Genome-Wide And Phase-Specific DNA-Binding Rhythms Of BMAL1 Control Circadian Output Functions In Mouse Liver

Using chromatin immuno-precipitation (ChIP) combined with deep sequencing (ChIP-seq) we obtained a time resolved and genome-wide map of BMAL1 binding in mouse liver, which allowed to identify over two thousand binding sites with peak binding narrowly centered around Zeitgeber time (ZT) 6. Annotation of BMAL1 targets confirms carbohydrate and lipid metabolism as the major output of the circadian clock in mouse liver. Moreover, transcription regulators are largely overrepresented, several of which also exhibit circadian activity. Genes of the core circadian oscillator stand out as strongly bound, often at promoter and distal sites. Genomic sequence analysis of the sites identified E- boxes and tandem E1-E2 consensus elements. Electromobility shift assays (EMSA) showed that E1-E2 sites are bound by a dimer of BMAL1/CLOCK heterodimers with a spacing-dependent cooperative interaction that was further validated in transactivation assays. BMAL1 target genes showed cyclic mRNA expression profiles with a phase distribution centered at ZT10. Importantly, sites with E1-E2 elements showed tighter phases both in binding and mRNA accumulation. Finally, comparing the temporal accumulation of precursor mRNA and mature mRNA helped distinguish direct BMAL1 targets from targets with more complex regulation, and showed how transcriptional and post-transcriptional regulation contribute differentially to circadian expression phase. Together, our analysis of a dynamic protein-DNA interactome uncovered how genes of the core circadian oscillator are wired together and drive phase-specific circadian output programs in a complex tissue. Overall design: ChIP-Seq of BMAL1 in mouse liver during one circadian cycle at 4 hour time resolution presented in this Series (GSE26602). mRNA profiling data used in this study are already published (Kornmann et al, PLoS Biol 2007) and have been deposited on ArrayExpress repository (accession number: E-MEXP-842).