Systematic Dissection of Sequence Features Affecting the Binding Specificity of a Pioneer Factor Reveals Binding Synergy Between FOXA1 and AP-1 [EMSA-seq]

Pioneer factors (PFs) are a subset of transcription factors (TFs) that can bind to nucleosomal DNA and invade closed chromatin. Despite that nucleosomes do not present a strong barrier to PF binding, PF can only bind to a small fraction of motifs in the genome. The underlying mechanism of such binding selectivity is not well understood. Here, we design a high-throughput assay named Chromatin Immunoprecipitation over Integrated Synthetic Oligonucleotides (ChIP-ISO) to systematically dissect the sequence features affecting the binding specificity of a classic PF, FOXA1, in A549 human lung carcinoma cells. This method involves integrating thousands of synthetic sequences containing FOXA1 motifs into a fixed genomic locus, followed by FOXA1 chromatin immunoprecipitation (ChIP) and amplicon sequencing. We find that 1) FOXA1 binding is affected by motif strength and co-binding TFs AP-1 and CEBPB, with AP-1 playing a major role in promoting FOXA1 binding, 2) FOXA1 binding in vivo and in vitro are poorly correlated, and FOXA1 and AP-1 show binding cooperativity in vitro, 3) FoxA1’s binding specificity is mostly determined by the local sequences, whereas chromatin context, including heterochromatin marks, only plays a minor role, and 4) AP-1 is at least partially responsible for differential binding of FOXA1 in different cell types. Finally, neural network analysis shows that AP-1 and CEBPB motifs are predictive of FOXA1 ChIP-seq peaks in A549, but not in some other cell types. In summary, combining ChIP-ISO with in vitro and in silico analyses, our study provides insights into the genetic rules underlying PF binding specificity and reveals a mechanism for its regulation during cell differentiation. Electrophoretic mobility shift assay with high-throughput sequencing (EMSA-seq) on a synthetic oligonucleotide library incubated with 1-60 nM purified FOXA1 and separated on a native polyacrylamide gel to separate FOXA1 bound and unbound oligonucleotides for purification followed by high-throughput sequencing