PBAF/cBAF reorganization on H3.3 chromatin regulates BMAL1 activity in the absence of circadian negative feedback.

Abstract

Circadian rhythms in gene expression are coincident with 24 hr dynamics in the recruitment of the core-clock transcription factor heterodimer CLOCK-BMAL1 to chromatin. In the liver, circadian chromatin is characterized by rhythmic histone modifications and the deposition of histone variant H2A.Z. However, other histone variants and remodelers that work in conjunction with CLOCK-BMAL1 on variant chromatin remain poorly understood. Here, we reveal that H3.3 variant histone deposition peaks during the daytime in liver chromatin and that CLOCK-BMAL1 is recruited to H3.3 nucleosomes. Moreover, H3.3:CLOCK-BMAL1 associates with PBAF and BRG1/cBAF complexes - members of the SWI/SNF remodeler family - only during the active phase of the circadian cycle. In clock-disrupted Per1^-/-; Per2^-/- livers, we observe a depletion in ARID2, the central cog in the molecular assembly of the PBAF complex, accompanied by an increase in H3.3 incorporation. Remarkably, a disassembly of PBAF complex and the concurrent reduction in BRG1 triggers a remodeler reorganization in Per knockout livers, where BRM/cBAF now targets BMAL1 at readily-accessible genomic sites. An abundance of fragile, acetylated H3.3 nucleosomes and a remodeler reorganization provide a mechanistic basis for BMAL1 activity in the absence of PER-mediated negative feedback.