A Natural System of Chromosome Transfer in Yersinia pseudotuberculosis Biliana Lesic1, Mohamed Zouine2, Magaly Ducos-Galand 2 , Christe` le Huon 1 , Marie-Laure Rosso1, Marie- Christine Pre´ vost3, Didier Mazel2*, Elisabeth Carniel1* 1 Yersinia Research Unit, Institut Pasteur, Paris, France, 2 Unite´ Plasticite´ du Ge´nome Bacte´rien, Institut Pasteur, CNRS UMR 3525, Paris, France, 3 Plateforme de Microscopie Ultrastructurale, Institut Pasteur, Paris, France Abstract The High Pathogenicity Island of Yersinia pseudotuberculosis IP32637 was previously shown to be horizontally transferable as part of a large chromosomal segment. We demonstrate here that at low temperature other chromosomal loci, as well as a non-mobilizable plasmid (pUC4K), are also transferable. This transfer, designated GDT4 (Generalized DNA Transfer at 4uC), required the presence of an IP32637 endogenous plasmid (pGDT4) that carries several mobile genetic elements and a conjugation machinery. We established that cure of this plasmid or inactivation of its sex pilus fully abrogates this process. Analysis of the mobilized pUC4K recovered from transconjugants revealed the insertion of one of the pGDT4–borne ISs, designated ISYps1, at different sites on the transferred plasmid molecules. This IS belongs to the IS6 family, which moves by replicative transposition, and thus could drive the formation of cointegrates between pGDT4 and the host chromosome and could mediate the transfer of chromosomal regions in an Hfr-like manner. In support of this model, we show that a suicide plasmid carrying ISYps1 is able to integrate itself, flanked by ISYps1 copies, at multiple locations into the Escherichia coli chromosome. Furthermore, we demonstrate the formation of RecA-independent cointegrates between the ISYps1- harboring plasmid and an ISYps1-free replicon, leading to the passive transfer of the non-conjugative plasmid. We thus demonstrate here a natural mechanism of horizontal gene exchange, which is less constrained and more powerful than the classical Hfr mechanism, as it only requires the presence of an IS6-type element on a conjugative replicon to drive the horizontal transfer of any large block of plasmid or chromosomal DNA. This natural mechanism of chromosome transfer, which occurs under conditions mimicking those found in the environment, may thus play a significant role in bacterial evolution, pathogenesis, and adaptation to new ecological niches. Citation: Lesic B, Zouine M, Ducos-Galand M, Huon C, Rosso M-L, et al. (2012) A Natural System of Chromosome Transfer in Yersinia pseudotuberculosis. PLoS Genet 8(3): e1002529. doi:10.1371/journal.pgen.1002529 Editor: Josep Casadesu´ s, Universidad de Sevilla, Spain Received September 15, 2011; Accepted December 23, 2011; Published March 8, 2012 Copyright: 2012 Lesic et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was funded by a grant from the French ‘‘Ministe`re de la Recherche et de la Technologie’’ to BL and by the Centre National de la Recherche Scientifique (CNRS-URA 2171), the Fondation pour la Recherche Me´dicale, and the EU (STREP CRAB, LSHM-CT-2005-019023, and NoE EuroPathoGenomics, LSHB- CT-2005-512061) to DM. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: didier.mazel@pasteur.fr (DM); elisabeth.carniel@pasteur.fr (EC) Introduction Horizontal gene transfer (HGT) is a driving force for bacterial evolution, as it allows the dispersion of adaptive loci between closely related and also phylogenetically distant bacterial species. Well-characterized mobile genetic elements such as conjugative plasmids, transposons, Integrative conjugative elements (ICE), pathogenicity islands (PAI), or phages are associated with HGT of specific adaptive functions (antibiotic resistance, virulence, meta- bolic pathways) and participate to genome plasticity. However, exchanges of chromosomal regions that form the core genome and are not part of the mobile genetic pool are also observed. While their importance in bacterial evolution and speciation is now well established, the underlying mechanisms are often loosely described and remain hypothetical in many cases. The Gram-negative enteropathogen Yersinia pseudotuberculosis carries a PAI termed High Pathogenicity Island (HPI) [1], which encodes the siderophore yersiniabactin [2]. The fact that this island is mobile within the genome of its host strain [3], and is present and often conserved both in terms of genetic organization and nucleotide sequence in various bacterial genera such as Escherichia coli (various pathotypes), Klebsiella or Citrobacter [4], suggested that it may have retained its ability to be horizontally transmitted to new bacterial hosts. Indeed, we evidenced the transfer of the HPI between natural Y. pseudotuberculosis isolates [3]. This phenomenon was observed only when the bacteria were incubated at low temperature (optimal at 4uC) and in broth, and was more efficient in an iron-poor medium [5]. However, this transfer did not require the integration/excision machinery encoded by the HPI, was RecA-dependent in the recipient strain, and involved not only the HPI but also adjacent sequences encompassing at least 46 kb of chromosomal DNA [3]. Similar results were recently obtained for the HPI of natural Escherichia coli isolates, using a multi locus sequence typing approach. The E. coli HPI was found to have been acquired simultaneously with the chromosomal flanking regions of the donor strains [6], indicating again that the island was transmitted as part of a larger chromosomal region. This phenomenon is not restricted to the HPI and to enterobacteria since it has been recently reported that movement of the Enterococcus faecalis PAI was invariably accompanied by transfer of flanking donor chromosome sequences [7]. PLoS Genetics | www.plosgenetics.org 1 March 2012 | Volume 8 | Issue 3 | e1002529