Control of Symbiotic Gene Expression in Sinorhizobium meliloti

描述（由申请人提供）：当细菌侵入宿主细胞时，它们可能利用宿主自身的细胞机制进行感染并持续存在。细菌也利用宿主分子作为信号。这些信号交换对致病机制和共生至关重要。我们研究了一种α -变形菌Sinorhizobium meliloti，它与宿主植物（包括模式豆科植物Medicago truncatula）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获得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。这种生物过程也很重要，因为它允许某些植物作物，如大豆和豌豆，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，其中：(1)寄主建立根瘤，这是一个多组织器官；(2)细菌通过根瘤层层侵入，作为一个长期的细胞内共生体占领目标寄主细胞。我们建议开展一项综合性项目，旨在识别共生中间阶段（感染、入侵和释放到靶细胞）中涉及的基因、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，并可用于前向遗传筛选。这在细菌和寄主中都产生了大量的突变体，我们的项目具有特殊的前景，利用植物发育突变体作为分析细菌在寄主组织中的感染和分化机制的明确工具。我们的目的是分析细菌如何在共生的几个连续步骤中分化。我们将以一些意想不到的最新结果为基础，包括：(1)发现成功的共生需要特定的RNA聚合酶sigma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细菌共生基因表达所需的宿主肽方面存在缺陷。这项工作将有助于深入了解细菌如何感染宿主细胞。

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.