Mesorhizobium is a genus of soil bacteria, some isolates of which form an endosymbiotic relationship with diverse legumes of the Loteae tribe. The symbiotic genes of these mesorhizobia are generally carried on integrative and conjugative elements termed symbiosis islands (ICESyms). Mesorhizobium strains that nodulate Lotus spp. have been divided into host-range groupings. Group I (GI) strains nodulate L. corniculatus and L. japonicus ecotype Gifu, while group II (GII) strains have a broader host range, which includes L. pedunculatus. To identify the basis of this extended host range, and better understand Mesorhizobium and ICESym genomics, the genomes of eight Mesorhizobium strains were completed using hybrid long- and short-read assembly. Bioinformatic comparison with previously sequenced mesorhizobia genomes indicated host range was not predicted by Mesorhizobium genospecies but rather by the evolutionary relationship between ICESym symbiotic regions. Three radiating lineages of Loteae ICESyms were identified on this basis, which correlate with Lotus spp. host-range grouping and have lineage-specific nod gene complements. Pangenomic analysis of the completed GI and GII ICESyms identified 155 core genes (on average 30.1 % of a given ICESym). Individual GI or GII ICESyms carried diverse accessory genes with an average of 34.6 % of genes unique to a given ICESym. Identification and comparative analysis of NodD symbiotic regulatory motifs – nod boxes – identified 21 branches across the NodD regulons. Four of these branches were associated with seven genes unique to the five GII ICESyms. The nod boxes preceding the host-range gene nodZ in GI and GII ICESyms were disparate, suggesting regulation of nodZ may differ between GI and GII ICESyms. The broad host-range determinant(s) of GII ICESyms that confer nodulation of L. pedunculatus are likely present amongst the 53 GII-unique genes identified.

中文翻译：

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Loteae-nodulating Mesorhizobium 的共生岛包括三个辐射谱系，具有一致的 nod 基因互补和 nodulation 宿主范围分组。

中根瘤菌 是土壤细菌的一个属，其中一些分离物与 Loteae 部落的多种豆类形成内共生关系。这些中生根瘤菌的共生基因通常携带在称为共生岛 (ICESyms) 的整合和结合元件上。使Lotus spp 结瘤的中生根瘤菌菌株。已分为主机范围分组。I组 (GI) 菌株在L. corniculatus和L. japonicus生态型 Gifu 上形成根瘤，而 II (GII) 组菌株的宿主范围更广，其中包括L. pedunculatus。为了确定这种扩展宿主范围的基础，并更好地了解中生根瘤菌和 ICESym 基因组学，八个 使用混合长读长和短读长组装完成中生根瘤菌菌株。与先前测序的中生根瘤菌基因组的生物信息学比较表明宿主范围不是由中生根瘤菌基因种预测的，而是由 ICESym 共生区域之间的进化关系预测的。在此基础上确定了 Loteae ICESyms 的三个辐射谱系，它们与Lotus spp相关。主机范围分组并具有特定于谱系的点头 基因互补。完整的 GI 和 GII ICESyms 的泛基因组分析确定了 155 个核心基因（平均为给定 ICESym 的 30.1%）。单个 GI 或 GII ICESyms 携带不同的辅助基因，平均 34.6% 的基因是给定 ICESym 独有的。NodD 共生调节基序（点头盒）的鉴定和比较分析确定了整个 NodD 调节子的 21 个分支。其中四个分支与五个 GII ICESyms 独有的七个基因相关。的点头的宿主范围的基因前述框nodZ在GI和GII ICESyms是完全不同的，表明调节nodZ可以GI和GII ICESyms之间不同。GII ICESyms 的广泛宿主范围决定因素赋予结瘤L. pedunculatus可能存在于已鉴定的 53 个 GII 独特基因中。