Evidence for a cytosolic-dependent light induction of chloroplastic glutamine synthetase during greening of etiolated rice leaves

During the greening of etiolated rice leaves, total glutamine synthetase activity increases about twofold, and after 48 h the level of activity usually observed in green leaves is obtained. A density-labeling experiment with deuterium demonstrates that the increase in enzyme activity is due to a synthesis of the enzyme. The enhanced activity obtained upon greening is the result of two different phenomena: there is a fivefold increase of chloroplastic glutamine synthetase content accompanied by a concommitant decrease (twofold) of the cytosolic glutamine synthetase. The increase of chloroplastic glutamine synthetase (GS2) is only inhibited by cycloheximide and not by lincomycin. This result indicates a cytosolic synthesis of GS2. The synthesis of GS2 was confirmed by a quantification of the protein by an immunochemical method. It was demonstrated that GS2 protein content in green leaves is fivefold higher than in etiolated leaves.