Kinetics of the cycloheximide-induced phase changes in the biological clock in Gonyaulax

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Abstract
Cycloheximide, an inhibitor of protein synthesis on cytoplasmic ribosomes in eukaryotes, is shown to shift the phase of the circadian rhythm in stimulated bioluminescence in the marine dinoflagellate G. polyedra. Kinetic analysis of the phase changes shows that this effect may be subdivided into 2 distinctly different and well-separated parts. The 1st (early) phase change occurs with 15-min exposure to cycloheximide and is saturated at low concentrations (.apprxeq. 10 nM). The 2nd (late) phase changes requires about 150 min of exposure to cycloheximide and is saturated at 0.36 .mu.M cycloheximide. Twenty-times-higher concentrations cause no further phase changes. The magnitude of both early and late phase changes depend on the time of day when the cells are exposed to cycloheximide. Early phase changes vary from 5 h advance at circadian time (CT) 20 to 1 h delay at CT 12; late phase changes are larger, the maximal advance being 12 h at CT 16 and the greatest delay, 10 h at CT 14. It is proposed that the early phase changes are caused by alterations in the ion distribution across membranes as a consequence of the permeation of cycloheximide. Late phase changes may be the result of inhibition of protein synthesis. The phase response curve for the late phase change is identical to that obtained with saturating light pulses in otherwise constant darkness in Gonyaulax. Maximal phase changes drive the clock into the part of the circadian cycle between CT 4 and 9. Perturbations in this part of the circadian cycle are without effect on phase. Incubation of Gonyaulax with cycloheximide for a critical duration at a critical time induces arhythmicity, but longer exposures to the inhibitor at the same time do not. This observation suggests the existence of a singularity in the circadian clock of Gonyaulax.