Bartling Pascal, Brinkmann Henner, Bunk Boyke, Overmann Jörg, Göker Markus, Petersen Jörn
Leibniz-Institute DSMZ-German Collection of Microorganisms and Cell CulturesBraunschweig, Germany.
Front Microbiol. 2017 Sep 21;8:1787. doi: 10.3389/fmicb.2017.01787. eCollection 2017.
A multipartite genome organization with a chromosome and many extrachromosomal replicons (ECRs) is characteristic for . The best investigated ECRs of terrestrial rhizobia are the symbiotic plasmids for legume root nodulation and the tumor-inducing (Ti) plasmid of . RepABC plasmids represent the most abundant alphaproteobacterial replicon type. The currently known homologous replication modules of rhizobia and 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 DSM 17316. 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 259-kb plasmid is only maintained in C58 () but not in DSM 17395 (). 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.
具有染色体和许多染色体外复制子（ECR）的多部分基因组组织是……的特征。陆地根瘤菌中研究最深入的ECR是用于豆科植物根瘤形成的共生质粒和……的致瘤（Ti）质粒。RepABC质粒代表最丰富的α-变形杆菌复制子类型。目前已知的根瘤菌和……的同源复制模块在系统发育上是不同的。在本研究中，我们调查了来自千种微生物基因组（KMG-I）项目的模式菌株基因组，并在海洋根瘤菌DSM 17316的基因组草图中鉴定出一个玫瑰杆菌特异性的RepABC型操纵子。PacBio基因组测序证明存在三个大小分别为593、259和170 kb的环状ECR。红杆菌属的RepABC模块与一个根瘤菌等效模块一起位于中等大小的质粒pMM259上，该质粒可能起源于一个预先存在的根瘤菌ECR与一个接合的玫瑰杆菌质粒的融合。259 kb质粒上存在玫瑰杆菌特异性的IV型分泌系统以及该质粒上62%的基因起源于红杆菌科，这进一步证明了水平基因转移（HGT）。功能测试表明，来自259 kb质粒的真正根瘤菌RepABC模块仅在C58（……）中得以维持，而在DSM 17395（……）中则不然。出乎意料的是，玫瑰杆菌样复制系统在两种宿主菌株中均具有功能且能稳定维持，从而为其宿主范围比先前提出的更广泛提供了证据。总之，pMM259是天然质粒可能介导玫瑰杆菌和根瘤菌之间遗传交换的首个实例。
