UV-B-INDUCED DEGRADATION OF THE D1 PROTEIN IN ISOLATED REACTION CENTERS OF PHOTOSYSTEM-II

Illumination of isolated Photosystem II reaction centres by low-intensity UV-B light in the absence of added quinones does not result in any detectable breakdown products of the D1 protein but instead induces the formation of a 41 kDa adduct of the D1 protein and a alpha-subunit of cytochrome b-559, and changes its electrophoretic mobility on SDS-PAGE. If, however, DBMIB is present, many breakdown fragments of the D1 protein are detected, indicative of several cleavage sites in its amino acid sequence. Low temperature or the presence of a cocktail of proteinase inhibitors did not prevent the UV-B-induced degradation, nor did anaerobic conditions. Other quinones were also found to be effective, but to a lesser extent, giving rise to the same degradation pattern. Since the pattern of D1 protein fragmentation was not observed when non-quinone electron accepters, such as silicomolybdate, were present, it was concluded that degradation of this protein involved UV B-induced formation of quinone radicals.