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.