Control of Symbiotic Gene Expression in Sinorhizobium meliloti

描述(申请人提供)：当细菌入侵宿主细胞时，它们可能会利用宿主自身的细胞机制来感染并持续存在。细菌也使用宿主分子作为信号。这些信号交换是致病和共生的关键。我们研究了苜蓿中华根瘤菌，一种α-蛋白细菌，它与包括模式豆科植物紫花苜蓿在内的寄主植物建立了有益的固氮共生关系。对这种细菌入侵宿主的分子研究已经产生了适用于布鲁氏菌等一系列哺乳动物疾病的基本信息和技术方法。这一生物过程也很重要，因为它允许大豆和豌豆等某些植物作物在没有外源氮肥的情况下生长：因此，这一过程对人类和动物的营养至关重要。细菌与寄主植物的相互作用具有物种特异性和发育过程的复杂性，在这个过程中：(1)寄主构建一个根瘤，根瘤是一个多组织器官；(2)细菌通过根瘤层入侵，并作为长期的细胞内共生体占据目标寄主细胞。我们建议开展一个全面的项目，旨在确定共生的中间阶段(感染、入侵和释放到靶细胞)所涉及的基因、调节因子和信号。该系统的一个特别优点是细菌和宿主可以各自独立生长，并可用于正向遗传筛选。这在细菌和宿主中都产生了大量的突变，我们的项目具有利用植物发育突变作为分析细菌在宿主组织中的感染和分化机制的明确工具的特殊前景。我们的目标是分析细菌在几个连续的共生步骤中是如何分化的。我们将建立在一些意想不到的最新结果的基础上，包括(1)发现成功的共生需要特定的RNA聚合酶西格玛因子(2)观察到一些先前已知的调节因子在多种细菌行为中具有意想不到的功能，以及(3)发现我们的一个植物突变体在产生表达细菌共生基因所需的宿主多肽方面存在缺陷。这项工作将深入了解细菌是如何感染宿主细胞的。 与公共卫生相关：当细菌与人类或其他高等生物体相互作用时，一些细菌会导致疾病，但另一些细菌会建立一种共生关系，使宿主受益。越来越多的证据表明，共生对人类健康至关重要。我们的工作将通过研究根瘤菌与豆科植物宿主的相互作用来促进对共生的理解；这是一个复杂、实用的系统，用于发现细菌及其宿主使用的复杂的沟通和协调机制。

DESCRIPTION (provided by applicant): When bacteria invade host cells, they may use the host's own cell machinery to infect and persist. Bacteria also use host molecules as signals. These signal exchanges are critical to pathogenesis and to symbiosis. We study Sinorhizobium meliloti, an alpha- proteobacterium, which establishes a beneficial nitrogen fixing symbiosis with host plants including the model legume Medicago truncatula. Molecular study of this bacteria's invasion into its host has yielded basic information and technical approaches applicable to series mammalian diseases such as Brucella. This biological process is also important because it allows certain plant crops such as soybean and pea to grow without exogenous N-fertilizer: the process is thus critical to human and animal nutrition. The interaction of bacterium and host plant is characterized by species specificity and by the complexity of a developmental process in which (1) the host build a root nodule, which is a multi-tissue organ, and (2) the bacteria invade through layers of nodule and occupy target host cells as a long-term intracellular symbiont. We propose to carry out a comprehensive project aimed to identify genes, regulators and signals involved in intermediate stages of symbiosis (infection, invasion, and release into target cells). A particular advantage of this system is that the bacterium and the host can each be grown independently, and can be used for forward genetic screens. This has yielded a bounty of mutants in both the bacterium and in the host, and our project has the special prospect of using plant developmental mutants as an explicit tool for analyzing the bacteria's mechanisms of infection and differentiation within the host tissues. We aim to analyze how the bacteria differentiate during several sequential steps of symbiosis. We will build on some unexpected recent results, including (1) the finding that specific RNA polymerase sigma factors are required for successful symbiosis (2) the observation that some previously known regulators have unexpected functions in multiple bacterial behaviors, and (3) the discovery that one of our plant mutants is defective in producing host peptides that are required for expression of bacterial symbiosis genes. This work will yield insights into how bacteria infect host cells. PUBLIC HEALTH RELEVANCE: When bacteria interact with humans or other higher organisms, some cause disease but others establish a symbiosis that benefits the host. Increasingly, evidence is showing that symbioses are crucial for human health. Our work will advance the understanding of symbiosis by studying the interaction of rhizobial bacteria with a legume plant host; this is a sophisticated, practical system for discovering the intricate communication and coordination mechanisms used by bacteria and their hosts.