The essence of the Drosophila circadian clock involves an autoregulatory feedback loop in which PERIOD (PER) and TIMELESS (TIM) inhibit their own transcription by association with the transcriptional activators dCLOCK (dCLK) and CYCLE (CYC). Because PER, dCLK, and CYC each contain a PAS domain, it has been assumed that these interaction domains are important for negative feedback. However, a critical role for PAS-PAS interactions in Drosophila clock function has not been shown. Nuclear transport of PER is also believed to be an essential regulatory step for negative feedback, but this has not been directly tested, and the relevant nuclear localization sequence (NLS) has not been functionally mapped. We evaluated these critical aspects of PER-mediated transcriptional inhibition in Drosophila Schneider 2 (S2) cells. We mapped the dCLK:CYC inhibition domain (CCID) of PER and discovered that it lies in the C terminus, downstream of the PAS domain. Using deletion mutants and site-directed mutagenesis, we identified a novel NLS in the CCID of PER that is a potent regulator of PER's nuclear transport in S2 cells. We further found that nuclear transport, primarily through this novel NLS, is essential for the inhibitory activity of PER. The data indicate that nuclear PER inhibits dCLK:CYC-mediated transcription through a novel domain that additionally contains a potent NLS.

Circadian clocks are important regulators of behavior and physiology. The circadian clock of Drosophila depends on an autoinhibitory feedback loop involving dCLOCK, CYCLE (also called dBMAL, for Drosophila brain and muscle ARNT-like protein), dPERIOD, and dTIMELESS. Recent studies suggest that the clock mechanism in other insect species may differ strikingly from that of Drosophila. We cloned Clock, Bmal, and Timeless homologs (apClock, apBmal, and apTimeless) from the silkmoth Antheraea pernyi, from which a Period homolog (apPeriod) has already been cloned. In Schneider 2 (S2) cell culture assays, apCLOCK:apBMAL activates transcription through an E-box enhancer element found in the 5 region of the apPeriod gene. Furthermore, apPERIOD can robustly inhibit apCLOCK: apBMAL-mediated transactivation, and apTIMELESS can augment this inhibition. Thus, a complete feedback loop, resembling that found in Drosophila, can be constructed from silkmoth CLOCK, BMAL, PERIOD, and TIMELESS. Our results suggest that the circadian autoinhibitory feedback loop discovered in Drosophila is likely to be widespread among insects. However, whereas the transactivation domain in Drosophila lies in the C terminus of dCLOCK, in A. pernyi, it lies in the C terminus of apBMAL, which is highly conserved with the C termini of BMALs in other insects (except Drosophila) and in vertebrates. Our analysis sheds light on the molecular function and evolution of clock genes in the animal kingdom.