Regulation of clock and NPAS2 DNA binding by the redox state of NAD cofactors

Clock:BMAL1 and NPAS2:BMAL1 are heterodimeric transcription factors that co ntrol gene expression as a function of the Light-dark cycle. Although built to fluctuate at or near a 24-hour cycle, the clock can be entrained by Lig ht, activity, or food. Here we show that the DNA-binding activity of the Cl ock:BMAL1 and NPAS2:BMAL1 heterodimers is regulated by the redox state of n icotinamide adenine dinucleotide (NAD) cofactors in a purified system. The reduced forms of the redox cofactors, NAD(H) and NADP(H), strongly enhance DNA binding of the Clock:BMAL1 and NPAS2:BMAL1 heterodimers, whereas the ox idized forms inhibit. These observations raise the possibility that food, n euronal activity, or both may entrain the circadian clock by direct modulat ion of cellular redox state.