Members of the Mesorhizobium genus are soil bacteria that often form nitrogen-fixing symbioses with legumes. Most characterised Mesorhizobium spp. genomes are ~8 Mb in size and harbour extensive pangenomes including large integrative and conjugative elements (ICEs) carrying genes required for symbiosis (ICESyms). Here, we document and compare the conjugative mobilome of 41 complete Mesorhizobium genomes. We delineated 56 ICEs and 24 integrative and mobilizable elements (IMEs) collectively occupying 16 distinct integration sites, along with 24 plasmids. We also demonstrated horizontal transfer of the largest (853,775 bp) documented ICE, the tripartite ICEMspSymAA22. The conjugation systems of all identified ICEs and several plasmids were related to those of the paradigm ICESym ICEMlSymR7A, with each carrying conserved genes for conjugative pilus formation (trb), excision (rdfS), DNA transfer (rlxS) and regulation (fseA). ICESyms have likely evolved from a common ancestor, despite occupying a variety of distinct integration sites and specifying symbiosis with diverse legumes. We found extensive evidence for recombination between ICEs and particularly ICESyms, which all uniquely lack the conjugation entry-exclusion factor gene trbK. Frequent duplication, replacement and pseudogenization of genes for quorum-sensing-mediated activation and antiactivation of ICE transfer suggests ICE transfer regulation is constantly evolving. Pangenome-wide association analysis of the ICE identified genes potentially involved in symbiosis, rhizosphere colonisation and/or adaptation to distinct legume hosts. In summary, the Mesorhizobium genus has accumulated a large and dynamic pangenome that evolves through ongoing horizontal gene transfer of large conjugative elements related to ICEMlSymR7A.

中文翻译：

---

中根瘤菌属综合和共轭移动遗传元件的比较分析

Mesorhizobium属的成员是土壤细菌，通常与豆科植物形成固氮共生体。最具特征的Mesorhizobium spp。基因组大小约为 8 Mb，包含广泛的泛基因组，包括携带共生所需基因 (ICESyms) 的大型整合和共轭元件 (ICE)。在这里，我们记录并比较了 41 个完整的中根瘤菌基因组的共轭移动组。我们描绘了 56 个 ICE 和 24 个可整合和可移动元件 (IME)，它们共同占据 16 个不同的整合位点，以及 24 个质粒。我们还展示了记录在案的最大（853,775 bp）ICE，即三方 ICE M spSym AA22的水平转移 . 所有已鉴定的 ICE 和几个质粒的结合系统都与 ICESym ICE Ml Sym R7A范式的结合系统相关，每个都携带用于结合菌毛形成 ( trb )、切除 ( rdfS )、DNA 转移 ( rlxS ) 和调节 ( fseA ) 的保守基因）。ICESyms 很可能是从一个共同的祖先进化而来的，尽管它占据了各种不同的整合位点并指定了与不同豆科植物的共生关系。我们发现了 ICE 和特别是 ICESyms 之间重组的广泛证据，它们都独特地缺乏共轭进入排除因子基因trbK. 群体感应介导的 ICE 转移激活和抗激活基因的频繁复制、替换和假基因化表明 ICE 转移调节正在不断发展。ICE 的泛基因组关联分析确定了可能参与共生、根际定殖和/或适应不同豆科植物宿主的基因。总之，Mesorhizobium属已经积累了一个庞大且动态的泛基因组，该泛基因组通过与 ICE M1 Sym R7A相关的大型共轭元件的持续水平基因转移而进化。