Abstract:Photosystem Ⅱ reaction center D1/Dg/Cyt b559 complex is very sensitive to light. Besides pigments, some amino acids, like histidine and methionine residues on the polypeptide chain, were damaged and D1 and D2 proteins were degraded by illumination. SDS-PAGE analysis demonstrated an increased content of the D1 and D2 protein dimers and a new band with molecular weight of 41 kD after light treatment. Meanwhile, the D1 and D2 bands were shifted to apparent positions of higher molecular weight. During the consequent incubation in the dark following illumination, although there was no change in the composition of amino acids, the degradation process of D1 and D2 proteins and the production of 41 kD fragment continued. It was proposed that degradation of D1 and D2 proteins was probably due to the photodamage of some amino acids via chemical splitting and co-valent cross-linkage in this process.