EVIDENCE THAT CYTOCHROME B(559) PROTECTS PHOTOSYSTEM-II AGAINST PHOTOINHIBITION

Light that exceeds the photosynthetic capacity of a plant can impair the ability of photosystem II to oxidize water. The light-induced inhibition is initiated by inopportune electron transport reactions that create damaging redox states. There is evidence that secondary electron transport pathways within the photosystem II reaction center can protect against potentially damaging redox states. Experiments using thylakoid membranes poised at different ambient redox potentials demonstrate that light-induced damage to photosystem II can be controlled by a redox component within the reaction center [Nedbal, L., et al. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 7929-7933]. The rate of photoinhibition is slow when the redox component is oxidized, but increases by more than 10-fold when the redox component is reduced. Here, using spinach thylakoid membranes, we provide evidence that the redox component is cytochrome b(559), an intrinsic heme protein of the photosystem II reaction center. The results support a model in which the low-potential (LP) form of cytochrome b(559) protects photosystem II by deactivating a rarely formed, but hazardous redox state of photosystem LI, namely, P680/Pheo(-)/Q(A)(-) Cytochrome b(559)LP is proposed to deactivate this potentially lethal redox state by accepting electrons from reduced pheophytin. The key observations supporting this proposal are as follows: (1) The oxidation-reduction potential of cytochrome b(559)LP is in the range predicted by redox titrations of photoinhibition. (2) If cytochrome b(550)LP is reduced prior to illumination, the rate of photoinhibition is fast, whereas if the cytochrome is oxidized prior to illumination, the rate of photoinhibition is slow. (3) Irradiation of thylakoid membranes results in the photoreduction of cytochrome b(550)LP, which is followed by the loss of water oxidation capacity. (4) The photoreduction of cytochrome b(559)LP in continuous light precedes the reversible loss of the variable fluorescence, which monitors the photoreduction of pheophytin, supporting the proposal that electrons can flow from pheophytin to cytochrome b(550)LP. (5) As predicted by the model, the protective pathway is effective over a broad range of light intensities, offering protection from 500 to 20 000 mu mol of photons m(-2) s(-1)