ACCEPTOR SIDE MECHANISM OF PHOTOINDUCED PROTEOLYSIS OF THE D1 PROTEININ PHOTOSYSTEM-II REACTION CENTERS

A 23-kDa breakdown product, containing the N terminus of the D1 protei n, has been detected after photoinhibitory treatment of isolated photo system II (PSII) reaction centers. The ability to induce charge separa tion in the reaction center and the presence of oxygen seem to be requ ired for the generation of this fragment. It is suggested that, under these conditions, the initial light-induced damage to the complex occu rs via singlet oxygen generated by the P680 triplet state and contrast s with the situation when an electron acceptor is present and donor-si de photoinhibition gives rise to a 24-kDa C-terminal fragment of the D 1 protein. The temperature sensitivity of the appearance of the 23-kDa N-terminal fragment suggests that the cleavage is not by a direct pho tochemical process but that it is proteolytic in nature, being trigger ed possibly by a conformational change induced by singlet oxygen-media ted photodestruction of the P680 chlorophylls. The existence of an int rinsic serine-type protease, within the reaction center itself, is sup ported by inhibition of the appearance of the 23-kDa N-terminal fragme nt by stoichiometric levels of soybean trypsin inhibitor. It seems lik ely that the 23-kDa N-terminal fragment which we have detected is the same as that identified in vivo by Greenberg et al. [Greenberg, B. M., Gaba, V., Mattoo, A. K., & Edelman, M. (1987) EMBO J. 6,2865-2869] an d originates from the acceptor-side mechanism advocated by Vass et al. [Vass, I., Styring, S., Hundal, T., Koivuniemi, A., Aro, E.-M., & And ersson, B. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 1408-1412].