Pioneer Factor-Nucleosome Binding Events during Differentiation Are Motif Encoded.

Although the in vitro structural and in vivo spatial characteristics of transcription factor (TF) binding are well defined, TF interactions with chromatin and other companion TFs during development are poorly understood. To analyze such interactions in vivo, we profiled several TFs across a time course of human embryonic stem cell differentiation and studied their interactions with nucleosomes and co-occurring TFs by enhanced chromatin occupancy (EChO), a computational strategy for classifying TF interactions with chromatin. EChO shows that multiple individual TFs can employ either direct DNA binding or "pioneer" nucleosome binding at different enhancer targets. Nucleosome binding is not exclusively confined to inaccessible chromatin but rather correlated with local binding of other TFs and degeneracy at key bases in the pioneer factor target motif responsible for direct DNA binding. Our strategy reveals a dynamic exchange of TFs at enhancers across developmental time that is aided by pioneer nucleosome binding.