EVIDENCE FOR A CENTRAL ROLE OF TRANSCRIPTION IN THE TIMING MECHANISM OF A CIRCADIAN CLOCK

The retinal circadian clock in the isolated in vitro eye of the marine mollusc Bulla gouldiana exhibits a phase-dependent requirement for tr anscription. The transcription-sensitive phase extends through most of the subjective day and therefore is substantially longer than the pre viously reported translation-sensitive phase. Lower concentrations of transcription inhibitors yield a significant dose-dependent lengthenin g of circadian period. Clock motion can be stopped by a high concentra tion of the transcription inhibitor 5,6-dichlorobenzimidazole riboside (DRB) when applied during the sensitive phase; after withdrawal of th e inhibitor, motion resumes from the phase at which it was stopped. In a double-pulse experiment, phase shifts to light pulses applied after DRB pulses, and not during the translation-sensitive phase, indicate that the inhibition of transcription has immediate effects on the phas e of the clock. These data suggest that DRB-induced phase shifts are i ndependent of translation, which implies that the rate of transcriptio n itself plays a significant role in the mechanism underlying the gene ration of the circadian cycle.