Cycloheximide-induced phase shifting of circadian clock of Neurospora.

Cycloheximide (CHX), an inhibitor of cytosolic (80S) protein synthesis in eucaryotes, causes phase shifts of the circadian clock of Neurospora crassa when administered as 4-h pulses to cultures in liquid medium. Differential effects of the pulses at different phases of the circadian cycle were observed and plotted as a phase-response curve (PRC). Nearly all phase shifts observed were phase advances, with maximum sensitivity in the middle of the subjective day. Inhibition of protein synthesis by CHX was the same at both phases of the cycle. The PRC was the same at 20 and 25 degrees C. Dose-response curves for the effects of CHX on phase shifting and inhibition of protein synthesis were determined and showed a striking parallel in the responses of these two phenomena to CHX. These results support the view that synthesis of one or more proteins at specific phases of the circadian cycle is necessary for the normal operation of the circadian clock of Neurospora.