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A multipartite genome organization with a chromosome and many extrachromosomal replicons (ECRs) is characteristic for  Alphaproteobacteria . The best investigated ECRs of terrestrial rhizobia are the symbiotic plasmids for legume root nodulation and the tumor-inducing (Ti) plasmid of  Agrobacterium tumefaciens . RepABC plasmids represent the most abundant alphaproteobacterial replicon type. The currently known homologous replication modules of rhizobia and  Rhodobacteraceae  are phylogenetically distinct. In this study, we surveyed type-strain genomes from the One Thousand Microbial Genomes (KMG-I) project and identified a roseobacter-specific RepABC-type operon in the draft genome of the marine rhizobium  Martelella mediterranea  DSM 17316 T . PacBio genome sequencing demonstrated the presence of three circular ECRs with sizes of 593, 259, and 170-kb. The rhodobacteral RepABC module is located together with a rhizobial equivalent on the intermediate sized plasmid pMM259, which likely originated in the fusion of a pre-existing rhizobial ECR with a conjugated roseobacter plasmid. Further evidence for horizontal gene transfer (HGT) is given by the presence of a roseobacter-specific type IV secretion system on the 259-kb plasmid and the rhodobacteracean origin of 62% of the genes on this plasmid. Functionality tests documented that the genuine rhizobial RepABC module from the  Martelella  259-kb plasmid is only maintained in  A. tumefaciens  C58 ( Rhizobiaceae ) but not in  Phaeobacter inhibens  DSM 17395 ( Rhodobacteraceae ). Unexpectedly, the roseobacter-like replication system is functional and stably maintained in both host strains, thus providing evidence for a broader host range than previously proposed. In conclusion, pMM259 is the first example of a natural plasmid that likely mediates genetic exchange between roseobacters and rhizobia.

frontiers in microbiology

The Composite 259-kb Plasmid of Martelella mediterranea DSM 17316T–A Natural Replicon with Functional RepABC Modules from Rhodobacteraceae and Rhizobiaceae