Molecular genetics of the Sinorhizobium meliloti- alfalfa symbiosis

紫花苜蓿根瘤菌是一种在紫花苜蓿上形成固氮根瘤的细菌。我的研究项目涉及了解紫花苜蓿和紫花苜蓿共生的各个方面，并将其扩展到将紫花苜蓿作为模式土壤微生物进行研究。我们研究了细菌在根瘤内的碳代谢途径，并在基因组水平上分析了1683kb的pSymB巨质粒的生物学作用(S)。这项工作使我们参与了苜蓿链霉菌基因组的测序，最近又参与了分析其基因组的基因组方法的开发。我们最近的NSERC研究集中在分析pSymB稳定复制所需的refABC基因簇，并将我们在苜蓿链霉菌中的磷酸盐运输方面的工作扩展到鉴定许多基因的表达，这些基因的表达受Pho-Regulon的调节。在这里，我们建议继续分析草地早熟禾中PI调节的代谢：i)通过研究它们诱导/抑制的生理基础；ii)通过研究这些基因在不同的生物中是否受到PI响应的调节(那些生物的基因组已经测序)。这项工作将导致对pI限制作出反应的核心基因集的识别，并由此推断出对pI限制的生物学反应。这代表了一种关键细胞成分的基本信息，该成分在细胞中含量丰富，但往往会限制土壤环境中的生长。我们将研究根瘤内细菌的碳代谢，并应用复杂的基因组学方法来解决在土壤和植物根周围的根际诱导哪些基因。这些信息将有助于理解细菌(包括病原体)如何在土壤环境中生存，以及如何改进在农业和经济上具有重要意义的固氮过程。

Sinorhizobium meliloti is a bacterium that forms N2-fixing root nodules on alfalfa. My research program has dealt with understanding various aspects of the symbiosis between S. meliloti and alfalfa and also extends to studying S. meliloti as a model soil microorganism. We have investigated the pathway of carbon metabolism employed by the bacteria within root nodules and at the genome level, we have analyzed the biological role(s) of the 1,683-kilobase pSymB megaplasmid. That work led to our involvement in the sequencing of the S. meliloti genome and more recently on the development of genomic approaches to the analysis of the it's genome. We focused our recent NSERC research towards the analysis of the repABC gene cluster which is required for stable replication of pSymB and have also extended our work on phosphate transport in S. meliloti towards the identification of many genes whose expression is regulated by PhoB in response to phosphate (Pi) limitation i.e. the Pho-regulon. Here we propose to continue our analysis of Pi-regulated metabolism in S. meliloti: i) by investigating the physiological bases for their induction/repression and ii) by investigating whether these genes are subject to Pi-responsive regulation in diverse organisms (those whose genomes are sequenced). This work will result in the recognition of a core gene set that responds to Pi-limitation and by inference a biological response to Pi-limitation. This represents fundamental information on a key cellular component that is abundant in cells but often limits growth in soil environments. We will investigate carbon metabolism in the bacteria within nodules and also apply sophisticated genomic approaches towards addressing what genes are induced in the soil and rhizosphere surrounding plant roots. This information will help in understanding how bacteria (including pathogens) survive in soil environments and how the agriculturally and economically important nitrogen fixation process can be improved.