The role of transcription factors in circadian gene expression.

There are three basic components present in all species which are essential for the circadian gene expression; an input pathway which connects the clock to the environment, the clock oscillator, and an output pathway which connects the pacemaker to the resulting biological phenomena. In this review, an attempt to separate the three processes will be made from the molecular biological stand point. In the pineal of birds, Drosophila, and algae, cAMP/PKA pathway is functional in the output, but in mammalian suprachiasmatic nucleus (SCN), this pathway including the CREB/c-fos pathway, is believed to function in the input pathway. We propose here a model that easily explains the controversial results of the CREB/c-fos story in the SCN by considering this signal transduction pathway as an output. Finally, we propose the importance of E-box and bHLH-PAS type transcription factors, in the clock oscillator in the SCN.