Degradation of the D1 Protein of Photosystem‐II Reaction Centre by Ultraviolet‐B Radiation Requires the Presence of Functional Manganese on the Donor Side

The in vivo effects of ultraviolet‐B radiation (280–320 nm) on photosystem‐II activity and degradation of the D1 protein are investigated and compared with the in vitro results on isolated thylakoids and other detergent‐extracted photosystem‐II preparations. A cleavage site in the 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 after irradiation of entire leaves as well as in all photosystem‐II preparations, irrespective of their actual ability to evolve oxygen but depending on the presence of Mn ions associated with the water‐splitting system. Damage to the plastoquinone moiety, observed by other authors, is confirmed and is proposed to be responsible for the impairment of electron‐transport activity, but not for the observed cleavage of the D1 protein.