Mechanism of Osmoregulation in Rhizobium meliloti

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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.