Hypothesis on the Control of D 1 Protein Turnover by Nuclear Coded Proteins in Chlamydomonas reinhardtii

Abstract

A hypothesis is presented on the events in the degradation of the D 1 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 D 1 protein degradation under photoinhibitory light is tested in Chlamydomonas reinhardtii grown under phosphate deficiency and pretreated with cyclo-heximide. Under these conditions the degradation of the D 1 protein is delayed whereas in Chlamydomonas reinhardtii grown in full medium the D 1 protein is rapidly disappearing in high light upon addition of chloramphenicol (CAP) or lincomycin for inhibiting the resynthesis of the D 1 protein . Cycloheximide (CHI) has little effect on photoinhibition in such control cells. In cells grown, however, for 20 h 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 D 1 protein even in 6 h 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 D 1 protein is due to a delay of degradation. The results are interpreted to show that excess high light which converts the D 1 protein into a potentially, degradable mode is not sufficient for degradation of the D 1 protein. A cleavage system is needed as well. It is postulated that under phosphate deficiency and pre-treatment with CHI or canavanine a nuclear coded cleavage system for the D 1 protein is depleted, i.e. the cleavage system for the rapidly turning over D 1 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