Photoreduction of Ubiquinone and Photooxidiation of Phenazine Methosulfate by Chromatophores of Photosynthetic Bacteria and Bacteriochlorophyll.

A stimulation by phenazine methosulfate (PMS) of photophosphorylation with extracts of Rhodospirillum rubrum was first reported by Geller and Gregory,' who at the same time presented spectroscopic evidence that PMS was oxidized during the reaction. Jagendorf and Avron2 subsequently reported that PMS serves as a cofactor for cyclic photophosphorylation in chloroplasts, and considerable interest and effort has since been concentrated on the action of PMS in the electron transfer reactions of photosynthetic systems. The activity of PMS in the photophosphorylation system of R. rubrum has been explained in two ways: (a) PMS bypasses inhibitor-sensitive and rate-limiting sites of the electron transfer sequence,3 4 and (b) PMS exerts an oxidation-reduction poising action on the system.5 The present communication shows that PMS participates directly in lightinduced electron transfer reactions of R. rubrum, Rhodopscudomonas spheroides, and Chromatium chromatophores. These reactions are characterized by a rapid, light-dependent oxidation of reduced PMS and an associated reduction of added ubiquinone (UQ). In a subsequent dark period PMS is reduced by the photoreduced UQ, forming a cyclic system much like that previously described for the coupled photooxidation of ferrocytochrome c and photoreduction of UQ by R. rubrum chromatophores.6 Furthermore, PMS can be replaced with the reduced forms of NNN',N'-tetramethyl-p-phenylenediamine (TMPD) and 2,6-dichlorophenolindophenol (DPIP), and a methanol extract of R. rubrum chromatophores (bacteriochlorophyll) also catalyzes the PMS-UQ photoreaction. Methods and Materials.-R. rubrum cells were grown in a medium containing malate, glutamate, acetate, and ammonium chloride as described previously.7 Chromatium was grown as described by Hendley,8 and the culture medium for Rps. spheroides was that reported by Lascelles.9 Chromatophores were prepared by a 2-min sonic oscillation (Raytheon, 10 kc) of twice-washed whole cells in 10% sucrose buffered at pH 7.8 with 0.1 M Tris-HCl. The particles sedimenting between 20,000 and 100,000 X g were washed once, suspended in the Tris-sucrose solution, and stored at 0-30C. Reactions were performed under anaerobic conditions using red light (650 mgt sharp cutoff filter) and followed with a modified Bausch and Lomb Spectronic 505 recording spectrophotometer as outlined by Vernon.7 UQ6 was obtained from Mann Research Laboratories, and UQ2 was a gift of Merck, Sharp and Dohme Research Laboratories. (UQ6 is the abbreviation used for the ubiquinone containing 6 isoprene units in the side chain. Likewise, UQ2 contains two such units.) Reduced ubiquinones were prepared by the method of Green and Burkhard. 10 Chlorophyll was determined by the method of van Niel and Arnold." Results.-The photooxidation of reduced PMS (PMSH2) in the presence of UQ was followed at 388 mq, a major absorption peak of PMS. Figure 1 shows the