CYCLOHEXIMIDE RETARDS HIGH LIGHT DRIVEN D1 PROTEIN-DEGRADATION IN CHLAMYDOMONAS-REINHARDTII

The D1 protein of Photosystem II (PS II) is degraded when a Chlamydomo nas reinhardtii culture is exposed to high light, as is well known. As degradation may be compensated by resynthesis of the protein, chloram phenicol (CAP) is added to visualize D1 protein disappearance in an im munoblot. A 2 h pretreatment with cycloheximide (CHI) of a culture gro wn photoautotrophically retards the D1 protein degradation and the los s of PS II activity in the subsequent high light exposure. Further add ition of CAP no longer leads to complete disappearance of the D1 prote in. [C-14]Leucine is not incorporated under these conditions of CHI CAP into the D1 protein, i.e. the D1 protein still present had not bee n resynthesized as in the control with no inhibitors, but had not been degraded. A turning over protease is postulated in which a nuclear co ded subunit gets depleted after CHI addition. The model is extended in which this protease gets accessibility to a relaxed state in the amin o acid sequence of the D1 protein that contains the cleavage site. Und er photoheterotrophic growth conditions (acetate present) CHI alone ha s not as much as influence on D1 protein stabilization as under photoa utotrophic conditions. As already reported phosphate deficiency has to occur as well. Canavanine has an even stronger retarding effect on D1 protein degradation than CHI in photoheterotrophically grown Chlamydo monas reinhardtii.