Purchase this article with an account.
CB Green, X Liu; Circadian Regulation of Nocturnin Transcription by Phosphorylated CREB in Xenopus Retinal Photoreceptor Cells . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1355.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
Purpose: Nocturnin is a gene that is transcriptionally regulated by a circadian clock with a peak in early to mid-night. In Xenopus retina, this gene is expressed specifically in photoreceptor cells and we have previously identified a novel enhancer element, PCEII, in the proximal promoter region of this gene that is necessary and sufficient for its photoreceptor-specific expression (Liu & Green, 2001, JBC 276:15146-54). In this study, we describe experiments designed to identify the promoter element that drives nocturnin's high amplitude circadian pattern of expression. Methods: We identified protein-binding sequences within the nocturnin 5'-flanking sequence using electromobility shift assays (EMSA) with nuclear extracts isolated from Xenopus retinas. Transient transfection assays were done in NIH3T3 cells to test the activation of the defined promoter element. Cyclic AMP response element binding protein (CREB) phosphorylation within the photoreceptor cells was assayed by standard immunohistochemistry techniques. Results: We identified a protein-binding site in the proximal nocturnin 5'-flanking region (-104/-95 bp) that resembled an E-box, but with one base pair change at the least conserved position (GACGTG instead of CACGTG). E-box sequences have been shown to bind to CLOCK/BMAL1 heterodimers and are important in the rhythmic transcription of a number of circadian clock-controlled genes. EMSA analysis demonstrated, however, that the nocturnin promoter element does not bind CLOCK/BMAL1, but instead binds to CREB (E-boxes share some sequence similarity to CREs). Transient transfection experiments demonstrated that phospho-CREB can activate transcription by binding to this nocturnin promoter element and dominant-negative CREBs can not. Examination of retinal sections revealed that presence of phospho-CREB is rhythmic in the photoreceptor cell nuclei with a peak in mid-night. Conclusion: CLOCK/BMAL1 proteins bind to E-box sequences and drive transcription of genes such as period that peak in early day, but it was not previously known how the circadian clock drives transcription that peaks in the night. The phase of phospho-CREB in Xenopus retinal photoreceptors suggests that nocturnin rhythmicity, and possible other night-time genes, may be driven by the phospho-CREB rhythm.
This PDF is available to Subscribers Only