title: Assembly and transfer of tripartite integrative and conjugative genetic elements.
abstract: Integrative and conjugative elements (ICEs) are ubiquitous mobile genetic elements present as "genomic islands" within bacterial chromosomes. Symbiosis islands are ICEs that convert nonsymbiotic mesorhizobia into symbionts of legumes. Here we report the discovery of symbiosis ICEs that exist as three separate chromosomal regions when integrated in their hosts, but through recombination assemble as a single circular ICE for conjugative transfer. Whole-genome comparisons revealed exconjugants derived from nonsymbiotic mesorhizobia received three separate chromosomal regions from the donor Mesorhizobium ciceri WSM1271. The three regions were each bordered by two nonhomologous integrase attachment (att) sites, which together comprised three homologous pairs of attL and attR sites. Sequential recombination between each attL and attR pair produced corresponding attP and attB sites and joined the three fragments to produce a single circular ICE, ICEMcSym1271A plasmid carrying the three attP sites was used to recreate the process of tripartite ICE integration and to confirm the role of integrase genes intS, intM, and intG in this process. Nine additional tripartite ICEs were identified in diverse mesorhizobia and transfer was demonstrated for three of them. The transfer of tripartite ICEs to nonsymbiotic mesorhizobia explains the evolution of competitive but suboptimal N2-fixing strains found in Western Australian soils. The unheralded existence of tripartite ICEs raises the possibility that multipartite elements reside in other organisms, but have been overlooked because of their unusual biology. These discoveries reveal mechanisms by which integrases dramatically manipulate bacterial genomes to allow cotransfer of disparate chromosomal regions.
authors.affiliation: Centre for Rhizobium Studies, Murdoch University, Perth, WA 6150, Australia.
authors.forename: Timothy L
authors.initials: TL
authors.lastname: Haskett
authors.affiliation: Centre for Rhizobium Studies, Murdoch University, Perth, WA 6150, Australia.
authors.forename: Jason J
authors.initials: JJ
authors.lastname: Terpolilli
authors.affiliation: Centre for Rhizobium Studies, Murdoch University, Perth, WA 6150, Australia.
authors.forename: Amanuel
authors.initials: A
authors.lastname: Bekuma
authors.affiliation: Centre for Rhizobium Studies, Murdoch University, Perth, WA 6150, Australia.
authors.forename: Graham W
authors.initials: GW
authors.lastname: O'Hara
authors.affiliation: Department of Microbiology and Immunology, University of Otago, Dunedin 9016, New Zealand.
authors.forename: John T
authors.initials: JT
authors.lastname: Sullivan
authors.affiliation: Centre for Rhizobium Studies, Murdoch University, Perth, WA 6150, Australia.
authors.forename: Penghao
authors.initials: P
authors.lastname: Wang
authors.affiliation: Department of Microbiology and Immunology, University of Otago, Dunedin 9016, New Zealand.
authors.forename: Clive W
authors.initials: CW
authors.lastname: Ronson
authors.affiliation: School of Biomedical Sciences and Curtin Health Innovation Research Institute, Curtin University, Perth, WA 6102, Australia josh.ramsay@curtin.edu.au.
authors.forename: Joshua P
authors.identifier: 0000-0002-1301-7077
authors.initials: JP
authors.lastname: Ramsay
mesh_terms: D003227:Conjugation, Genetic / Q000235:genetics; D004251:DNA Transposable Elements / Q000235:genetics*; D007887:Fabaceae / Q000235:genetics* / Q000254:growth & development; D022761:Gene Transfer, Horizontal / Q000235:genetics*; D016680:Genome, Bacterial; D044404:Genomic Islands / Q000235:genetics; D019426:Integrases / Q000235:genetics; D059128:Mesorhizobium / Q000235:genetics / Q000254:growth & development; D010957:Plasmids; D011995:Recombination, Genetic; D013559:Symbiosis / Q000235:genetics
chemical_list: D004251:DNA Transposable Elements; D019426:Integrases
keywords: conjugation; integrase; integrative and conjugative elements; recombination; symbiosis