Two Sites for Adenosine Triphosphate Formation in Photosynthetic Electron Transport Mediated by Photosystem I. Evidence from Digitonin Subchloroplast Particles

TWO types of photophosphorylation, one supported by phenazine methosulfate and one by electron transport from reduced dichlorophenolindophenol (DPIPH₂) to methylviologen, have been studied in photosystem I digitonin subchloroplast particles. Phenazine methosulfate supported ATP formation is inhibited by a wide range of uncouplers, but it is resistant to the action of amines or nigericin. ATP formation supported by electron transport from DPIPH₂ to methylviologen is also inhibited by several uncouplers, including m-chlorocyanocarbonyl phenylhydrazone, citrate, and atebrin, but is stimulated by amines or nigericin at saturating light intensities. Electron transport from DPIPH2 to methylviologen is stimulated by all the uncouplers tested including amines and nigericin. These results are explained on the basis of an hypothesis that electron transport from DPI-PH2 to methylviologen supports ATP formation at two sites (A and B) on the linear pathway between the two photosystems, each with different properties. Only one of these sites (site A) participates in ATP formation catalyzed by phenazine methosulfate. In digitonin system I subchloroplast particles site B is inhibited by amines whereas site A is not. The net stimulation of ATP formation in the DPIPH₂ system by amines or nigericin is a result of an increased rate of electron transport through site A (which is not effected by amines or nigericin) due to a release by these compounds of a limiting step in electron flow at coupling site B. The use of a new inhibitor of photoreactions 2,3-dimethyl-5-hydroxy-6-phytolbenzoquinone provided evidence for two sites of oxidation of reduced dichlorophenolindophenol by the photosynthetic electron-transport chain, one preceding the ATP-forming sites and the other bypassing phosphorylation sites.