Synthesis and possible character of a high-energy intermediate in bacterial photophosphorylation

Abstract

In photophosphorylation with chromatophores from Rhodospirilium rubrum, evidence is presented for the synthesis of activated precursors of adenosine triphosphate (ATP) in the energy-conversion system coupled to photosynthetic electron transport. A significant amount of ATP is synthesized when a reaction mixture containing chromatophores and adenosine diphosphate (ADP) is illuminated and then incubated with Pi in the dark; ATP is not synthesized to an appreciable extent, either when a reaction mixture containing chromatophores and Pi is illuminated and then incubated with ADP in the dark, or when one containing chromatophores alone is illuminated and then incubated with ADP and Pi in the dark. The amount of ATP thus synthesized is influenced markedly by concentrations of ADP. The chromatophores illuminated with ADP, if allowed to stand in the dark at 30[degree], gradually lose the ability to form ATP with Pi in the dark. No loss of the ability occurs when the chromatophores illuminated with ADP are allowed to stand in the dark at 13[degree] or in a frozen state. Mg2+ 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 Pi in the dark. Antimycin-A, 2-heptyl-4-hydroxyquinoline N-oxide and o-phenanthroline strongly inhibit the light-dependent acquisition of the ability to form ATP with Pi in the dark, but not the consequent ATP formation with Pi in the dark. Arsenate, 2,4-dinitrophenol, quinacrlne hydrochloride, quinine hydrochloride and pyrophosphate inhibit the former or the latter, or both. Oligomycin inhibits the former somewhat more than the latter. 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 Pi.