EVIDENCE FOR CONCURRENT DONOR AND ACCEPTOR SIDE PHOTOINDUCED DEGRADATION OF THE D1-PROTEIN IN ISOLATED REACTION CENTERS OF PHOTOSYSTEM-II

By varying the period of illumination or by changing the concentration of the quinone electron acceptor, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), it has been possible to demonstrate, using immunoblottimg, that the photoinduced degradation of the D1-protein follows two distinct pathways in isolated Photosystem II (PS II) reaction centres. During the initial illumination period, with an intermediate level of DBMIB present (0.2 mM), a 23 kDa fragment containing the N-terminus of the D1-protein appears. Further illumination facilitates the accumulation of C-terminal fragments of apparent molecular masses 24 kDa and 16 kDa. The 24 kDa C-terminal fragment is not observed in the absence of DBMIB or when this quinone is present at high levels (1 mM). In the latter condition, the photoinduced degradation of the DI-protein is significantly protected and only the 23 kDa N-terminal fragment is produced. The immuno-detected band at 16 kDa is weaker than those of the higher molecular mass breakdown fragments and its appearance seems to follow conditions similar, but not identical, to those needed to generate the 24 kDa C-terminal fragment. A closer examination of the 16 kDa band revealed that it consists of both C-terminal and N-terminal portions of the D1-protein, suggesting it arises from a cleavage about midway in the protein. We conclude that both donor and acceptor side photoinhibition can occur in the isolated PS II reaction centre, the balance between them being controlled by the extent of the illumination and the level of DBMIB present. The 23 kDa N-terminal fragment is generated during acceptor side photoinhibition, while the 24 kDa C-terminal fragment is induced by damage on the donor side.