CLOCK/BMAL1 regulates human nocturnin transcription through binding to the E-box of nocturnin promoter

Nocturnin has been identified as a clock-controlled gene based on its rhythmic expression and night-time peak of transcript level in Xenopus retina. Further studies show that the widespread expression and rhythmicity of nocturnin mRNA level parallel the expression of clock genes. In Xenopus, nocturnin transcription is regulated by cAMP response element-binding protein (CREB) binding the nocturnin element (NE). However, mechanism(s) underlying the regulation of nocturnin transcription in human cells is unknown at present. In this study, we demonstrated that the transcription of human nocturnin gene displayed circadian oscillations in Huh7 cells (a human hepatoma cell line) and was regulated by CLOCK/BMAL1 heterodimer via the E-box of nocturnin promoter. In addition, E-box2 is more efficient than E-box1 in the regulation of CLOCK/BMAL1 on nocturnin transcription in vitro.