Mechanism of Osmoregulation in Rhizobium meliloti

环境中盐分或水分含量的变化(即渗透胁迫)是所有形式的生命都必须处理的问题。事实上，渗透应激会导致细胞脱水和死亡，如果没有适应的过程可用。该项目研究了在微生物、植物和动物(包括人类)中发现的一种常见的适应渗透胁迫的机制。在这一机制中，细胞中积累了小的有机化合物，以平衡环境中盐浓度的增加(或水含量的减少)。具体地说，人们正在研究紫花苜蓿根瘤菌的渗透适应机制，这种细菌附着在紫花苜蓿的根上，为植物提供固定的氮素(从而消除了对氮肥的需求)。利用核磁共振技术对盐胁迫下苜蓿积累的有机化合物(N-乙酰谷氨酰谷氨酰胺)进行了鉴定。这种化合物以前没有被观察到，它代表了一类新的渗透调节化合物。该建议的具体目的如下：1)利用细胞提取物和活培养物的核磁共振波谱确定N-乙酰谷氨酰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制；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上积累的化合物(S)。本研究的重点是对紫花苜蓿渗透胁迫适应机制的识别，这将有助于发展耐盐/干旱的紫花苜蓿，并可能有助于其他含有根瘤菌的豆科植物的发展。然而，由于在大量物种中发现的一种共同的适应机制正在被研究，这种机制的特征在许多重要的研究领域可能是关键的。最突出的例子是任何耐盐/耐旱作物的发展。

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.