CAREER: Role of Exopolysaccharides in Rhizobium meliloti/ alfalfa Symbiosis

小行星9733532 苜蓿根瘤菌与其寄主植物苜蓿（Medicago sativa）之间的固氮共生关系需要细菌胞外多糖的参与，但胞外多糖在这种相互作用中的确切作用尚不清楚。苜蓿根瘤菌菌株1021的酸性Calcofluoro结合胞外多糖琥珀聚糖是有效发酵所必需的。R.不能合成琥珀聚糖的苜蓿属突变体形成没有细菌和类细菌并且不能固氮的空根瘤。RM 1021还具有产生第二胞外多糖（EPS II）的隐蔽能力，所述胞外多糖抑制苜蓿根瘤入侵上的琥珀聚糖缺陷菌株的共生缺陷。从这些研究中出现的一个共同主题是，少量的低分子量胞外多糖足以介导R.苜蓿属突变体不能产生胞外多糖，这意味着低分子量胞外多糖可能在此过程中充当信号分子。由15-18个EPS II二糖亚基组成的低分子量EPS II级分有效地允许非感染菌株以低至7皮摩尔/株的量存在时的根瘤侵入，这表明低分子量EPS II在感染期间可能充当共生信号。 本项目将对产胞外多糖的红球藻进行遗传和生化分析。苜蓿属植物，并确定参与生产生物活性低分子量形式的EPS II的基因。本项目还将研究琥珀多糖和EPS II在共生中的作用。 这些结果将为鉴定具有生物活性的根瘤菌胞外多糖的假定植物受体及其在共生中的作用奠定基础。 多糖越来越被认为是植物和哺乳动物病原体毒力的重要决定因素。 琥珀聚糖是最具特征的细菌多糖之一，并且已经推进了对其他细菌（包括人类病原体）中多糖的合成和生物学作用的理解。 多糖生物合成的基础研究可能最终有助于复杂碳水化合物合成中酶促步骤的设计，以控制连接特异性和立体化学。 这些研究可能会导致深入了解可能更广泛的使用寡糖作为信号在生物系统中比一般公认的。 这些研究有望导致更好地了解植物-细菌共生，这是必要的生物固氮。

9733532 Gonzalez Bacterial exopolysaccharide is required for establishment of the nitrogen fixing symbiosis between Rhizobium meliloti and its host plant Medicago sativa (alfalfa), but the precise role of exopolysaccharides in this interaction is not well defined. Succinoglycan, the acidic Calcofluor-binding exopolysaccharide of Rhizobium meliloti strain 1021 is required for effective nodulation. R. meliloti mutants that fail to synthesize succinoglycan form empty nodules that are devoid of bacteria and bacteroids and unable to fix nitrogen. Rm l021 has also the cryptic ability to produce a second exopolysaccharide (EPS II) that suppresses the symbiotic defects succinoglycan-deficient strains on alfalfa nodule invasion. A common theme emerging from these studies is that small quantities of low molecular weight exopolysaccharides are sufficient to mediate successful invasion by R. meliloti mutants which fail to produce exopolysaccharides, implying that low molecular weight exopolysaccharide may act as a signaling molecule during this process. A low molecular weight EPS II fraction consisting of 15-18 EPS II disaccharide subunits efficiently allowed nodule invasion by non-infective strains when present in amounts as low as 7 picomoles per plant, suggesting that low molecular weight EPS II may act as a symbiotic signal during infection. This project will perform genetic and biochemical analyses of exopolysaccharide production by R. meliloti and identify the genes involved in the production of the biologically-active low molecular weight form of EPS II. This project will also investigate the roles of succinoglycan and EPS II in symbiosis. The results will set the stage for the identification of the putative plant receptors for the biologically-active Rhizobium exopolysaccharides and their role in symbiosis. Polysaccharides are increasingly being recognized as important determinants of virulence in both plant and mammalian pathogens. Succingoglycan is one of the best-characterized bacterial polysaccharides and has already advanced understanding of the synthesis and biological roles of polysaccharides in other bacteria, including human pathogens. Basic studies of polysaccharide biosynthesis might eventually contribute to the design of enzymatic steps in the synthesis of complex carbohydrates to control linkage specificity and stereochemistry. These studies may lead to insights into the possible broader use of oligosaccharides as signals in biological systems than has generally been recognized. These studies promise to lead to a better understanding of plant-bacterial symbiosis which is necessary for biological nitrogen fixation.