Control of Photosynthetic Electron Transfer from the Reaction Center to Electron Carriers of Photosystem II Studied by Fluorescence Induction

Abstract

Abstract. The relationships between the primary electron acceptor (0) and the secondary pool (AI) are examined from fluorescence induction curves, at various illumination times and relaxation dark times. The fast-phase in the fluorescence rise curve, obtained after insufficient relaxation time, indicates the occurrence of "unconnected" 0, i.e. the reaction between 0-and AI is blocked. Various treatments bring about similar disconnection: high pH, ethylene glycol, low temperature (all reversible), trypsin and reduction of A I by dithionite. All these treatments are similar to DCMU treatment. A scheme is suggested in which 0 is mostly unconnected in the dark and becomes immediately connected as it is reduced by light. The above treatments inhibit the reconnection reaction in the light. The thermodynamic equilibrium between the 0 populations that can or cannot be reconnected was analyzed from the fluorescence curves at various temperatures. The connection-disconnection switching reaction is probably controlled by conformational changes of an associated protein. This scheme explains the difference of apparent redox equilibria of 0 and A in the light or in the dark, as well as the lOP wave in the fluorescence induction of intact cells.

Photochemical Reactions of Photosystem II in Ethylene Glycol

“…In the 30-60% EG range, there is a reversible slowing in the Q Arecombination kinetics (Table 1) as well as a reversible loss in the nanosecond kinetics for P680 + reduction (Figure 6). One possible explanation for these effects may be that EG causes a lowering of the midpoint potentials of the intermediate S n states and/or of the electron acceptor side (Malkin, 1981). Earlier work has shown, for example, that the S 2 state in PSII samples prepared without the 33 kDa protein (Miyao et al, 1987) and in 33 kDa (psbO) deletion mutants (Philbrick et al, 1991;Burnap et al, 1992) has a much higher stability, presumably because of altered solvent interactions.…”