Cloning and characterization of a nodule-enhanced glutamine synthetase-encoding gene from Lupinus luteus

Glutamine synthetase (GS)-encoding genes in Lupinus luteus constitute a small family of genes showing different expression patterns [Boroń et al., Acta Biochim. Polon. 36 (1989) 295-301]. One member of this family, the LlNGS1 gene, is strongly induced in root nodules close to the onset of nitrogen fixation and is referred to as a nodule-enhanced GS gene. We present here the structure of the nodule-enhanced LlNGS1 gene, the first gene of this class which has been sequenced. LlNGS1 is composed of twelve exons and shows structural similarity to the GS gene from Medicago sativa, indicating structure conservation of GS genes in legumes. Comparison of protein coding regions, as well as 5'-untranslated regions derived from LlNGS1 and a Lupinus angustifolius pGS5 GS cDNA clone [Grant et al., Plant Mol. Biol. 13 (1989) 481-490], revealed a high degree of shared identity between both genes, indicating that they are orthologous. The sequence of the LlNGS1 5'-flanking region (2.3 kb) contains several elements implicated in regulation of nodulin genes, as well as other characteristic DNA motifs. RNA blot hybridization analysis carried out using a probe corresponding to the LlNGS1 3'-untranslated region revealed that this gene is also transcribed in leaves, but at a barely detectable level.