A cDNA sequence encoding glutamine synthetase is preferentially expressed in nodules of Vigna aconitifolia

GS (EC 6.3.1.2) occupies a key position in controlling nitrogen metabolism, a pathway directly affecting plant productivity. It catalyzes the first step in the assimilation of ammonia in higher plants, converting inorganic ammonia produced by different metabolic routes into organic nitrogen. GS is encoded by a small gene family in higher plants, the members of which are expressed in different tissues, cell types, and subcellular compartments under the control of both metabolic status and developmental programs of the plant (Hirel et al., 1993). This temporal and spatial regulation of GS genes meets the requirements of ammonia assimilation under various physiological conditions. In legume-Rhizobium symbiosis, the symbiotically reduced ammonia is assimilated by host root nodule-specific and/or root/nodule-specific GS. Both nodule-specific (alfalfa, Dunn et al., 1988; and soybean, Roche et al., 1993) and nodule-enhanced (French bean, Cullimore et al., 1984) GS isoforms have been reported. In pea no nodule-specific GS has been found (Tingey et al., 1987). In addition to the nodule-specific GS, soybean also contains a root/nodule GS, the expression of which is strongly enhanced in nodules. The latter occurs in direct response to the availability of ammonia (Hirel et al., 1987; Mia0 et al., 1991). We report here the isolation of a full-length cDNA sequence encoding mothbean (Vigna aconitifolia) GS, expression of which appears to be nodule specific. Mothbean is a diploid tropical legume that can be easily transformed by Agrobacterium-mediated transformation methods (Lee et al., 1993) and regenerated (our unpublished results). Thus, mothbean is a plant of choice toward understanding the regulation and role of specific GS isoforms in plant nitrogen metabolism. Using a soybean cytosolic GS sequence (GS20; Mia0 et al., 1991) as a probe, we isolated severa1 GS cDNA sequences from a mothbean nodule cDNA library (Table I). One of these sequence, VGSn-1, contains an open reading frame of 1068 nucleotides and encodes a GS polypeptide of 356 amino acids. Sequence homology comparison showed that VGSn-1 shares more than 85% similarity at the amino acid level