A High-Throughput Genome-Integrated Assay Reveals Spatial Dependencies Governing Tcf7l2 Binding

Abstract

Predicting where transcription factors bind in the genome from their in vitro DNA-binding affinity is confounded by the large number of possible interactions with nearby transcription factors. To characterize the in vivo binding logic for the Wnt effector Tcf7l2, we developed a high-throughput screening platform in which thousands of synthesized DNA phrases are inserted into a specific genomic locus, followed by measurement of Tcf7l2 binding by DamID. Using this platform at two genomic loci in mouse embryonic stem cells, we show that while the binding of Tcf7l2 closely follows the in vitro motif-binding strength and is influenced by local chromatin accessibility, it is also strongly affected by the surrounding 99 bp of sequence. Through controlled sequence perturbation, we show that Oct4 and Klf4 motifs promote Tcf7l2 binding, particularly in the adjacent ∼50 bp and oscillating with a 10.8-bp phasing relative to these cofactor motifs, which matches the turn of a DNA helix. © 2020 The Authors Transcription factor binding in cells depends on interactions with other proteins. We have developed a high-throughput screening platform to study transcription factor binding strength at thousands of variable sequences in a fixed genomic locus. We find that binding of the Wnt effector Tcf7l2 in mouse embryonic stem cells depends on proximity and phasing that matches the turn of the DNA helix relative to its cofactors Oct4 and Klf4. © 2020 The Authors