A comparison of electron transport and photophosphorylation systems of Rhodopseudomonas capsulata and Rhodospirillum rubrum. Effects of antimycin A and dibromothymoquinone.

The photophosphorylation systems of Rhodopseudomonas capsulata and Rhodospirillum rubrum chromatophores have been compared in respect to the effects of artificial electron carriers [N-methylphenazonium methosulfate (PMS) and diaminodurene], reducing agents (ascorbate in particular), and various quinones in the absence and presence of the electron transport inhibitors antimycin A and dibromothymoquinone (DBMIB). In addition, the effects of both inhibitors on photosynthetic electron transport through cytochromes b and c has been followed. From the results obtained, it appears that in both organisms: a) ubiquinone functions as an electron carrier between the cytochromes, and b) both antimycin A and DBMIB inhibit cyclic electron flow in the segment...cytochrome b leads to ubiquinone leads to cytochrome c..., but at different sites. The systems apparently differ mainly in respect to the nature of the electron flow by-pass "shunt" that is evoked in the presence of PMS; thus, in R. rubrum, PMS catalyzes a shunt that by-passes both cytochrome b and ubiquinone, whereas in Rps. capsulata the PMS shunt seems to circumvent only ubiquinone.