DESTRUCTIVE ROLE OF SINGLET OXYGEN DURING AEROBIC ILLUMINATION OF THE PHOTOSYSTEM-II CORE COMPLEX

Strong illumination of the Photosystem II (PS II) core complex, at 35 degrees C under aerobic conditions, resulted in rapid inactivation of electron transport activity, and pigment photobleaching which was followed by the degradation of the D1 polypeptide. Concomitant with D1 degradation there was a significant disappearance of the 43 and 29 kDa Chl binding proteins and an appearance of high-molecular-weight species originating from the cross-linking of other PS II proteins with the D1 polypeptide and/or D1 fragments. Strict anaerobic conditions during illumination almost completely prevented pigment photobleaching and protein degradation and subsequent cross-linking. The presence of singlet oxygen scavengers, histidine and rutin, during illumination, significantly protected against photoinduced damage to the PS II complex, suggesting an involvement of singlet oxygen in the destructive process. Singlet oxygen-generating chemicals led to similar pigment bleaching and protein cross-linking. We propose that singlet oxygen, which is generated during aerobic illumination of the PS II complex, is responsible for the photobleaching of photosynthetic pigments, D1 protein degradation and protein cross-linking.