McpT, a Broad-Range Carboxylate Chemoreceptor in Sinorhizobium meliloti
McpT, a Broad-Range Carboxylate Chemoreceptor in Sinorhizobium meliloti
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DOI：
10.1128/jb.00216-21
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发表时间：
2021-09-01
影响因子：
3.2
通讯作者：
Scharf, Birgit E.
中科院分区：
文献类型：
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作者：
Baaziz, Hiba;Compton, K. Karl;Scharf, Birgit E.
化学感受器使豆类共生菌Sinorhizobium Meliloti能够检测并响应从寄主植物紫花苜蓿释放的特定化学物质，这使得建立固氮共生关系成为可能。跨膜化学受体的周质区域(PR)通过直接或间接地与特定的配体结合，作为趋化系统的感觉输入模块。紫花苜蓿有6个跨膜化学受体和2个细胞质化学感受器。然而，到目前为止，只有三种跨膜受体的功能得到了表征，McpU、McpV和McpX分别作为一般氨基酸、短链羧酸盐和季铵化合物传感器。在本研究中，我们分析了紫花苜蓿化学受体McpT。高通量差示扫描荧光分析，使用Biolog表型微阵列平板，确定了McpT(PR)的15个潜在配体，其中大多数被归类为单羧酸盐、二羧酸盐和三羧酸盐。紫花苜蓿对7种羧酸类化合物表现出正趋化作用，即α-酮丁酸、柠檬酸、乙醛、苹果酸、丙二酸、草酸和琥珀酸。在紫花苜蓿寄主的种子分泌物中检测到这些羧基化合物。McpT的缺失导致对除柠檬酸以外的所有羧酸盐的趋化性显著降低。等温滴定量热法表明，McpT(PR)优先与单羧酸盐乙醛酸盐结合，与苹果酸、丙二酸盐和草酸盐的亲和力较低。然而，没有检测到引起McpT依赖的趋化反应的其余三种羧酸盐的直接结合。综上所述，这些结果表明McpT是一种广泛的羧酸盐化学受体，通过直接配体结合和间接机制介导趋化反应，这一机制还需要确定。重要的硝酸盐污染是最普遍和最具挑战性的环境问题之一，主要由农业过量使用氮肥引起。内共生菌Sinorhizobium Meliloti的生物固氮促进了其宿主紫花苜蓿的生长，并有效地为土壤提供了氮素。紫花苜蓿-紫花苜蓿共生关系的建立有赖于化学信号的早期交换和识别。本研究通过研究紫花苜蓿中羧酸感觉的潜在机制，有助于揭示这种复杂的分子对话。了解控制紫花苜蓿分子串扰的各个步骤有助于开发高效的商业细菌接种剂，促进紫花苜蓿的生长，并提高土壤肥力。紫花苜蓿是世界上种植最多的豆科牧草。
Chemoreceptors enable the legume symbiont Sinorhizobium meliloti to detect and respond to specific chemicals released from their host plant alfalfa, which allows the establishment of a nitrogen-fixing symbiosis. The periplasmic region (PR) of transmembrane chemoreceptors act as the sensory input module for chemotaxis systems via binding of specific ligands, either directly or indirectly. S. meliloti has six transmembrane and two cytosolic chemoreceptors. However, the function of only three of the transmembrane receptors have been characterized so far, with McpU, McpV, and McpX serving as general amino acid, short-chain carboxylate, and quaternary ammonium compound sensors, respectively. In the present study, we analyzed the S. meliloti chemoreceptor McpT. High-throughput differential scanning fluorimetry assays, using Biolog phenotype microarray plates, identified 15 potential ligands for McpT(PR), with the majority classified as mono-, di-, and tricarboxylates. S. meliloti exhibited positive chemotaxis toward seven selected carboxylates, namely, alpha- ketobutyrate, citrate, glyoxylate, malate, malonate, oxalate, and succinate. These carboxylates were detected in seed exudates of the alfalfa host. Deletion of mcpT resulted in a significant decrease of chemotaxis to all carboxylates except for citrate. Isothermal titration calorimetry revealed that McpT(PR) bound preferentially to the monocarboxylate glyoxylate and with lower affinity to the dicarboxylates malate, malonate, and oxalate. However, no direct binding was detected for the remaining three carboxylates that elicited an McpT-dependent chemotaxis response. Taken together, these results demonstrate that McpT is a broad-range carboxylate chemoreceptor that mediates chemotactic response via direct ligand binding and an indirect mechanism that needs to be identified.IMPORTANCE Nitrate pollution is one of the most widespread and challenging environmental problems that is mainly caused by the agricultural overapplication of nitrogen fertilizers. Biological nitrogen fixation by the endosymbiont Sinorhizobium meliloti enhances the growth of its host Medicago sativa (alfalfa), which also efficiently supplies the soil with nitrogen. Establishment of the S. meliloti-alfalfa symbiosis relies on the early exchange and recognition of chemical signals. The present study contributes to the disclosure of this complex molecular dialogue by investigating the underlying mechanisms of carboxylate sensing in S. meliloti. Understanding individual steps that govern the S. meliloti-alfalfa molecular cross talk helps in the development of efficient, commercial bacterial inoculants that promote the growth of alfalfa, which is the most cultivated forage legume in the world, and improves soil fertility.