Assembly and transfer of tripartite integrative and conjugative genetic elements

Abstract

Significance Integrative and conjugative elements (ICEs) are one of the most prevalent but least-characterized families of mobile genetic elements in bacteria. We identified a family of ICEs that exists as three separate parts integrated within the single chromosomes of symbiotic mesorhizobia. These “tripartite ICEs,” through a series of chromosomal recombinations mediated by integrase proteins, assemble into a single circular ICE. Following transfer to nonsymbiotic mesorhizobia, tripartite ICEs integrate and disassemble into three parts in the recipient genome and exconjugant mesorhizobia gain the ability to form a symbiosis with legumes. These discoveries expand our appreciation of the potential for gene transfer in bacteria and demonstrate how mobile genetic elements can dramatically manipulate the bacterial genome.