THE QUINONE ELECTRON-ACCEPTORS ARE NOT THE MAIN SENSITIZERS OF UV-B INDUCED PROTEIN DAMAGE IN ISOLATED PHOTOSYSTEM-II REACTION-CENTER AND CORE COMPLEXES

The effect of UV-B radiation on the protein structure of the photosyst em II reaction centre and core complexes has been investigated in rela tion to the putative role of quinones as UV sensitizers of protein dam age. 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 pr oteins are significantly lost upon exposure to UV-B radiation. In cont rast, the alpha and beta subunits of cytochrome b-559 are much less af fected. UV-B induced loss of the D1 and D2 proteins is also observed i n the absence of oxygen or at 0 degrees C. The quinone analog, 2,5-dib romo-3-methyl-6-isopropyl-p-benzoquinone, has only a minor effect on t he protein loss up to 500 mu M concentration. UV-B irradiation of isol ated core complexes, in which Q(A) is present but the Q(B) site is emp ty, also induces D1 and D2 protein loss. However, the D2 protein is le ss affected by UV-B than the D1 protein, which is in contrast to their parallel degradation observed in the quinone-free reaction centre com plexes. It is concluded that in isolated reaction centre and core comp lexes there exists a quinone independent mechanism for UV-B induced da mage of the D1 and D2 proteins which also proceeds in the absence of o xygen via a probably non-proteolytic mechanism.