Degradation and release from the thylakoid membrane of Photosystem II subunits after UV-B irradation of the liverwort Conocephalum conicum

The effect of ultraviolet-B (UV-B) radiation on the amount of various Photosystem (PS) II subunits has been studied in the thalloid liverwort Conocephalum conicum. UV-B irradiation led to a drastic decrease of the reaction center proteins D1 and D2 and the outer light harvesting antenna (LHC II). A minor reduction was found in the levels of the CP 43 polypeptide of the inner antenna and the 33, 23 and 16 kDa extrinsic polypeptides of PS II. During UV-B irradiation, the extrinsic polypeptides accumulated in the soluble protein fraction, but D1 and D2 were not dedectable. Streptomycin, but not cycloheximide inhibited the repair process of PS II, indicating that only protein synthesis in the chloroplast is necessary for recovery. This indicates that the extrinsic proteins of PS II dissociate from the membrane during UV-B treatment and reassociate with PS II in the course of the repair process. We conclude that the reaction center core is a target of UV-B radiation in C. concicum. The extrinsic proteins of PS II are not directly affected by UV-B, but their release is the consequence of UV-B-induced degradation of the D1 and D2 proteins.