A cytochrome containing fraction virtually devoid of the photosynthetic apparatus (bacteriochlorophyll and/or chromatophores) was isolated from Rps. palustris grown photolithotrophically with $203= as the exogenous electron donor. This fraction contained predominantly cytochromes of c, a and o type and exhibited thiosulfate: cytochrome c oxidoreductase and ferrocytoehrome c:02 oxidoreductase activities. Under anaerobic conditions the enzyme preparation catalyzed an ATP-dependent NAD+ reduction by $203 = in the dark involving a reversal of electron transfer from cytochrome c and yielding a. molar stoichiometry of approximately 2:1 for the ferrocytoehrome c oxidized and NAD+ reduced. In this process approximateIy 4 to 7 molar equivalents of ATP were utilized/equivalent of NAD+ reduced. The optimal reaction occurred at pH 8.0 and in the presence of 55 ~1-~ added mammalian cyt. c, 1.7 mM Mg ++, 1.7 mM ATP and 7.0 mM $20~ ~. The S20a=-linked ATP-driven reduction of NAD+ as well as the coupled oxidation of cyt. c were inhibited completely by 5 9m CCCP or 10 ~M DNP and the reaction was also markedly sensitive to other uncouplers of the energy transfer reactions. The pathway of electron transfer from S~03 = to NAD+ appears to involve cyt. c, b, and flavoprotein systems as evidenced by the complete inhibition of the process by low concentrations of antimycin A, NOQNO, rotenone and amytal.
Among members of the Athiorhodaccae, Rhodopseudomonas palustris is unique in its ability to assimilate CO~ or formate photosynthetically with thiosulfate as the electron donor (van Niel, 1944). Even under photoheterotrophic growth conditions the presence of thiosulfate results in the increased cell yield, indicating that the organic electron donors not do compete with or suppress the photoautotrophic metabolism of the bacterium (Rolls and Lindstrom, 1967~, 1967b). Quite obviously the thiosulfate-linked photoautotrophie metabolism in Rps. ioalustris must * Post doctorate fellow of the Deutsche Forschungsgemeinschaft; permanent address-