Circadian regulation of nocturnin transcription by phosphorylated CREB in Xenopus retinal photoreceptor cells

Although CLOCK/BMAL1 heterodimers have been implicated in transcriptional regulation of several rhythmic genes in vitro through E-box sequence elements, little is known about how the circadian clock regulates rhythmic genes with diverse phases in vivo. The gene nocturnin is rhythmically transcribed in Xenopus retinal photoreceptor cells, which contain endogenous circadian clocks. Transcription of nocturnin peaks in these cells in the middle of the night, while CLOCK/BMAL1 activity peaks during the early morning. We have identified a novel protein-binding motif within the nocturnin promoter, which we designated the nocturnin element (NE). Although the NE sequence closely resembles an E-box, our data show that it functions as a cyclic AMP response element (CRE) by binding CREB. Furthermore, phosphorylated CREB (P-CREB) levels are rhythmic in Xenopus photoreceptors, with a phase similar to that of nocturnin transcription. Our results suggest that P-CREB controls the rhythmic regulation of nocturnin transcription and perhaps that of other night phase genes. The NE may be an evolutionary intermediate between the E-box and CRE sequences, both of which seem to be involved in the circadian control of transcription, but have evolved to drive transcription with different phases in these clock-containing cells.