LIGHT-DEPENDENT DEGRADATION OF THE D1 PROTEIN IN PHOTOSYSTEM-II IS ACCELERATED AFTER INHIBITION OF THE WATER SPLITTING REACTION

Strong illumination of oxygen-evolving organisms inhibits the electron transport through photosystem II (photoinhibition). In addition the illumination leads to a rapid turnover of the D1 protein in the reaction center of photosystem II. In this study the light-dependent degradation of the D1 reaction center protein and the light-dependent inhibition of electron-transport reactions have been studied in thylakoid membranes in which the oxygen evolution has been reversibly inhibited by Cl− depletion. The results show that Cl−-depleted thylakoid membranes are very vulnerable to damage induced by illumination. Both the D1 protein and the inhibition of the oxygen evolution are 15–20 times more sensitive to illumination than in control thylakoid membranes. The presence, during the illumination, of the herbicide 3-(3,4-dichlorophenyl)-l, l-dimethylurea (DCMU) prevented both the light-dependent degradation of the D1 protein and the inhibition of the electron transport. The protection exerted by DCMU is seen only in Cl−-depleted thylakoid membranes. These observations lead to the proposal that continuous illumination of Cl−-depleted thylakoid membranes generates anomalously long-lived, highly oxidizing radicals on the oxidizing side of photosystem II, which are responsible for the light-induced protein damage and inhibition. The presence of DCMU during the illumination prevents the formation of these radicals, which explains the protective effects of the herbicide. It is also observed that in Cl−-depleted thylakoid membranes, oxygen evolution (measured after the readdition of Cl−) is inhibited before electron transfer from diphenylcarbazide to dichlorophenolindophenol. The latter activity is dependent on functional electron transfer in photosystem II between the electron donor tyrosine Z and the first quinone acceptor, QA. The kinetics for the inhibition of the electron transfer from diphenylcarbazide to dichlorophenolindophenol were approximately similar to the kinetics for the degradation of the D1 protein. Together these results indicate that the light-dependent degradation of the D1 protein is triggered by the accumulation of P680 + and/or tyrosine Z+, both of which are highly oxidizing. It is proposed that similar reactions also trigger the degradation of the D1 protein in vivo and possible mechanisms for this are discussed. © 1990, American Chemical Society. All rights reserved.