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Insect circadian rhythms are generated by a circadian clock consisting of transcriptional/translational feedback loops, in which CYCLE and CLOCK are the key elements in activating the transcription of various clock genes such as  timeless  ( tim ) and  period  ( per ). Although the transcriptional regulation of  Clock  ( Clk ) has been profoundly studied, little is known about the regulation of  cycle  ( cyc ). Here, we identify the orphan nuclear receptor genes  HR3  and  E75 , which are orthologs of mammalian clock genes,  Rorα  and  Rev-erbα , respectively, as factors involved in the rhythmic expression of the  cyc  gene in a primitive insect, the firebrat  Thermobia domestica . Our results show that  HR3  and  E75  are rhythmically expressed, and their normal, rhythmic expression is required for the persistence of locomotor rhythms. Their RNAi considerably altered the rhythmic transcription of not only  cyc  but also  tim . Surprisingly, the RNAi of  HR3  revealed the rhythmic expression of  Clk , suggesting that this ancestral insect species possesses the mechanisms for rhythmic expression of both  cyc  and  Clk  genes. When either  HR3  or  E75  was knocked down,  tim ,  cyc , and  Clk  or  tim  and  cyc , respectively, oscillated in phase, suggesting that the two genes play an important role in the regulation of the phase relationship among the clock genes. Interestingly,  HR3  and  E75  were also found to be involved in the regulation of ecdysis, suggesting that they interconnect the circadian clock and developmental processes.

The Nuclear Receptor Genes HR3 and E75 Are Required for the Circadian Rhythm in a Primitive Insect