REGULATION OF D1 PROTEIN-DEGRADATION DURING PHOTOINHIBITION OF PHOTOSYSTEM-II IN-VIVO - PHOSPHORYLATION OF THE D1 PROTEIN IN VARIOUS PLANT GROUPS

Photoinhibition of PSII and turnover of the D1 reaction-centre protein in vivo were studied in pumpkin leaves (Cucurbita pepo L.) acclimated to different growth irradiances and in low-light-grown moss, Ceratodo n purpureus (Hedw.) Brid. The low-light-acclimated pumpkins were most susceptible to photoinhibition. The production rate of photoinhibited PSII centres (k(PI)), determined in the presence of a chloroplast-enco ded protein-synthesis inhibitor, showed no marked difference between t he high- and low-light-grown pumpkin leaves. On the other hand, the ra te constant for the repair cycle (k(REC)) of PSII was nearly three tim es higher in the high-light-grown pumpkin when compared to low-light-g rown pumpkin. The slower degradation rate of the damaged D1 protein in the low-light-acclimated leaves, determined by pulse-chase experiment s with [S-35]methionine suggested that the degradation of the D1 prote in retards the repair cycle of PSII under photoinhibitory light. Slow degradation of the D1 protein in low-light-grown pumpkin was accompani ed by accumulation of a phosphorylated form of the D1 protein, which w e postulate as being involved in the regulation of D1-protein degradat ion and therefore the whole PSII repair cycle. In spite of low growth irradiance the repair cycle of PSII in the moss Ceratodon was rapid un der high irradiance. When compared to the high- or low-light-acclimate d pumpkin leaves, Ceratodon had the highest rate of D1-protein degrada tion at 1000 mu mol photons m(-2) s(-1). In contrast to the higher pla nts, the D1 protein of Ceratodon was not phosphorylated either under h igh irradiance in vivo or under in-vitro conditions, which readily pho sphorylate the D1 protein of higher plants. This is consistent with th e rapid degradation of the D1 protein in Ceratodon. Screening experime nts indicated that D1 protein can be phosphorylated in the thylakoid m embranes of angiosperms and conifers but not in lower plants. The post ulated regulation mechanism of D1-protein degradation involving phosph orylation and the role of thylakoid organization in the function of PS II repair cycle are discussed.