Mechanism of Osmoregulation in Rhizobium meliloti

环境中盐或水含量的变化（即渗透压力）是所有生命形式都必须应对的问题。事实上，如果没有适应过程，渗透压力会导致细胞脱水和死亡。本项目研究了在微生物、植物和动物（包括人类）中发现的一种适应渗透胁迫的共同机制。在这种机制中，小的有机化合物在细胞中积累，以平衡环境中增加的盐浓度（或减少的含水量）。具体来说，正在研究的渗透适应机制的根瘤菌meliloti，细菌附着在苜蓿的根和提供固定氮的植物（从而避免需要氮肥）。利用核磁共振波谱技术鉴定了盐胁迫下褐藻积累的有机化合物（N-乙酰谷氨酰胺酰谷氨酰胺）。该化合物以前未被观察到，代表了一类新的渗透调节化合物。具体目的如下：1)利用细胞提取液和活培养物的核磁共振波谱测定n -乙酰-谷氨酰胺-谷氨酰胺在渗透胁迫适应中的生理作用；2)利用物理技术鉴定该化合物的分子作用机理；3)确定该化合物的生物合成途径；4)测定胁迫条件下紫花苜蓿根部附着后积累的化合物。本研究的重点是确定黑草的渗透胁迫适应机制，这将有助于开发耐盐/干旱的紫花苜蓿和其他可能携带根瘤菌的豆科植物。然而，由于在许多物种中发现的一种共同的适应机制正在被研究，这种机制的特征可能在许多重要的研究领域至关重要。最突出的例子是开发任何耐盐/耐旱作物。

Change in the salt or water content of the environment (i.e. osmotic stress) is a problem with which all forms of life must deal. Indeed, osmotic stress causes cells to dehydrate and die if no adaptive process are available. This project examines one common mechanism of adaptation to osmotic stress found in microbes, plants, and animals, including man. In this mechanism, small organic compounds are accumulated in the cell to balance the increased salt concentration (or decreased water content) in the environment. Specifically, the mechanism of osmotic adaptation in Rhizobium meliloti, the bacterium which attaches to the roots of alfalfa and supplies fixed nitrogen to the plant (thus obviating the need for nitrogenous fertilizers) is being studied. Using nuclear magnetic resonance spectroscopy the organic compound (N- acetylglutaminylglutamine amide) accumulated by salt-stressed R. meliloti has been identified. This compound has not been previously observed and represents a new class of an osmotically regulated compound. The specific aims of the proposal are as follows: 1) Determination of the physiological role of N-acetylglutaminylglutamine amide in osmotic stress adaptation using nuclear magnetic resonance spectroscopy on cellular extracts and living cultures; 2) Identification of the molecular mechanism of action of this compound using physical techniques; 3) Determination of the pathway of biosynthesis of this compound; and 4) Determination of the compound(s) accumulated in stressed R. meliloti when attached to alfalfa roots. The identification of the mechanism of osmotic stress adaptation in R. meliloti, which is the focus of this proposal, should contribute to the development of salt/drought hardy alfalfa and possibly other legumes that harbor Rhizobia. However, since a common adaptive mechanism that is found in a great variety of species is being studied, features of this mechanism may be critical in a number of important fields of study. The most prominent example is the development of any salt/drought tolerant crop.