Function of three cytochromes in photosynthesis of whole cells of Rhodospirillum rubrum as studied by flash spectroscopy. Evidence for two types of reaction center

Abstract

1. Changes in the absorption spectrum induced by 10-mus flashes and continuous light of various intensities were studied in whole cells of Rhodospirillum rubrum in the presence and absence of 2-n-heptyl-4-hydroxyquinoline-N-oxide(HOQNO) and antimycin A. 2. Three cytochromes, c-420 (cytochrome c2), c-560 (cytochrome b) and c-428 were photoactive and gamma and alpha peaks at 420 and 550, 428 and 560, and 428 and 551 nm, respectively; they were photooxidized following the flash with half times of 0.3, 0.6 and 7 ms in the approximate ratios of 1/100, 1/300 and 1/1000 (cytochrome oxidized/antenna chlorophyll) and became reduced with half times of 12 ms, 60 ms and 0.7 s, respectively. c-428 and c-560 have not been distinguished before. 3. From a detailed analysis of the kinetics of P+ (oxidized reaction center chlorophyll) and the cytochromes, we conclude that 5% of the P+ (P2+) oxidizes c-428, whereas the remaining 95% of P+ (P1+) oxidizes c-420. At actinic light intensities low enough to keep c-420 fully reduced, approx. 4-5% of P becomes oxidized, accompanied by all c-428. The P2+ -P2 difference spectrum induced by this weak light is, when corrected for a shift to longer wavelengths of the bacteriochlorophyll absorption band at 878 nm, identical to the difference spectrum caused by the photooxidation of the remaining P1. At low flash intensity, c-428 becomes preferentially photooxidized, which suggests that the reaction centers where c-428 functions as a secondary donor contain much more antenna pigments compared to the centers where c-420 serves this purpose. 4. c+-420 is reduced in a competitive way by reduced c-560 (t 1/2=7 ms), and by an electron donor pool, (t 1/2=15 ms). HOQNO inhibits both pathways; antimycin A only the first. In the presence of HOQNO, c-560 is in the oxidized state in the dark, and is reduced in a light flash (t 1/2=100 ms), indicating that c-560 acts in a cyclic electron transport chain connected to P1. 5. The ratio of numbers of molecules P1 and antenna bacteriochlorophyll, transferring excitation energy to P1, is P1/bacteriochlorophyll1=1/30 P2: bacteriochlorophyll2=1/300; c-420/P1=1:2; c-560/P1=1/6; C-428/P2=1/1; bacteriochlorophyll2=7:3. If P2 is oxidized, excitation energy is transferred from bacteriochlorophyll2 to bacteriochlorophyll1.