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。它们允许这些植物在不添加合成化肥的情况下生长，而合成化肥的生产和应用在化石燃料消耗方面的成本很高，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。根瘤菌在细菌中的不同寻常之处在于，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科根瘤菌VF39和3841的质粒上，这两个菌株都有六个质粒，重点是决定质粒复制、稳定性、细胞功能和通过细菌菌株之间接合转移的基因。这项研究建立在实验室以前处理质粒的专业知识和两个菌株的基因组序列的可用性的基础上。其中两个主要主题将是1)研究如何通过分配和其他机制确保质粒从一代到下一代稳定地保持，以及2)VF39中的新的质粒转移系统如何在菌株之间转移其他质粒。此外，我们计划继续我们正在进行的关于质粒编码基因在根瘤菌在土壤中的生存以及在竞争寄主植物的定植和感染中的作用的工作。

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 fertilizers, whose production and application has a high cost in fossil fuel consumption, 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 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 strains VF39 and 3841, both of which have six plasmids, with an emphasis on the genes that determine plasmid replication, stability, function in cells, and potential for transfer by conjugation between bacterial strains. The research builds on previous expertise in the lab for dealing with plasmids, and the availability of genome sequences for both strains. Two of the main themes will be 1) examining how plasmids ensure, by partitioning and other mechanisms, that they are stably maintained from one generation to the next, and 2) how a novel plasmid transfer system in VF39 functions to move other plasmids between strains. In addition, we plan to continue our ongoing work on the role of plasmid-encoded genes in survival of rhizobia in the soil and in competition for colonization and infection of host plants.