Enhancers harbor binding motifs that recruit transcription factors (TFs) for gene activation. While cooperative binding of TFs at enhancers is known to be critical for transcriptional activation of a handful of developmental enhancers, the extent of TF cooperativity genome-wide is unknown. Here, we couple high-resolution nuclease footprinting with single-molecule methylation profiling to characterize TF cooperativity at active enhancers in the Drosophila genome. Enrichment of short micrococcal nuclease (MNase)-protected DNA segments indicates that the majority of enhancers harbor two or more TF-binding sites, and we uncover protected fragments that correspond to co-bound sites in thousands of enhancers. From the analysis of co-binding, we find that cooperativity dominates TF binding in vivo at the majority of active enhancers. Cooperativity is highest between sites spaced 50 bp apart, indicating that cooperativity occurs without apparent protein-protein interactions. Our findings suggest nucleosomes promoting cooperativity because co-binding may effectively clear nucleosomes and promote enhancer function.
Keywords: chromatin dynamics; enhancer; enhancer cooperativity; nucleosome; transcription factor.
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