THE GLUTAMINE-SYNTHETASE GENE FAMILY OF ARABIDOPSIS-THALIANA - LIGHT REGULATION AND DIFFERENTIAL EXPRESSION IN LEAVES, ROOTS AND SEEDS

Glutamine synthetase (GS) plays an important role in the assimilation of nitrogen by higher plants. We present here a molecular analysis of the GS polypeptides, mRNAs, and genes of Arabidopsis thaliana. Western blot analysis of leaf and root protein extracts revealed at least two distinct GS polypeptides; 43 kDa and 39 kDa GS polypeptides were present in leaves, while only a 39 kDa GS was detected in roots. The 43 kDa GS polypeptide is light-inducible. In etiolated seedlings only the 39 kDa GS was detected. However, upon greening the 43 kDa GS increased to levels comparable to those observed in light-grown plants. Four distinct GS cDNA clones, lambda-Atgsl1, lambda-Atgsr1, lambda-Atgsr2 and lambda-Atkb6 were isolated and characterized. Their complete nucleotide and deduced amino acid sequences are presented. The coding sequences of the four clones are 70-88% similar while their 5' and 3' untranslated regions exhibit less than 50% similarity. Northern blots of leaf, root and germinated seed RNA revealed that the four cDNAs hybridize to mRNAs which are differentially expressed in the organs of Arabidopsis thaliana. Lambda-Atgsl1 is leaf-specific and hybridizes to a 1.6 kb mRNA. Both lambda-Atgsr1 and lambda-Atgskb6 hybridize to 1.4 kb mRNAs which are expressed in both roots and germinated seeds. Lambda-Atgsr2 hybridizes to a 1.4 kb mRNA, which is primarily expressed in roots with low levels of expression in seeds and leaves. Lambda-Atgsl1, which represents the leaf-specific mRNA, is induced by light. Lambda-Atgsl1 mRNA levels increase during the greening of etiolated seedlings while lambda-Atgsr1 levels remain constant. Southern blot analysis indicated that the Arabidopsis genome contains at least four and possibly five distinct GS genes.