On the Origin of the Slow Electric Potential Component Induced by Ferredoxin-Mediated Cyclic Electron Transfer in Photosystem I-Enriched Subchloroplast Vesicles

Abstract

Cyclic electron transfer around photosystem I (PSI) can be considered as an important energy-conserving pathway in plant photosynthesis. Its operation requires ferredoxin as a cofactor while its activity is strongly dependent on a proper redox balance of the electron carriers. The arrangement of electron carriers has been only partly characterized (Crowter, Hind, 1980; Chain, 1982) and is still under investigation. In particular, schemes of non-serial transfer of electrons from ferredoxin to P-700 (i.e. Q- and b-cycles) have been proposed to account for an extra electrogenic step as monitored by the electrochromic carotenoid absorbance changes. We have studied ferredoxin-mediated cyclic electron transfer and electric potential generation in PSI-enriched subchloroplast vesicles that contain all components involved in the native system except ferredoxin, in amounts originally present in the stroma lamellae (Peters et al., 1983a). In the experimental system of PSI vesicles, supplemented with ferredoxin, the activity of cyclic electron transfer can be adjusted by varying the concentrations of NADPH, NADP+ and O2 (Peters et al., 1983b). In this way we studied the flash-induced slow carotenoid response in relation to redox changes of cytochromes b-563 and c-554. We present evidence for a reductant-induced oxidation of cyt b-563, but the slow potential component has its origin before the reoxidation of b-563.