title: Three pathogenicity islands of Vibrio cholerae can excise from the chromosome and form circular intermediates.
abstract: Vibrio pathogenicity island-2 (VPI-2) is a 57-kb region integrated at a transfer RNA (tRNA)-serine locus that encompasses VC1758 to VC1809 on the V. cholerae N16961 genome and is present in pandemic isolates. VPI-2 encodes a P4-like integrase, a restriction modification system, a Mu phage-like region, and a sialic acid metabolism region, as well as neuraminidase (VC1784), which is a glycosylhydrolase known to release sialic acid from sialoglycoconjugates to unmask GM1 gangliosides, the receptor for cholera toxin. We examined the tRNA-serine locus among the sequenced V. cholerae genomes and identified five variant VPI-2 regions, four of which retained the sialometabolism region. Three variant VPI-2 regions contained a type three secretion system. By using an inverse nested PCR approach, we found that the VPI-2 region can form an extrachromosomal circular intermediate (CI) molecule after precise excision from its tRNA-serine attachment site. We constructed a knockout mutant of VC1758 (int) with V. cholerae strain N16961 and found that no excision PCR product was produced, indicating that a functional cognate, VPI-2 integrase, is required for excision. The Vibrio seventh pandemic island-I (VSP-I) and VSP-II regions are present in V. cholerae O1 El Tor and O139 serogroup isolates. Novel regions are present at the VSP-I insertion site in strain MZO-3 and at the VSP-II insertion site in strain 623-39. VSP-II is a 27-kb region that integrates at a tRNA-methionine locus, is flanked by direct repeats, and encodes a P4-like integrase. We show that VSP-II can excise and form a CI and that the cognate VSP-II integrase is required for excision. Interestingly, VSP-I is not inserted at a tRNA locus and does encode a XerDC-like recombinase, but similar to VPI-2 and VSP-II, VSP-I does excise from the genome to form a CI. These results show that all three pathogenicity islands can excise from the chromosome, which is likely a first step in their horizontal transfer.
authors.affiliation: Department of Biological Sciences, University of Delaware, Newark, DE 19716, USA.
authors.forename: Ronan A
authors.initials: RA
authors.lastname: Murphy
authors.forename: E Fidelma
authors.initials: EF
authors.lastname: Boyd
mesh_terms: D002876:Chromosomes, Bacterial; D004269:DNA, Bacterial / Q000378:metabolism*; D004270:DNA, Circular / Q000378:metabolism*; D017353:Gene Deletion; D044404:Genomic Islands; D019426:Integrases / Q000235:genetics / Q000502:physiology; D011995:Recombination, Genetic; D041241:Vibrio cholerae O1 / Q000235:genetics*; D041242:Vibrio cholerae O139 / Q000235:genetics*; D044163:Vibrio cholerae non-O1 / Q000235:genetics*
chemical_list: D004269:DNA, Bacterial; D004270:DNA, Circular; D019426:Integrases