PDF(Pubmed)
Abstract：
BACKGROUND: Mercury (Hg) is highly toxic and has the potential to cause severe health problems for humans and foraging animals when transported into edible plant parts. Soil rhizobia that form symbiosis with legumes may possess mechanisms to prevent heavy metal translocation from roots to shoots in plants by exporting metals from nodules or compartmentalizing metal ions inside nodules. Horizontal gene transfer has potential to confer immediate de novo adaptations to stress. We used comparative genomics of high quality de novo assemblies to identify structural differences in the genomes of nitrogen-fixing rhizobia that were isolated from a mercury (Hg) mine site that show high variation in their tolerance to Hg.
RESULTS: Our analyses identified multiple structurally conserved merA homologs in the genomes of Sinorhizobium medicae and Rhizobium leguminosarum but only the strains that possessed a Mer operon exhibited 10-fold increased tolerance to Hg. RNAseq analysis revealed nearly all genes in the Mer operon were significantly up-regulated in response to Hg stress in free-living conditions and in nodules. In both free-living and nodule environments, we found the Hg-tolerant strains with a Mer operon exhibited the fewest number of differentially expressed genes (DEGs) in the genome, indicating a rapid and efficient detoxification of Hg from the cells that reduced general stress responses to the Hg-treatment. Expression changes in S. medicae while in bacteroids showed that both rhizobia strain and host-plant tolerance affected the number of DEGs. Aside from Mer operon genes, nif genes which are involved in nitrogenase activity in S. medicae showed significant up-regulation in the most Hg-tolerant strain while inside the most Hg-accumulating host-plant. Transfer of a plasmid containing the Mer operon from the most tolerant strain to low-tolerant strains resulted in an immediate increase in Hg tolerance, indicating that the Mer operon is able to confer hyper tolerance to Hg.
CONCLUSIONS: Mer operons have not been previously reported in nitrogen-fixing rhizobia. This study demonstrates a pivotal role of the Mer operon in effective mercury detoxification and hypertolerance in nitrogen-fixing rhizobia. This finding has major implications not only for soil bioremediation, but also host plants growing in mercury contaminated soils.
RESULTS: Our analyses identified multiple structurally conserved merA homologs in the genomes of Sinorhizobium medicae and Rhizobium leguminosarum but only the strains that possessed a Mer operon exhibited 10-fold increased tolerance to Hg. RNAseq analysis revealed nearly all genes in the Mer operon were significantly up-regulated in response to Hg stress in free-living conditions and in nodules. In both free-living and nodule environments, we found the Hg-tolerant strains with a Mer operon exhibited the fewest number of differentially expressed genes (DEGs) in the genome, indicating a rapid and efficient detoxification of Hg from the cells that reduced general stress responses to the Hg-treatment. Expression changes in S. medicae while in bacteroids showed that both rhizobia strain and host-plant tolerance affected the number of DEGs. Aside from Mer operon genes, nif genes which are involved in nitrogenase activity in S. medicae showed significant up-regulation in the most Hg-tolerant strain while inside the most Hg-accumulating host-plant. Transfer of a plasmid containing the Mer operon from the most tolerant strain to low-tolerant strains resulted in an immediate increase in Hg tolerance, indicating that the Mer operon is able to confer hyper tolerance to Hg.
CONCLUSIONS: Mer operons have not been previously reported in nitrogen-fixing rhizobia. This study demonstrates a pivotal role of the Mer operon in effective mercury detoxification and hypertolerance in nitrogen-fixing rhizobia. This finding has major implications not only for soil bioremediation, but also host plants growing in mercury contaminated soils.
摘要：
背景：汞（Hg）具有很高的毒性，当运输到可食用植物部分时，有可能对人类和觅食动物造成严重的健康问题。与豆科植物共生的土壤根瘤菌可能具有通过从根瘤中输出金属或将根瘤内的金属离子分隔来防止重金属从植物的根转移到芽的机制。水平基因转移有可能使压力立即从头适应。我们使用高质量从头装配体的比较基因组学来鉴定从汞（Hg）矿山中分离出的固氮根瘤菌基因组的结构差异，这些固氮根瘤菌对Hg的耐受性表现出高度差异。
结果：我们的分析确定了在中华根瘤菌和豆科根瘤菌基因组中多个结构保守的merA同源物，但只有具有Mer操纵子的菌株对汞的耐受性提高了10倍。RNAseq分析显示，在自由生活条件和结节中，Mer操纵子中的几乎所有基因都响应Hg胁迫而显着上调。在自由生活和结节环境中，我们发现具有Mer操纵子的汞耐受菌株在基因组中表现出最少数量的差异表达基因（DEGs），表明汞从细胞中快速有效地解毒，从而降低了对汞处理的一般应激反应。在S.medicae和类细菌中的表达变化表明，根瘤菌菌株和寄主植物耐受性都会影响DEGs的数量。除了Mer操纵子基因，在S.medicae中参与固氮酶活性的nif基因在汞含量最高的宿主植物中，在汞含量最高的耐性菌株中显示出显着的上调。将含有Mer操纵子的质粒从最耐受的菌株转移到低耐受的菌株导致汞耐受性立即增加，表明Mer操纵子能够赋予汞高耐受性。
结论：Mer操纵子在固氮根瘤菌中尚未报道。这项研究证明了Mer操纵子在固氮根瘤菌的有效汞解毒和高耐受性中的关键作用。这一发现不仅对土壤生物修复具有重要意义，但也寄主植物生长在汞污染的土壤。
结果：我们的分析确定了在中华根瘤菌和豆科根瘤菌基因组中多个结构保守的merA同源物，但只有具有Mer操纵子的菌株对汞的耐受性提高了10倍。RNAseq分析显示，在自由生活条件和结节中，Mer操纵子中的几乎所有基因都响应Hg胁迫而显着上调。在自由生活和结节环境中，我们发现具有Mer操纵子的汞耐受菌株在基因组中表现出最少数量的差异表达基因（DEGs），表明汞从细胞中快速有效地解毒，从而降低了对汞处理的一般应激反应。在S.medicae和类细菌中的表达变化表明，根瘤菌菌株和寄主植物耐受性都会影响DEGs的数量。除了Mer操纵子基因，在S.medicae中参与固氮酶活性的nif基因在汞含量最高的宿主植物中，在汞含量最高的耐性菌株中显示出显着的上调。将含有Mer操纵子的质粒从最耐受的菌株转移到低耐受的菌株导致汞耐受性立即增加，表明Mer操纵子能够赋予汞高耐受性。
结论：Mer操纵子在固氮根瘤菌中尚未报道。这项研究证明了Mer操纵子在固氮根瘤菌的有效汞解毒和高耐受性中的关键作用。这一发现不仅对土壤生物修复具有重要意义，但也寄主植物生长在汞污染的土壤。
