Inhibition of electron-transport in chloroplasts by a quinone analogue: evidence for two sites of DPIPH₂ oxidation

A new inhibitor of photoreactions in chloroplasts, 2,3-dimethyl 5-dybroxy 6-phytol benzoquinone is shown to be an electron transfer inhibitor which blocks both cyclic and non-cyclic electron flow. Basal levels of electron transport from reduced dichlorophenol-indophenol to methyl viologen are not affected by the inhibitor, but uncoupler stimulated electron transport in the same system is inhibited. It is concluded that reduced dichlorophenol-indophenol can be oxidized by the photosynthetic electron transport chain in isolated chloroplasts at two sites: site I proximal to P₇₀₀ and site II distal to P₇₀₀. Site I has a low affinity for the electron donor. Electron flow from this site to methyl viologen does not suppert ATP formation and it is resistant to inhibition by the quinone analogue. Electron donation at site II, located on the linear portion of the electron transport chain between the two photosystems, has a higher affinity for reduced dichlorophenol-indophenol and precedes a phosphorylation site. The electron flow from this site is stimulated by uncouplers and inhibited by the quinone analogue.