Figure 4.—

Circadian cycles as negative feedback loops. Transcription factors are heterodimers: two proteins interacting with each other via mutual PAS domains. These heterodimers drive expression of proteins that act as negative elements, turning down the activity of their activators. E-box is the cis-acting element through which trans-acting heterodimers act; P signifies phosphorylation. Several elements/aspects were educated guesses. (Figure is from Dunlap 1998.)