In photophosphorylation with chromatophores from Rhodospirillum rubrum, evidence is presented for the synthesis of activated precursors of ATP in the energy-conversion system coupled to photosynthetic electron transport. 2. A significant amount of ATP is synthesized when a reaction mixture containing chromatophores and ADP is illuminated and then incubated with P(i) in the dark. ATP is not synthesized to an appreciable extent, either when a reaction mixture containing chromatophores and P(i) is illuminated and then incubated with ADP in the dark, or when one containing chromatophores alone is illuminated and then incubated with ADP and P(i) in the dark. The amount of ATP thus synthesized is influenced markedly by concentrations of ADP. 3. The chromatophores illuminated with ADP, if allowed to stand in the dark at 30 degrees , gradually lose the ability to form ATP with P(i) in the dark. No loss of the ability occurs when the chromatophores illuminated with ADP are allowed to stand in the dark at 13 degrees or in a frozen state. 4. Mg(2+) is absolutely required for chromatophores to form ATP in the dark after illumination in the presence of ADP, and for the chromatophores to achieve ATP formation with P(i) in the dark. 5. Antimycin A, 2-heptyl-4-hydroxyquinoline N-oxide and o-phenanthroline strongly inhibit the light-dependent acquisition of the ability to form ATP with P(i) in the dark, but not the consequent ATP formation with P(i) in the dark. Arsenate, 2,4-dinitrophenol, quinacrine hydrochloride, quinine hydrochloride and pyrophosphate inhibit the former or the latter, or both. Oligomycin inhibits the former somewhat more than the latter. 6. From these findings it is suggested that a high-energy intermediate is formed in photosynthetic ATP formation, and that its formation is dependent on ADP but not P(i).
