Plasmid biology of Rhizobium leguminosarum

根瘤菌是一种在豆科植物（如豌豆、黄豆、扁豆和苜蓿）根部形成固氮根瘤的能力显著的细菌。它们能够使这些植物在不添加合成肥料的情况下生长，而合成肥料的生产和应用在化石燃料消耗方面成本很高，这使得它们在可持续农业中极其重要。生产更有效和更具竞争力的根瘤菌菌株作为豆科植物的接种剂是生物技术的主要目标。然而，引入的根瘤菌菌株之间的相互作用，以及菌株之间在环境中遗传转移的可能性，还没有得到很好的了解。根瘤菌在细菌中是不寻常的，因为它们具有大而复杂的基因组，通常由一条染色体和几个非常大的染色体外遗传元件（称为质粒）组成。质粒往往非常稳定，其中一些表现出介于质粒和染色体之间的特性。本文将重点研究豆科根瘤菌菌株VF39和3841的质粒，这两种菌株都有6个质粒，重点研究决定质粒复制、稳定性、细胞功能和菌株间偶联转移潜力的基因。这项研究建立在实验室先前处理质粒的专业知识和两种菌株的基因组序列的可用性的基础上。两个主要主题将是1)研究质粒如何通过分裂和其他机制确保它们从一代到下一代的稳定维持，以及2)VF39中的新型质粒转移系统如何在菌株之间移动其他质粒。此外，我们计划继续我们正在进行的工作，质粒编码基因在土壤中根瘤菌的生存和寄主植物的定植和感染竞争中的作用。

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.