HYPOTHESIS ON THE CONTROL OF D1 PROTEIN-TURNOVER BY NUCLEAR CODED PROTEINS IN CHLAMYDOMONAS-REINHARDTII

A hypothesis is presented on the events in the degradation of the D1 p rotein of photosystem II in the light. It proposes the existence of a nuclear encoded cleavage system that is turning over and which is modu lated by its phosphorylation state. A new experimental approach is pre sented in which the D1 protein degradation under photoinhibitory light is tested in Chlamydomonas reinhardtii grown under phosphate deficien cy and pretreated with cycloheximide. Under these conditions the degra dation of the D1 protein is delayed whereas in Chlamydomonas reinhardt ii grown in full medium the D1 protein is rapidly disappearing in high light upon addition of chloramphenicol (CAP) or lincomycin for inhibi ting the resynthesis of the D1 protein. Cycloheximide (CHI) has little effect on photoinhibition in such control cells. In cells grown, howe ver, for 20 h in phosphate deficiency - such that there is not yet los s of photosynthesis capacity - pretreatment with cycloheximide or cana vanine in low light the degradation of the D1 protein even in 6 h high light is prevented to an appreciable extent. Further addition of CAP or lincomycin has only a small effect. [C-14] leucine incorporation wa s used to show that there is no resynthesis and that the presence of t he D1. protein is due to a delay of degradation. The results are inter preted to show that excess high light which converts the D1 protein in to a potentially, degradable mode is not sufficient for degradation of the D1 protein. A cleavage system is needed as well. It is postulated that under phosphate deficiency and pretreatment with CHI or canavani ne 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 phosphat ase activity in the chloroplast and the thylakoid membrane of Chlamydo monas reinhardtii is increased. It is proposed that the ratio of kinas e/phosphatase converts an active, stable phosphorylated cleavage syste m into a labile unphosphorylated and turning over state.