Photosynthesis, Rhodopseudomonas palustris, Respiration Chain, Electron Retardation, ATPase Rhodopseudomonas palustris is grown photosynthetically on thiosulfate. As pointed out earlier, the chromatophore deficient cell-free fraction S-144,000 catalyzes the thiosulfate-linked ATP-dependent reversal of electron transfer in anaerobiosis, thus providing reducing equivalents in the form of NADH. Under aerobic conditions, this fraction also catalyzes the oxidation of NADH, ferro-cytochrome c, or ascorbate. ATP, ADP, and PPi are active in retarding the aerobic electron flow. The electron retardation is stimulated by the addition of Mg2+ due to a Mg2+-stimulated ATPase present. The ATPase system in S-144,000 hydrolyzes ATP, ADP, and P Pi. Similarly, ATP or ADP or even PPi can function as energy sources in order to achieve the reduction of pyridine nucleotide. The ATPase turnover is diminished by NADH or NAD+. Admixture of ascorbate results in an increased ATPase activity. Exactly the enhanced amount of adenine nucleotide hydrolysis caused by the addition of ascorbate is inhibited by cyanide.