Abstract
Light serves as a key environmental signal for synchronizing the circadian clock with the day night cycle. The zebrafish represents an attractive model for exploring how light influences the vertebrate clock mechanism. Direct illumination of most fish tissues and cell lines induces expression of a broad range of genes including DNA repair, stress response and key clock genes. We have previously identified D- and E-box elements within the promoter of the zebrafish per2 gene that together direct light-induced gene expression. However, is the combined regulation by E- and D-boxes a general feature for all light-induced gene expression? We have tackled this question by examining the regulation of additional light-inducible genes. Our results demonstrate that with the exception of per2, all other genes tested are not induced by light upon blocking of de novo protein synthesis. We reveal that a single D-box serves as the principal light responsive element within the cry1a promoter. Furthermore, upon inhibition of protein synthesis D-box mediated gene expression is abolished while the E-box confers light driven activation as observed in the per2 gene. Given the existence of different photoreceptors in fish cells, our results implicate the D-box enhancer as a general convergence point for light driven signaling.
Highlights
The circadian clock is a highly conserved, physiological timing mechanism that allows organisms to anticipate and adapt to daily environmental changes [1]
In addition to the clock genes per2 and cry1a, many other genes are acutely induced upon light exposure in zebrafish [19,20,24]
Light exposure of most tissues and cells triggers expression of a set of genes that is not restricted to components of the circadian clock [19,20,24]
Summary
The circadian clock is a highly conserved, physiological timing mechanism that allows organisms to anticipate and adapt to daily environmental changes [1]. As the levels of PER and CRY rise, they enter the nucleus as heterodimers and repress their own transcription by inhibiting the action of the CLOCK:BMAL complex, closing the feedback loop [3]. This mechanism involves additional stabilizing loops [4,5] as well as complex posttranslational regulation. This additional regulation confers robustness and ensures that the mechanism requires circa 24 hours to complete one cycle [6] Given that this is not a precise 24 hours clock, it is vital that its phase is reset regularly by signals that reliably indicate the time of day, to ensure its synchronization with the natural day-night cycle. Given the importance of light as an environmental timing cue, most organisms have evolved dedicated photoreceptors and associated signalling pathways that relay this lighting information to the core clock machinery
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