Abstract
Photosystem II (PSII) catalyses the photoinduced oxygen evolution and, by producing reducing equivalents drives, in concert with PSI, the conversion of carbon dioxide to sugars. Our knowledge about the architecture of the reaction centre (RC) complex and the mechanisms of charge separation and stabilisation is well advanced. However, our understanding of the processes associated with the functioning of RC is incomplete: the photochemical activity of PSII is routinely monitored by chlorophyll-a fluorescence induction but the presently available data are not free of controversy. In this work, we examined the nature of gradual fluorescence rise of PSII elicited by trains of single-turnover saturating flashes (STSFs) in the presence of a PSII inhibitor, permitting only one stable charge separation. We show that a substantial part of the fluorescence rise originates from light-induced processes that occur after the stabilisation of charge separation, induced by the first STSF; the temperature-dependent relaxation characteristics suggest the involvement of conformational changes in the additional rise. In experiments using double flashes with variable waiting times (∆τ) between them, we found that no rise could be induced with zero or short ∆τ, the value of which depended on the temperature - revealing a previously unknown rate-limiting step in PSII.
Highlights
Photosystem II (PSII), or water-plastoquinone oxidoreductase, is a large multi-subunit homodimeric protein complex embedded in the thylakoid membranes of cyanobacteria, algae and vascular plants
In order to discriminate between the Foto-F1 and the F1-to-Fm rises and to elucidate the origin of the F1-to-Fm increment, we recorded the singleturnover saturating flashes (STSFs)-induced variations of the fluorescence yield on dark-adapted PSII core particles, isolated from Thermosynechococcus vulcanus cells, and isolated spinach thylakoid membranes in the presence of DCMU
The major aim of this work was to resolve some of the controversies over the origin and kinetics of the chlorophyll-a fluorescence induction of PSII, perhaps the most widely used probe of PSII activity
Summary
Photosystem II (PSII), or water-plastoquinone oxidoreductase, is a large multi-subunit homodimeric protein complex embedded in the thylakoid membranes of cyanobacteria, algae and vascular plants. Recent time-resolved serial femtosecond crystallography experiments on PSII crystals uncovered structural changes accompanying the reactions around the QB/non-heme iron and the Mn4CaO5 cluster[5] This raises the question if the variable chlorophyll fluorescence, which is proportional to the quantum efficiency of PSII6, contains any component originating from similar conformational changes, as proposed earlier by some authors[7,8]. In contrast to the Fo-to-F1 rise, leading to the reduction of QA, the consecutive flash-induced rises are associated with non-electrogenic reactions - showing their different physical origin. These data have been explained[18] by assuming the presence of another quencher, different from QA, called Q2.
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