Novel mechanisms operating in the central pacemaker and in the light-synchronization pathway of Drosophila’s circadian clock

Abstract

Most organisms display circadian rhythms of approximately 24 hours in many aspects of their physiology and behaviour. The synchronization between their internal rhythm and the environmental light-dark cycles is essential for an organism’s survival and fitness. In the fruit fly, Drosophila melanogaster, circadian locomotor activity is controlled by central pacemaker neurons, in which the circadian oscillation of the molecular clock is built on the negative feedback regulation of period (per) and timeless (tim) gene expression. After transcription and translation, PER and TIM proteins form stable heterodimers in the cytoplasm and transfer into the nucleus to suppress their own transcription. Whether other processes including PER homodimerisation and nuclear trafficking are involved in circadian feedback control remains largely unknown. To study the functions of these processes, I attempted to specifically disrupt PER homodimers and nuclear export sequences (NES). I found that PER homodimers are required for PER nuclear translocation, period transcriptional repression, and normal circadian behaviour in flies. I also demonstrated that the potential NES of PER contributes to the repressor activity of PER and temperature compensation of the circadian clock. Light can phase-shift and even disrupt the molecular clock by degrading TIM. Light-dependent TIM degradation in Drosophila is mainly mediated by the photoreceptor CRYPTOCHROME (CRY). However, CRY-independent light-input pathways are also utilized by the Drosophila circadian clock. To explore these pathways, I investigated the function of a novel gene quasimodo (qsm) and found that QSM is a light-responsive protein. In constant light, qsm mutants maintain oscillation of clock proteins and show abnormal behavioral rhythms, indicating impaired photoreception. 6 Functional analysis suggests that qsm may mediate TIM degradation in the absence of CRY, and constitutes part of a novel light-input pathway to the clock. In addition, I found that in conjunction with a functional circadian clock QSM may suppress light induced ultradian rhythms.