ELECTRON PARAMAGNETIC RESONANCE SIGNAL II IN SPINACH CHLOROPLASTS—II. INFLUENCE OF PHOSPHORYLATION AND ELECTRON-TRANSPORT INHIBITORS

Abstract —The influence of several groups of inhibitors on the electron paramagnetic resonance Signal II of spinach chloroplasts was investigated. It was found that phosphorylation uncouplers that do not inhibit electron transport (gramicidin, valinomycin) have no effect. Likewise, inhibitors that block electron transport between the two photosystems are ineffective. However, reagents that can inhibit electron transport and also uncouple phosphorylation, such as phenylhydrazones (FCCP) or anilino-thiophenes (ANT 2p), will completely abolish Signal II in the dark. The signal is regenerated by red light and also by far-red light, but in the latter case only if cyclic electron transport is possible. These agents, in addition to discharging the water-splitting enzyme [ADRY effect, G. Renger, Biochim. Biophys. Acta (1972) 256, 428] were also found to inhibit cyclic electron flow. Light activation of Signal II in ANT 2p- or FCCP-treated chloroplasts by either red or far-red light can be suppressed by simazine or dichlorophenyl dimethylurea (DCMU) but not by other inhibitors that also inhibit linear electron flow. Evidence is provided which indicates that one functioning electron-transport system, either linear or cyclic, is necessary and sufficient for the enhancement of Signal II. The requirement for Signal II activation seems to be the reduction of plastoquinone. In ANT 2p- and FCCP-treated chloroplasts, a link probably exists between the water-splitting enzyme and the plastoquinone pool, bypassing Photosystem II; the unknown substance giving rise to Signal II may be located on such a sidepath.