INHIBITION OF ELECTRON-TRANSFER FROM A0 TO A1 IN PHOTOSYSTEM-I AFTER TREATMENT IN DARKNESS AT LOW REDOX POTENTIAL

Electron transfer reactions in Photosystem I (PS I) were investigated by flash-absorption spectroscopy under highly reducing conditions in the cyanobacterium Synechocystis PCC 6803. Under strong illumination, the inhibition of electron transfer from A0- to A1 has been previously put forward and was attributed to the double reduction of phylloquinone (Setif, P. and Bottin, H. (1989) Biochemistry 28, 2689-2697). The same inhibition, characterized by the appearance of fast recombination kinetics between P-700+ and A0- (t1/2 = 40 ns) is found, in the present work, to be induced in the dark, when Photosystem I is incubated at a very low redox potential (below -550 mV) in the presence of low-potential redox mediators. The kinetics of appearance of this inhibition process are studied at various pH values and redox potentials, in the presence of different mediators. The inhibition is found to be slowly induced (from minutes to hours) and is slowly reversible at potential values above -350 mV. These results are interpreted by assuming that phylloquinol (the inhibiting species) can be formed in the dark as well as under illumination. They also indicate that in Photosystem I, the redox potential of the semiphylloquinone/phylloquinol couple is higher than that of the phylloquinone/semiphylloquinone couple.