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 [Boron e t al., Acta Biochim. Polon. 36 (1989) 295-301]. One member of this fam ily, 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 gen e, the first gene of this class which has been sequenced. LlNGS1 is co mposed 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'-untransl ated regions derived from LlNGS1 and a Lupinus angustifolius pGS5 GS c DNA clone [Grant et al., Plant Mel. 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 g enes, as well as other characteristic DNA motifs. RNA blot hybridizati on analysis carried out using a probe corresponding to the LlNGS1 3'-u ntranslated region revealed that this gene is also transcribed in leav es, but at a barely detectable level.