title: Primary structure and differential expression of glutamine synthetase genes in nodules, roots and leaves of Phaseolus vulgaris.
abstract: 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.
authors.affiliation: Biochemistry Department, Rothamsted Experimental Station, Harpenden Herts. AL5 2JQ, UK.
authors.forename: C
authors.initials: C
authors.lastname: Gebhardt
authors.forename: J E
authors.initials: JE
authors.lastname: Oliver
authors.forename: B G
authors.initials: BG
authors.lastname: Forde
authors.forename: R
authors.initials: R
authors.lastname: Saarelainen
authors.forename: B J
authors.initials: BJ
authors.lastname: Miflin