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

Rhizobial communities in symbiosis with legumes: genetic diversity, competition and interactions with host plants

“…Phylogenetic analysis of repABC sequences enabled the investigation of the history of a plasmid set present in specific rhizobial strains and tracing of subsequent events of plasmid acquisition [25][26][27]. Moreover, some of the rhizobial plasmids are unique in that they cannot be eliminated from the genome, due to the presence of genes participating in the primary metabolism, and they are sometimes regarded as `secondary chromosomes` [28].…”

Rhizobial plasmids — replication, structure and biological role

“…In both cases, one of them is located upstream of repA and the other within repB and in the repB-repC intergenic region, respectively. In turn, no such elements were detected in the proximity of repABC in pRleTA1d and pRleTA1a [30]. However, it is known that only one or at least one par-site located near or within the repABC operon is needed for normal operon activity and adequate plasmid partitioning [20].…”

“…The secondary structure forming ctRNA is a trans incompatibility factor, modulating RepC levels and thereby the plasmid copy number [28,29]. It was shown that the ctRNA region introduced in trans exerts incompatibility against the parental plasmid [15,18,30].…”

“…Firstly, the maintenance of multiple plasmids in the cell indicates the presence of several incompatibility groups within the repABC plasmid family [19,30,48]. Secondly, phylogenetic analyses of rep regions showed that repABC plasmids coexisting in the same strain most probably arose through separate events of lateral gene transfer, further forcing the appearance of different new incompatibility groups and allowing co-residence of plasmids equipped with the same type of replication/ partition machinery in one bacterial cell [30,48,49]. In regards to the evolution of the repABC operon itself, it was hypothesized that it presumably originated through integration of the ctRNA-repC cassette between the repAB genes and the par-site located immediately downstream of the repB, separating the repAB genes from the parS.…”

“…viciae (Rlv) and trifoli (Rlt) are closely related species differing in their host-specificity but displaying very similar genome organization: a single circular chromosome and several large plasmids. Each of them carries the majority of nod-nif-fix genes on the plasmid, named pSym [17,36,46,47,82], but the pSyms of Rhe, Rlv and Rlt belong to different incompatibility groups and have quite different genes content other than those related to symbiotic performance [14,30,83]. In contrast to Sme, several major cluster of genes engaged in the biosynthesis of various surface polysaccharides are chromosomal.…”