Cyclic Beta Glucans of the Rhizobiaceae

9505706米勒革兰氏阴性细菌在其周质体内积累了高水平的低聚糖。这些低聚糖的生物合成受到严格的渗透调节，当细胞在低渗透介质中生长时，周质浓度可能达到100 mM以上。这些分子的积累被证明是在低渗透环境中生长的一种适应性策略，人们认为这种积累为细胞调节周质体积提供了一种机制。在拟议的研究计划中，将以紫花苜蓿根瘤菌为模式菌，直接研究周质低聚糖在低渗适应过程中的作用。提出了三个主要目标：1)鉴定不利于周质低聚糖生物合成的新型苜蓿突变体；2)鉴定苜蓿周质低聚糖突变体(低渗生长、周质体积和细胞表面性质)；3)进一步表征苜蓿周质低聚糖的生物合成(渗透调节和转运)。拟议的研究将对根际生物多样性与生态系统稳定性的互动动态中的微生物周质的性质提供重要的见解，涉及基本的微生物/植物宿主/寄生虫关系。由于生物膜对水是自由渗透的，所有活细胞都必须应对环境中水可获得性的变化。对于细菌细胞来说，这是一个特别的问题，因为它们直接遇到了广泛的渗透强度环境(例如，从非常稀薄的条件到高盐条件)。某些细菌会受到渗透胁迫的进一步挑战，因为它们含有一个被称为周质的外部细胞室。该隔室与许多基本的细胞过程(包括营养获取、蛋白质输出和运动)密切相关，可能占细胞总体积的40%。尽管如此，对周质的了解令人惊讶地有限。最近的研究表明，在稀薄环境中生长的细菌在其周质体内积累了高浓度的结构独特的低聚糖(一类碳水化合物)。此外，已有研究表明，细菌间存在多种生物合成途径，以确保周质低聚糖的合成和积累。虽然周质低聚糖的确切功能仍不清楚，但人们普遍认为，当细胞受到渗透胁迫时，它们的积累为细胞提供了一种调节周质体积的机制。在拟议的研究计划中，一种在农业上具有重要意义的土壤细菌--苜蓿根霉将被用作模式微生物，直接研究渗透胁迫环境中周边低聚糖在生长过程中的作用。这项研究的结果将提供有关周质的结构和功能的重要信息，周质是一个与几个重要的细胞过程有关的动态间隔。***

9505706 Miller Gram-negative bacteria accumulate high levels of oligosaccharides within their periplasmic compartments. The biosynthesis of these oligosaccharides is strictly osmoregulated, and periplasmic concentrations may reach levels greater than 100 mM when cells are grown in hypoosmotic media. The accumulation of these molecules has been shown to represent an adaptive strategy for growth in hypoosmotic environments, and it is believed that this accumulation provides a mechanism for the cell to regulate periplasmic volume. In the proposed research program, Rhizobium meliloti will be used as a model bacterium to directly investigate the roles of the periplasmic oligosaccharides during hypoosmotic adaptation. Three major objectives are proposed: 1) identification of new classes of R. meliloti mutants defective for periplasmic oligosaccharide biosynthesis; 2) characterization of R. meliloti periplasmic oligosaccharide mutants (hypoosmotic growth, periplasmic volume, and cell surface properties); 3) further characterization of periplasmic oligosaccharide biosynthesis by R. meliloti (osmotic regulation and transport). The proposed studies will provide important insight concerning the nature of the microbial periplasm in the interactive dynamics of biological diversity of the rhizosphere and ecosystem stability involving essential microbe/plant host/parasite relationships. %%% Because biological membranes are freely permeable to water, all living cells must cope with changes in the availability of water in their environment. This is particularly a problem for bacterial cells which directly encounter wide ranges of osmotic strength environments (e.g., from very dilute to hypersaline conditions). Certain bacteria are further challenged by osmotic stress because they contain an outer cellular compartment called the periplasm. The compartment is integrally linked to many fundamental cellular processes (including nutrient acquisition, protein export, and motility) and may represent up to 4 0% of the total cellular volume. Nonetheless, understanding of the periplasm is surprisingly limited. Recently, it was shown that a great diversity of bacteria accumulate high concentrations of structurally unique oligosaccharides (a class of carbohydrate) within their periplasmic compartments during growth in dilute environments. Furthermore, it has been shown that a variety of biosynthetic pathways have evolved among bacterial to insure the synthesis and accumulation of periplasmic oligosaccharides. Although the precise functions of the periplasmic oligosaccharides remain undefined, it is generally believed that their accumulation provides a mechanism for the cell to regulate periplasmic volume when cells are stressed osmotically. In the proposed research program, R. meliloti, an agriculturally important soil bacterium, will be used as a model microbe to directly investigate the roles of the periplsmic oligosaccharides during growth in osmotically stressed environments. The results of this study will provide important information concerning the structure and function of the periplasm, a dynamic compartment linked to several important cellular processes. ***