On the molecular mechanism of the circadian clock: The 64000-Mr protein of Chlamydomonas reinhardtii might be related to the biological clock.

Labelling of Chlamydomonas reinhardtii cells with [(35)S] methionine led to the detection of a 64-kDa polypeptide which is synthesized according to a circadian rhythm. The change in synthesis rate could be demonstrated to exist under constant dim-light conditions as well as in darkness. Maximum synthesis of the 64-kDa polypeptide occurred at about 10 h after onset of constant conditions, and the period length of its oscillation was about 29 h. The 64-kDa polypeptide was synthesized on 80S ribosomes as shown by experiments in which cycloheximide and chloramphenicol were supplied to the cultures. Peptide-microsequence analysis yielded an N-terminal sequence of 14 amino acids. No significant homology to any other known polypeptide could be demonstrated in searches of current databases. The possible role of the 64-kDa polypeptide and its relationship to the biological clock is discussed.