranded genome of phage CTX is the form used for integration into the genome of Vibrio cholerae.
A major determinant of Vibrio cholerae pathogenicity, the cholera enterotoxin, is encoded in the genome of an integrated phage, CTXvarphi. CTXvarphi integration depends on two host-encoded tyrosine recombinases, XerC and XerD. It occurs at dif1, a 28 bp site on V. cholerae chromosome 1 normally used by XerCD for chromosome dimer resolution. The replicative form of the phage contains two pairs of binding sites for XerC and XerD in inverted orientations. Here we show that in the single-stranded genome of the phage, these sites fold into a hairpin structure, which creates a recombination target for XerCD. In the presence of XerD, XerC can catalyze a single pair of strand exchanges between this target and dif1, resulting in integration of the phage. This integration strategy explains why the rules that normally apply to tyrosine recombinase reactions seemed not to apply to CTXvarphi integration and, in particular, why integration is irreversible.
Val
Marie-Eve
ME
Centre de Génétique Moléculaire, CNRS UPR2167, Avenue de la Terrasse, 91198 Gif sur Yvette Cedex, France.
Bouvier
Marie
M
Campos
Javier
J
Sherratt
David
D
Cornet
François
F
Mazel
Didier
D
Barre
François-Xavier
FX
D001483:Base Sequence; D004277:DNA, Single-Stranded / Q000235:genetics*; D016680:Genome, Bacterial; D016679:Genome, Viral; D008954:Models, Biological; D008969:Molecular Sequence Data; D015394:Molecular Structure; D017105:Pseudomonas Phages / Q000235:genetics*; D014734:Vibrio cholerae / Q000235:genetics*; D016662:Virus Integration / Q000235:genetics*
D004277:DNA, Single-Stranded