Quorum-Sensing and Growth Control in Rhizobium sp. NGR234

变形菌门中的革兰氏阴性细菌通常使用酰化高丝氨酸内酯（酰基-HSL）作为群体感应（QS）过程中的分子信号。 群体感应细菌释放可扩散的信号分子，随着细胞数量的增加而积累，最终触发适应性反应。 LuxI和LuxR家族的调节蛋白通常分别是合成酰基-HSL和响应酰基-HSL所需的。 扩散的细胞群体通常会产生恒定、低水平的酰基-HSL，并且这些物质沿着浓度梯度迅速扩散出细胞。 升高的群体密度增加了相对酰基-HSL浓度，最终促进信号与LuxR型蛋白的相互作用，这反过来控制靶基因的转录。 虽然这种基本机制是保守的，但QS调节的背景及其控制下的细胞功能在不同的细菌中是高度可变的。 该项目的重点是固氮植物共生根瘤菌NGR 234所采用的LuxI-LuxR型QS系统的调控背景，其作用机制及其对细胞生长速率的影响。 NGR 234在多种豆科植物的根部刺激形成固氮共生根瘤。 这种混杂的主机相互作用的分子基础已被广泛研究。 协调植物相互作用的许多功能在536 kb pNGR 234 a质粒上进行。 pNGR 234 a质粒还携带与来自其它细菌的质粒复制（rep）和接合转移（trb/tra）基因同源的大基因簇。 rep/trb/tra集群包括一个LuxI-LuxR类型的监管对TraI和TraR，以及附加的QS监管器TraM。 TraI合成3-氧代-辛酰基-L-高丝氨酸内酯，TraR与该酰基HSL相互作用以调节tra/trb和rep操纵子表达。 TraM通过形成抗活化复合物来抑制TraR。 分子遗传学方法正在被用来研究NGR 234中的QS机制。 将研究控制traR表达的调控信号和途径，以确定促进QS的条件。 基于类似的系统，宿主植物可能在这种调节中发挥作用。 将鉴定TraR控制下的QS调节的pNGR 234 a基因，并阐明TraR控制其表达的机制。 最后，将检查QS对细胞生长速率的控制，QS是几种根瘤菌中常见的QS调节靶标。 该研究项目产生的结果将提供有关重要植物共生体中细胞间通信的基本信息，以及这种通信在其高度可塑的宿主相互作用中的作用。 更普遍地说，这些研究将有助于理解细胞间通讯如何直接或间接地影响与宿主生物的相互作用，并使群体感应微生物能够平衡其生理活动与宿主环境。根瘤菌NGR 234在与高等植物共生关系期间使用细胞间通讯。 该研究项目研究了这种细胞间通讯的生化和遗传机制，以及它对这种细菌和宿主植物相互作用的影响。 这项工作产生的发现将提供利用这些细菌通信系统所需的基础知识，用于对抗植物和动物中的传染病，以及促进有益的微生物相互作用。

Gram-negative bacteria within the Proteobacteria group commonly use acylated homoserine lactones (acyl-HSLs) as molecular signals in the process of quorum sensing (QS). Quorum-sensing bacteria release diffusible signal molecules that accumulate with increasing cell number, eventually triggering adaptive responses. Regulatory proteins of the LuxI and LuxR families are usually required for synthesis of, and response to acyl-HSLs, respectively. Diffuse populations of cells generally produce a constant, low level of acyl-HSLs, and these rapidly diffuse out of cells down their concentration gradient. Elevated population density increases the relative acyl-HSL concentration, eventually fostering interaction of the signals with LuxR-type proteins, which in turn, control the transcription of target genes. Although this basic mechanism is well conserved, the context of QS regulation and the cellular functions under its control are highly variable among different bacteria. This project focuses on the regulatory context of a LuxI-LuxR-type QS system employed by the nitrogen-fixing plant symbiont Rhizobium sp. NGR234, its mechanism of action, and its effect on cellular growth rate. NGR234 incites the formation of nitrogen-fixing, symbiotic nodules on the roots of a wide range of leguminous plants. The molecular basis of this promiscuous host interaction has been extensively studied. Many of the functions that orchestrate the plant interaction are carried on the 536 kb pNGR234a plasmid. The pNGR234a plasmid also carries a large cluster of genes homologous to plasmid replication (rep) and conjugal transfer (trb/tra) genes from other bacteria. The rep/trb/tra cluster includes a LuxI-LuxR-type regulatory pair, TraI and TraR, and the additional QS regulator TraM. TraI synthesizes 3-oxo-octanoyl-L-homoserine lactone and TraR interacts with this acyl HSL to regulate tra/trb and rep operon expression. TraM acts to inhibit TraR through formation of an anti-activation complex. A molecular genetic approach is being employed to study the QS mechanism in NGR234. The regulatory signals and pathways that control traR expression will be investigated to determine the conditions that foster QS. Based on analogous systems, the host plant is likely to play a role in this regulation. The QS-regulated pNGR234a genes under TraR control will be identified and the mechanism by which TraR controls their expression elucidated. Lastly, control of cellular growth rate by QS, a common QS regulatory target among several Rhizobium species, will be examined. Findings generated from the research project will provide fundamental information on cell-to-cell communication in an important plant symbiont, and the role of this communication in its highly plastic host interactions. More generally, these studies will add to the understanding of how cell-to-cell communication directly and indirectly influences interactions with host organisms, and enables quorum-sensing microbes to balance their physiological activity with the host environment.The bacterium Rhizobium sp. NGR234 uses cell-to-cell communication during its symbiotic relationships with higher plants. The research project examines the biochemical and genetic mechanisms underlying this intercellular communication, and its influence on the interaction of this bacterium and host plants. The findings generated from this work will provide fundamental knowledge required to take advantage of these bacterial communication systems, for combating infectious disease in plants and animals, as well as promoting beneficial microbial interactions.