Characterization of a cDNA clone for barley leaf glutamine synthetase

Abstract

A barley cDNA clone (1182 bp) encoding chloroplastic glutamine synthetase was isolated with a heterologous cDNA probe of the gene specifying the enzyme from alfalfa. The clone, named pGS8, was found in a lambda gtII cDNA library prepared from dark grown barley leaves even though the chloroplastic glutamine synthetase is absent from such leaves. In agreement therewith the clone hybridized in Northern blot analyses with a 1.7 kb mRNA species present the in poly A+ mRNA fraction of both dark grown and greened primary leaves of barley. The nucleotide sequence of the barley clone reveals 75% identity to the Phaseolus vulgaris and Pisum sativum clones encoding chloroplastic glutamine synthetase, while only 69% identity is observed in comparisons with the clones specifying the cytosolic isozymes. At the amino acid level 85% identity is found between the deduced barley glutamine synthetase sequence and that of the corresponding chloroplastic isoenzymes from bean and pea. The chloroplastic glutamine synthetases contain cysteins in the putative ATP and and substrate binding sites. In the cytosolic forms these positions are occupied by alanine residues.