Plasmid Biology of Rhizobium leguminosarum

根瘤菌是一种细菌，它能在豌豆、豆类、小扁豆和紫花苜蓿等豆科植物的根部形成固氮根瘤，这一点值得注意。它们能够让这些植物在不添加合成含氮化肥的情况下生长，而合成含氮化肥的生产和应用在化石燃料消耗和温室气体排放方面成本很高，这使得它们在可持续农业中极其重要。生产更高效、更具竞争力的根瘤菌作为豆科植物的接种剂是生物技术的主要目标。然而，引进的根瘤菌之间的相互作用，以及环境中菌株之间遗传转移的潜力，还没有被很好地了解。根瘤菌在细菌中相对不寻常，因为它们拥有庞大而复杂的基因组，通常由一条染色体和几个被称为质粒的非常大的染色体外遗传元件组成。这些质粒往往非常稳定，其中一些性质介于质粒和染色体的性质之间。这项建议将集中在豆类根瘤菌生物量viciae菌株VF39的质粒上，该菌株是豌豆、小扁豆、蚕豆、豌豆和各种山毛豆物种的共生体。重点将放在决定细菌菌株之间通过接合转移的机制和后果的基因，以及质粒编码基因在细菌在土壤、根际和根瘤环境中生存和繁衍的能力中的其他作用。这项研究建立在实验室以前研究大质粒的专业知识的基础上，以及细菌和感染它的多个噬菌体的基因组序列的可用性。主要主题将是：1)详细分析pRleVF39b和pRleVF39a上编码的新的质粒转移和IV型分泌系统；2)鉴定调控质粒转移基因的因素；3)鉴定与质粒转移有关的蛋白质-蛋白质相互作用；4)继续探索编码质粒的分解代谢基因在根瘤菌生态成功中的作用；以及5)鉴定与限制噬菌体感染有关的质粒编码的基因座。关于根瘤菌水平基因转移的信息对于理解未来任何优化菌株(包括遗传增强菌株)作为豆类接种剂释放的后果至关重要。了解包括噬菌体抗性和分解代谢能力在内的属性，使单个菌株能够在土壤中持续存在，成功地在根部区域定居，并与其他菌株竞争，对于选择和设计更好的接种菌非常重要。

Rhizobia are bacteria that are notable for their ability to form nitrogen-fixing nodules on the roots of legume plants, such as peas, beans, lentils and alfalfa. Their ability to allow these plants to grow without addition of synthetic nitrogen-containing fertilizers, whose production and application has a high cost in fossil fuel consumption and greenhouse gas emissions, makes them extremely important in sustainable agriculture. Production of more efficient and more competitive rhizobial strains for use as legume inoculants is a major goal in biotechnology. However, the interactions of introduced rhizobial strains, and the potential for genetic transfer between strains in the environment, are not well understood. Rhizobia are relatively unusual among bacteria in that they have large and complex genomes that usually consist of one chromosome and several very large extrachromosomal genetic elements known as plasmids. The plasmids tend to be very stable, and some of them exhibit properties intermediate between those expected of a plasmid and those of a chromosome. This proposal will focus on the plasmids of Rhizobium leguminosarum biovar viciae strain VF39, a symbiont of peas, lentils, faba beans, vetches, and various Lathyrus species. The emphasis will be on the genes that determine the mechanisms and consequences of transfer by conjugation between bacterial strains, and other roles of plasmid-encoded genes in the ability of the bacterium to survive and thrive in the soil, rhizosphere, and nodule environments. The research builds on previous expertise in the lab studying large plasmids, and the availability of genome sequences of the bacterium and multiple bacteriophages that infect it. The main themes will be 1) detailed analysis of the novel plasmid transfer and type IV secretion systems encoded on plasmids pRleVF39b and pRleVF39a and 2) identification of factors regulating plasmid transfer genes 3) identification of the protein-protein interactions involved in plasmid transfer 4) continued exploration of the role of plasmid-encoded catabolic genes in the ecological success of rhizobia, and 5) identification of plasmid-encoded loci that are involved in limiting phage infections. Information on horizontal gene transfer in rhizobia is vital to understanding the consequences of release of any future optimized strains (included genetically enhanced strains) as legume inoculants. Understanding the attributes, including phage resistance and catabolic capacity, that allow individual strains to persist in soil, successfully colonize the root region, and compete with other strains, is of great importance in selecting and designing better inoculant strains.