repABC-based replication systems of Rhizobium leguminosarum bv. trifolii TA1 plasmids: incompatibility and evolutionary analyses

Soil bacteria of the genus Rhizobium possess complex genomes consisting of a chromosome and in addition, often, multiple extrachromosomal replicons, which are usually equipped with repABC genes that control their replication and partition. The replication regions of four plasmids of Rhizobium leguminosarum bv. trifolii TA1 (RtTA1) were identified and characterized. They all contained a complete set of repABC genes. The structural diversity of the rep regions of RtTA1 plasmids was demonstrated for parS and incα elements, and this was especially apparent in the case of symbiotic plasmid (pSym). Incompatibility assays with recombinant constructs containing parS or incα demonstrated that RtTA1 plasmids belong to different incompatibility groups. Horizontal acquisition was plausibly the main contributor to the origin of RtTA1 plasmids and pSym is probably the newest plasmid of this strain. Phylogenetic and incompatibility analyses of repABC regions of three closely related strains: RtTA1, R. leguminosarum bv. viciae 3841 and Rhizobium etli CFN42, provided data on coexistence of their replicons in a common genomic framework.