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Journal of Bacteriology, January 2008, p. 636-647, Vol. 190, No. 2
0021-9193/08/$08.00+0     doi:10.1128/JB.00562-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Three Pathogenicity Islands of Vibrio cholerae Can Excise from the Chromosome and Form Circular Intermediates{triangledown} ,{dagger}

Ronan A. Murphy1 and E. Fidelma Boyd2*

Department of Microbiology, UCC, National University of Ireland, Cork, Ireland,1 Department of Biological Sciences, University of Delaware, Newark, Delaware 197162

Received 12 April 2007/ Accepted 1 November 2007

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.

* Corresponding author. Mailing address: Department of Biological Sciences, University of Delaware, Newark, DE 19716. Phone: (302) 831-1088. Fax: (302) 831-2281. E-mail: fboyd{at}udel.edu

{triangledown} Published ahead of print on 9 November 2007.

{dagger} Supplemental material for this article may be found at http://jb.asm.org/.

Journal of Bacteriology, January 2008, p. 636-647, Vol. 190, No. 2
0021-9193/08/$08.00+0     doi:10.1128/JB.00562-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



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