DEGRADATION OF D2 PROTEIN DUE TO UV-B IRRADIATION OF THE REACTION-CENTER OF PHOTOSYSTEM-II

Exposure of isolated reaction centres of photosystem II to UV-B radiat ion generates specific breakdown products of the D2 protein. When the quinone, 2,5-dibromo-3 -methyl- 6-isopropyl-p-benzoquinone is present a 22 kDa fragment containing the N-terminus of the mature protein is g enerated. Concomitant with the appearance of the N-terminal fragment, two fragments containing the C-terminus of the D2 protein having appar ent molecular masses around 10-12 kDa are observed. It is concluded th at the primary cleavage occurs in the hydrophilic loop linking putativ e transmembrane segments IV and V. No such cleavage was observed when silicomolybdate was used as an electron acceptor, suggesting that this UV-B damage is dependent on binding of the added quinone to the Q(A) site.