Identification of a high molecular weight polypeptide that may be part of the circadian clockwork in Acetabularia

In the chloroplast fraction of the unicellular and uninucleate green alga Acetabularia, we have detected a M(r) approximately 230,000 protein (p230) whose synthesis exhibits a pronounced endogenous diurnal rhythm. As judged by scanning densitometry of fluorographs of NaDodSO(4)/polyacrylamide gels, the synthesis of other proteins in the same fraction was independent of the time in the cycle. The incorporation of [(35)S]methionine into p230 was completely inhibited by cycloheximide, whereas chloramphenicol had no effect. This strongly suggests that p230 is translated on 80S ribosomes. Eighthour periods of exposure to cycloheximide produced a shift in the phase of the oscillation of p230 synthesis. The results are consistent with the hypothesis that p230 is essential for expression of circadian rhythms in Acetabularia.