Glutamine synthetase isoforms in leaves of a C4 plant: Sorghum vulgare

In Sorghum vulgare L. (cv. INRA 450) leaves, two isoforms of glutamine synthetase GS₁ and GS₂ were identified by DEAE Sephacel chromatography and polyac-rylamide gel electrophoresis. GS₁ was present both in etiolated and green leaves and its activity remained constant during the greening process. In green leaves another isoform GS₂ was identified that was responsible for a 33% increase in total glutamine synthetase activity after light exposure. The two isoforms differed in their heat stability but exhibited similar pH optima and Km values for L-glutamate. The total glutamine synthetase activity was equally distributed between mesophyll and bundle sheath cells; moreover in the two types of cells both GS₁ and GS₂ were present in the proportions of 67% and 33% of the total respectively. In mesophyll cells the two isoforms had a different subcellular distribution: GS₂ was localized in the chloroplast and GS₁ in the cytosol.