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Ref ID : 4559

E. Bracht and A. Trebst; Hypothesis on the Control of D1 Protein Turnover by Nuclear Coded Proteins in Chlamydomonas reinhardtii. Z.Naturforsch. 49c:439-446, 1994

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A hypothesis is presented on the events in the degradation of the D1 protein of photosystem II in the light. It proposes the existence of a nuclear encoded cleavage system that is turning over and which is modulated by its phosphorylation state. A new experimental approach is presented in which the D1 protein degradation under photoinhibitory light is tested in Chlamydomonas reinhardtii grown under phosphate deficiency and pretreated with cycloheximide. Under these conditions the degradation of the D1 protein is delayed whereas in Chlamydomonas reinhardtii grown in full medium the D1 protein is rapidly disappearing in high light upon addition of chloramphenicol (CAP) or lincomycin for inhibiting the resynthesis of the D1 protein. Cycloheximide (CHI) has little effect on photoinhibition in such control cells. In cells grown, however, for 20 hr in phosphate deficiency -such that there is not yet loss of photosynthesis capacity- pretreatment with cycloheximide or canavanine in low light the degradation of the D1 protein even in 6 hr high light is prevented to an appreciable extent. Further addition of CAP or lincomycin has only a small effect. [14C]leucine incorporation was used to show that there is no resynthesis and that the presence of the D1 protein is due to a delay of degradation. The results are interpreted to show that excess high light which converts the D1 protein into a potentially, degradable mode is not sufficient of degradation of the D1 protein. A cleavage system is needed as well. It is postulated that under phosphate deficiency and pretreatment with CHI or canavanine a nuclear coded cleavage system for the D1 protein is depleted, i.e. the cleavage system for the rapidly turning over D1 is also turning over. It is shown that under phosphate deficiency an alkaline phosphatase activity in the chloroplast and the thylakoid membrane of Chlamydomonas reinhardtii is increased. It is proposed that the ratio of kinase/phosphatase converts an active, stable phosphorylated cleavage system into a labile unphosphorylated and turning over state.