Control of Symbiotic Gene Expression in Sinorhizobium meliloti

描述(申请人提供)：细菌和多细胞宿主生物如何相互识别和响应？这是健康和疾病研究中的一个基本问题。本项目研究了α-蛋白细菌种紫花苜蓿与其寄主紫花苜蓿属(如紫花苜蓿/紫花苜蓿和木桶/紫花苜蓿)共生过程中基因表达的调控。在这一复杂的共生和发育过程的早期阶段，我们的实验室曾率先发现了宿主细菌信号。我们发现，细菌基因在共生中的表达可以遵循共生过程中的六种主要模式之一(即基因何时何地开启和关闭)。在下一个项目期间，我们将确定这些阶段特定基因的启动子，并将确定在发育过程中控制表达的调控基因和蛋白质。基因表达控制的主要参与者之一是RNA聚合酶，它识别DNA并仅读出(表达)某些特定的基因。在细菌中，决定表达哪些基因的关键成分是这种核糖核酸聚合酶的西格玛(σ)因子。我们发现胞外功能σ因子是共生所必需的：我们现在计划分析胞外σ因子在多阶段发育过程中的调控机制。细菌的严格反应通常发生在营养缺乏的反应中；在中华根瘤菌中，这是正确的基因共生表达所必需的。我们假设这是程序化的，而不仅仅是对养分转移的反应。我们将直接测试关于严格反应如何参与分化机制的几个假说。同时，我们还将研究中华根瘤菌细胞如何对细胞表面的变化做出反应：了解这一点很重要，因为细菌表面与其真核宿主的相互作用不可避免地成为两种生物发出信号并相互响应的舞台。

DESCRIPTION (provided by applicant): How do bacteria and multicellular host organisms recognize and respond to each other? This is a fundamental question in studies of health and disease. Our project investigates the control of gene expression in a symbiosis between the alpha-proteobacterial species Sinorhizobium meliloti and its host, genus Medicago (such as alfalfa/M. sativa and barrel-medic/M. truncatula). Our lab has previously pioneered discoveries about host-bacterial signaling during the early steps of this complex symbiotic and developmental process. We have discovered that bacterial gene expression in symbiosis can follow one of six major patterns during symbiosis (that is, when and where the genes are turned on and off). In the next project period, we will identify promoters for these stage-specific genes and will determine the regulatory genes and proteins that control expression during development. One of the major players in gene expression control is the enzyme RNA polymerase, which recognizes DNA and reads out (expresses) only certain specific genes. In bacteria, a key component that determines which genes to express is the sigma (σ) factor of this RNA polymerase. We found that ECF (extra-cytoplasmic function) σ factors are required for symbiosis: we now plan to analyze the mechanisms by which ECF σ factors are controlled in the context of a multi-stage developmental process. The bacterial stringent response usually occurs in reaction to nutrient deprivation; in Sinorhizobium, it is required for correct gene expression in symbiosis. We hypothesize this is programmed, and not merely a response to nutrient shift. We will directly test several hypotheses about how the stringent response is involved in the mechanism of differentiation. In parallel we will also look at how Sinorhizobium cells respond to alterations at the cell surface: This is important to understand because bacterial surface interactions with its eukaryotic host inevitably become an arena in which the two organisms signal and respond to each other.