Light-induced degradation of cytochrome b559 during photoinhibition of thephotosystem II reaction center

The behaviour of cytochrome (cyt) b559 during acceptor- and donor-side phot oinhibition has been investigated in oxygen-evolving and non-evolving photo system II (PSII) membranes. Strong illumination at 20 degrees C under aerob iosis induced a strong decrease in the absorbance of the cyt b559 alpha-ban d in the two preparations. This absorbance decline was observed only in non -oxygen-evolving PSII samples when illumination was performed under aerobio sis but at 4 degrees C, or under anaerobiosis at 20 degrees C. These result s suggest that acceptor-side photoinhibition induces the degradation of cyt b559 by a mechanism related to an enzymatic reaction mediated by singlet o xygen. Donor-side photoinhibition may induce, however, a nonenzymatic photo cleavage of the protein. (C) 1999 Federation of European Biochemical Societ ies.