Sinorhizobium meliloti Chemoreceptor McpV Senses Short-Chain Carboxylates via Direct Binding
Sinorhizobium meliloti Chemoreceptor McpV Senses Short-Chain Carboxylates via Direct Binding
复制标题
DOI：
10.1128/jb.00519-18
复制
发表时间：
2018-12-01
影响因子：
3.2
通讯作者：
Scharf, Birgit E.
中科院分区：
文献类型：
--
作者：
Compton, K. Karl;Hildreth, Sherry B.;Scharf, Birgit E.
苜蓿中华根瘤菌是一种生长在土壤中的苜蓿内共生菌，它有八个化学感受器，可以在自由生活状态下感知环境刺激。两种受体的功能已得到表征，其中McpU和McpX分别作为通用氨基酸和季铵化合物传感器。两种受体均使用双 Cache（钙通道和趋化性受体）结构域进行配体结合。我们发现 McpV 的配体结合周质区 (PR) 包含单个缓存域。同源模型显示，McpV（PR）在结构上与脱卤厌氧杆菌的功能未知的化学感受器的传感器结构域相似，该化学感受器在其结合口袋中与乙酸盐结晶。因此，我们分析了 McpV 的羧酸盐结合和苜蓿中华根瘤菌的羧酸盐传感。差示扫描荧光法鉴定出 McpV(PR) 的 10 种潜在配体。其中九种是链长在二到四个碳之间的单羧酸盐。我们选择了七种化合物进行毛细管测定分析，建立了苜蓿中华根瘤菌野生型的正趋化性，乙酸盐、丙酸盐、丙酮酸盐和乙醇酸盐的峰值吸引浓度为1 mM，甲酸酸盐和乙酰乙酸盐的峰值浓度为100 mM。 mcpV 的缺失或配体协调所必需的残基的突变消除了对羧酸盐的正趋化性。使用微量热法，我们确定七个配体与 McpV(PR) 的解离常数在微摩尔范围内。配体配位位点发生突变的 McpV(PR) 变体未表现出与异丁酸或丙酸的结合。在作为引诱剂测试的所有羧酸盐中，在苜蓿种子渗出物中仅检测到乙醇酸盐。这项工作研究了羧酸盐及其传感器与根瘤菌-豆科植物相互作用的相关性。重要性豆科植物与某些土壤中的细菌（广泛称为根瘤菌）具有独特的关联。通过协调一致的生物体间通讯，豆科植物允许其同源根瘤菌物种进行细胞内感染。然后植物形成一个器官，即根瘤，专门为细菌提供栖息地和供应固定碳和营养物质。作为回报，被吞噬的根瘤菌分化成类菌，将大气中的 N-2 固定为植物宿主所需的铵。这种相互作用对于苜蓿和大豆等豆科植物的种植非常有利，并且是由对宿主植物的趋化性引发的。这项关于羧酸趋化性的研究有助于了解根瘤菌在根际的生存和竞争，并有助于商业接种剂的开发。
Sinorhizobium meliloti is a soil-dwelling endosymbiont of alfalfa that has eight chemoreceptors to sense environmental stimuli during its free-living state. The functions of two receptors have been characterized, with McpU and McpX serving as general amino acid and quaternary ammonium compound sensors, respectively. Both receptors use a dual Cache (calcium channels and chemotaxis receptors) domain for ligand binding. We identified that the ligand-binding periplasmic region (PR) of McpV contains a single Cache domain. Homology modeling revealed that McpV(PR) is structurally similar to a sensor domain of a chemoreceptor with unknown function from Anaeromyxobacter dehalogenans, which crystallized with acetate in its binding pocket. We therefore assayed McpV for carboxylate binding and S. meliloti for carboxylate sensing. Differential scanning fluorimetry identified 10 potential ligands for McpV(PR). Nine of these are monocarboxylates with chain lengths between two and four carbons. We selected seven compounds for capillary assay analysis, which established positive chemotaxis of the S. meliloti wild type, with concentrations of peak attraction at 1 mM for acetate, propionate, pyruvate, and glycolate, and at 100 mM for formate and acetoacetate. Deletion of mcpV or mutation of residues essential for ligand coordination abolished positive chemotaxis to carboxylates. Using microcalorimetry, we determined that dissociation constants of the seven ligands with McpV(PR) were in the micromolar range. An McpV(PR) variant with a mutation in the ligand coordination site displayed no binding to isobutyrate or propionate. Of all the carboxylates tested as attractants, only glycolate was detected in alfalfa seed exudates. This work examines the relevance of carboxylates and their sensor to the rhizobium-legume interaction.IMPORTANCE Legumes share a unique association with certain soil-dwelling bacteria known broadly as rhizobia. Through concerted interorganismal communication, a legume allows intracellular infection by its cognate rhizobial species. The plant then forms an organ, the root nodule, dedicated to housing and supplying fixed carbon and nutrients to the bacteria. In return, the engulfed rhizobia, differentiated into bacteroids, fix atmospheric N-2 into ammonium for the plant host. This interplay is of great benefit to the cultivation of legumes, such as alfalfa and soybeans, and is initiated by chemotaxis to the host plant. This study on carboxylate chemotaxis contributes to the understanding of rhizobial survival and competition in the rhizosphere and aids the development of commercial inoculants.