CLOCK (Circadian locomoter output cycles protein kaput) heterodimers activate E-bo, Xenopus/Amphibian, element (3'-CACGTG-5') transcription of a number of proteins of the circadian clock. CLOCK activates transcription of PER1 and PER2. This transcription is inhibited in a feedback loop by PER and CRY proteins. CLOCK has intrinsic histone acetyltransferase activity and this enzymatic function contributes to chromatin-remodeling events implicated in circadian control of gene e, Xenopus/Amphibian,pression. CLOCK acetylates primarily histones H3 and H4.