Basic helix-loop-helix transcription factors MYC-MAX and CLOCK-BMAL1 associate with histones for E-box access

Basic helix-loop-helix (bHLH) transcription factors (TF) recognize E-boxes (CANNTG) and include over 100 members. Here we investigated how chromatinised E-boxes are engaged by two structurally diverse bHLH family members; the proto-oncogene MYC-MAX and the circadian TF, CLOCK-BMAL1. Both bind E-boxes preferentially near the nucleosomal entry/exit sites. Structural studies with artificial or endogenous nucleosome sequences illustrate that MYC-MAX/CLOCK-BMAL1 trigger DNA release from histones to gain access. The CLOCK-BMAL1 PAS dimerization domains engage the histone-octamer disc atop the H2A/H2B acidic patch, an interaction critical for circadian cycling. Binding of tandem E-boxes at endogenous DNA sequences is similarly achieved through direct interactions between two CLOCK-BMAL1 protomers and histones. At internal E-boxes, the MYC-MAX leucine zipper can also interact with histones H2B/H3, and its binding is indirectly enhanced by OCT4 elsewhere on the nucleosome. The nucleosomal E-box position and the type of bHLH dimerisation domain jointly determine the histone contact, the affinity, and the degree of competition/cooperativity with other nucleosome-bound factors. ChiP-seq profiles of MYC:MAX and CLOCK:BMAL1 on in vitro reconstituted chromatin.