AUTHOR=Bartling Pascal , Brinkmann Henner , Bunk Boyke , Overmann Jörg , Göker Markus , Petersen Jörn 

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

JOURNAL=Frontiers in Microbiology

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2017.01787

DOI=10.3389/fmicb.2017.01787

ISSN=1664-302X

ABSTRACT=<p>A multipartite genome organization with a chromosome and many extrachromosomal replicons (ECRs) is characteristic for <italic>Alphaproteobacteria</italic>. The best investigated ECRs of terrestrial rhizobia are the symbiotic plasmids for legume root nodulation and the tumor-inducing (Ti) plasmid of <italic>Agrobacterium tumefaciens</italic>. RepABC plasmids represent the most abundant alphaproteobacterial replicon type. The currently known homologous replication modules of rhizobia and <italic>Rhodobacteraceae</italic> 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 <italic>Martelella mediterranea</italic> DSM 17316<sup>T</sup>. 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 <italic>Martelella</italic> 259-kb plasmid is only maintained in <italic>A. tumefaciens</italic> C58 (<italic>Rhizobiaceae</italic>) but not in <italic>Phaeobacter inhibens</italic> DSM 17395 (<italic>Rhodobacteraceae</italic>). 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.</p>