ELECTRON-TRANSPORT THROUGH PHOTOSYSTEM-I STIMULATES LIGHT ACTIVATION OF RIBULOSE BISPHOSPHATE CARBOXYLASE OXYGENASE (RUBISCO) BY RUBISCO ACTIVASE

The activation state of ribulose bisphosphate carboxylase/ oxygenase (rubisco) in a lysed chloroplast system is increased by light in the presence of a saturating concentration of ATP and a physiological concentration of CO2 (10 micromolar). Electron transport inhibitors and artificial electron donors and acceptors were used to determine in which region of the photosynthetic electron transport chain this light-dependent reaction occurred. In the presence of DCMU and methyl viologen, the artificial donors durohydroquinone and 2,6-dichlorophenolindophenol (DCPIP) plus ascorbate both supported light activation of rubisco at saturating ATP concentrations. No light activation occurred when DCPIP was used as an acceptor with water as electron donor in the presence of ATP and dibromothymoquinone, even though photosynthetic electron transport was observed. Nigericin completely inhibited the light-dependent activation of rubisco. Based on these results, we conclude that stimulation of light activation of rubisco by rubisco activase requires electron transport through PSI but not PSII, and that this light requirement is not to supply the ATP needed by the rubisco activase reaction. Furthermore, a pH gradient across the thylakoid membrane appears necessary for maximum light activation of rubisco even when ATP is provided exogenously.