CIRCADIAN REGULATION OF A DROSOPHILA HOMOLOG OF THE MAMMALIAN CLOCK GENE - PER AND TIM FUNCTION AS POSITIVE REGULATORS

The Clock gene plays an essential role in the manifestation of circadi an rhythms (congruent to 24 h) in mice and is a member of the basic he lix-loop-helix (bHLH) PER-ARNT-SIM (PAS) superfamily of transcription factors. sere we report the characterization of a novel Drosophila bHL H-PAS protein that is highly homologous to mammalian CLOCK. (Similar f indings were recently described by Allada et al. Cell 93:791-804, 1998 , and Darlington et al,, Science 280:1599-1603, 1998.) Transcripts fro m this putative Clock ortholog (designated dClock) undergo daily rhyth ms in abundance that are antiphase to the cycling observed for the RNA products from the Drosophila melanogaster circadian clock genes perio d (per) and timeless (tim). Furthermore, dClock RNA cycling is abolish ed and the levels are at trough values in the absence of either PER or TIM, suggesting that these two proteins can function as transcription al activators, a possibility which is in stark contrast to their previ ously characterized role in transcriptional autoinhibition, Finally, t he temporal regulation of dClock expression is quickly perturbed by sh ifts in light-dark cycles, indicating that this molecular rhythm is cl osely connected to the photic entrainment pathway. The isolation of a Drosophila homolog of Clock together with the recent discovery of mamm alian homologs of per indicate that there is high structural conservat ion in the integral components underlying circadian oscillators in Dro sophila and mammals. Nevertheless, because mammalian Clock mRNA is con stitutively expressed, our findings are a further example of striking differences in the regulation of putative circadian clock orthologs in different species.