Plant Physiol, January 2002, Vol. 128, pp. 182-193

Cytosolic Glutamine Synthetase in Soybean Is Encoded by a Multigene Family, and the Members Are Regulated in an Organ-Specific and Developmental Manner¹

Department of Agronomy and Horticulture (C.S.-G., J.L.O.) and Graduate Program in Molecular Biology (K.J.M.), New Mexico State University, Las Cruces, New Mexico 88003

Gln synthetase (GS) is the key enzyme in N metabolism and it catalyzes the synthesis of Gln from glutamic acid, ATP, and NH₄⁺. There are two major isoforms of GS in plants, a cytosolic form (GS₁) and a chloroplastic form (GS₂). In leaves, GS₂ functions to assimilate ammonia produced by nitrate reduction and photorespiration, and GS₁ is the major isoform assimilating NH₃ produced by all other metabolic processes, including symbiotic N₂ fixation in the nodules. GS₁ is encoded by a small multigene family in soybean (Glycine max), and cDNA clones for the different members have been isolated. Based on sequence divergence in the 3'-untranslated region, three distinct classes of GS₁ genes have been identified (, , and ). Genomic Southern analysis and analysis of hybrid-select translation products suggest that each class has two distinct members. The  forms are the major isoforms in the cotyledons and young roots. The  forms, although constitutive in their expression pattern, are ammonia inducible and show high expression in N₂-fixing nodules. The 1 gene appears to be more nodule specific, whereas the 2 gene member, although nodule enhanced, is also expressed in the cotyledons and flowers. The two members of the  and  class of GS₁ genes show subtle differences in the expression pattern. Analysis of the promoter regions of the 1 and 2 genes show sequence conservation around the TATA box but complete divergence in the rest of the promoter region. We postulate that each member of the three GS₁ gene classes may be derived from the two ancestral genomes from which the allotetraploid soybean was derived.