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

The behaviour of cytochrome (cyt) b559 during acceptor- and donor-side photoinhibition has been investigated in oxygen-evolving and non-evolving photosystem II (PSII) membranes. Strong illumination at 20 degrees C under aerobiosis induced a strong decrease in the absorbance of the cyt b559 alpha-band in the two preparations. This absorbance decline was observed only in non-oxygen-evolving PSII samples when illumination was performed under aerobiosis but at 4 degrees C, or under anaerobiosis at 20 degrees C. These results suggest that acceptor-side photoinhibition induces the degradation of cyt b559 by a mechanism related to an enzymatic reaction mediated by singlet oxygen. Donor-side photoinhibition may induce, however, a non-enzymatic photocleavage of the protein.