The quinone electron acceptors are not the main sensitizers of UV-B induced protein damage in isolated photosystem II reaction centre and core complexes

The effect of UV-B radiation on the protein structure of the photosystem II reaction centre and core complexes has been investigated in relation to the putative role of quinones as UV sensitizers of protein damage. In isolated reaction centre complexes, in which neither the Q(A) nor the Q(B) quinone electron accepters are retained on their binding sites (on the D2 and D1 subunits, respectively), both the D1 and D2 proteins are significantly lost upon exposure to UV-B radiation. In contrast, the alpha and beta subunits of cytochrome b-559 are much less affected. UV-B induced loss of the D1 and D2 proteins is also observed in the absence of oxygen or at 0 degrees C. The quinone analog, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, has only a minor effect on the protein loss up to 500 mu M concentration. UV-B irradiation of isolated core complexes, in which Q(A) is present but the Q(B) site is empty, also induces D1 and D2 protein loss. However, the D2 protein is less affected by UV-B than the D1 protein, which is in contrast to their parallel degradation observed in the quinone-free reaction centre complexes. It is concluded that in isolated reaction centre and core complexes there exists a quinone independent mechanism for UV-B induced damage of the D1 and D2 proteins which also proceeds in the absence of oxygen via a probably non-proteolytic mechanism.