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pubmed:abstractText |
In plants, glutamine synthetase (GS) is the enzyme primarily responsible for the assimilation of ammonia into organic nitrogen. In Phaseolus vulgaris a number of isoenzymic forms of GS are found, each of which consists of eight subunits of mol. wt 41 000-45 000. The GS subunits of P. vulgaris have previously been shown to be encoded by a small multigene family and a partial cDNA clone for a nodule-specific GS subunit has been obtained. We report here the isolation and nucleotide sequencing of two essentially full-length GS cDNA clones (pR-1 and pR-2) from a root cDNA library and the deduced amino acid sequences of the corresponding GS subunits (355 amino acid residues each). The coding sequences of pR-1 and pR-2 are closely related (80% nucleotide homology, 88% amino acid homology), but their 5'- and 3'-untranslated regions have diverged almost completely. Both pR-1 and pR-2 are related to, but distinct from, the nodule GS clone, pcPvNGS-01 (or pN-1). Hybridization to genomic Southern blots showed that the three GS mRNAs are encoded by three seperate genes and indicated the existence of a fourth class of GS gene. An S1 nuclease protection assay demonstrated the presence of R-1 and R-2 mRNA in both roots and leaves and confirmed that expression of the N-1 gene is nodule-specific. Expression of the R-1 and R-2 genes in the roots did not change significantly during nodulation. However, only the R-1 gene is expressed in the nodules themselves, indicating that the R-2 gene is specifically repressed during nodule development.
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