The role of photosynthetic electron transport in the oxidative degradationof chloroplastic glutamine synthetase

The stability of chloroplastic glutamine synthetase (CS; EC 6.3.1.2) was in vestigated under photooxidative stress using wheat (Triticum aestivum L.) l eaves, chloroplasts, and chloroplast lysates, Illuminated seedlings sprayed with the superoxide radical (O-2(radical anion)) propagator methyl viologe n showed rapid GS decline dependent on MV concentration and exposure time. Degradation products of approximately 39 and 31 kD were detected when chlor oplast lysates containing both stroma and thylakoids were illuminated in th e presence of MV or H2O2. In all cases, GS cleavage was prevented by the ad dition of the electron transport inhibitor 3-(3,4-dichlorophenyl)-1,1 -dime thylurea. Full protection against degradation could also be obtained by the incorporation of chelators or antioxidant enzymes. Maximal rates of degrad ation required the presence of transition metals and reducing compounds suc h as NADPH or dithiothreitol. Similar patterns of CS cleavage were obtained when seedlings were exposed to high doses of irradiation. The results indi cate that chloroplastic CS is extremely prone to oxidative cleavage, and th at reduced transition metals, presumably resulting from the destruction of iron-sulfur clusters by light-generated O-2(radical anion), play a crucial role in the degradation process. The physiological implications of CS labil ity to oxidative stress are discussed.