RESETTING THE DROSOPHILA CLOCK BY PHOTIC REGULATION OF PER AND A PER-TIM COMPLEX

Circadian clocks can be reset by light stimulation. To investigate the mechanism of this phase shifting, the effects of light pulses on the protein and messenger RNA products of the Drosophila clock gene period (per) were measured. Photic stimuli perturbed the timing of the PER p rotein and messenger RNA cycles in a manner consistent with the direct ion and magnitude of the phase shift. In addition, the recently identi fied clock protein TIM (for timeless) interacted with PER in vivo, and this association was rapidly decreased by light. This disruption of t he PER-TIM complex in the cytoplasm was accompanied by a delay in PER phosphorylation and nuclear entry and disruption in the nucleus by an advance in PER phosphorylation and disappearance. These results sugges t a mechanism for how a unidirectional environmental signal elicits a bidirectional clock response.