Rhythmic SAF-A binding underlies circadian transcription of the Bmal1 gene.

Although Bmal1 is a key component of the mammalian clock system, little is understood about the actual mechanism of circadian Bmal1 gene transcription, particularly at the chromatin level. Here we discovered a unique chromatin structure within the Bmal1 promoter. The RORE region, which is a critical cis element for the circadian regulation of the Bmal1 gene, is comprised of GC-rich open chromatin. The 3'-flanking region of the promoter inhibited rhythmic transcription in the reporter gene assay in vitro even in the presence of RORalpha and REV-ERBalpha. We also found that the nuclear matrix protein SAF-A binds to the 3'-flanking region with circadian timing, which was correlated with Bmal1 expression by footprinting in vivo. These results suggest that the unique chromatin structure containing SAF-A is required for the circadian transcriptional regulation of the Bmal1 gene in cells.