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.