DEGRADATION OF THE DI-PROTEIN OF PHOTOSYSTEM-II REACTION-CENTER BY ULTRAVIOLET-B RADIATION REQUIRES THE PRESENCE OF FUNCTIONAL MANGANESE ONTHE DONOR SIDE

The in vivo effects of ultraviolet-B radiation (280-320 nm) on photosy stem-II activity and degradation of the D1 protein are investigated an d compared with the in vitro results on isolated thylakoids and other detergent-extracted photosystem-II preparations. A cleavage site in th e second transmembrane segment of the D1 protein, giving rise to a 20- kDa C-terminal and a 13-kDa N-terminal fragment pair, is detected afte r irradiation of entire leaves as well as in all photosystem-II prepar ations, irrespective of their actual ability to evolve oxygen but depe nding on the presence of Mn ions associated with the water-splitting s ystem. Damage to the plastoquinone moiety, observed by other authors, is confirmed and is proposed to be responsible for the impairment of e lectron-transport activity, but not for the observed cleavage of the D 1 protein.