Identification of methylation-sensitive human transcription factors using meSMiLE-seq

Transcription factors (TFs) are key players in eukaryotic gene regulation, but the DNA binding specificity of many TFs remains unknown. Here, we assay 284 mostly uncharacterized putative human TFs using selective microfluidics-based ligand enrichment followed by sequencing (SMiLE-seq), revealing 74 new DNA binding motifs. To investigate whether TFs lacking detectable motifs preferably bind epigenetically modified DNA, we develop methylation-sensitive SMiLE-seq (meSMiLE-seq), a microfluidic assay that simultaneously probes binding to methylated and unmethylated DNA. Using meSMiLE-seq, we assay 114 TFs and identify DNA-binding models for 48 proteins, including known methylation-sensitive binding modes for POU5F1 and RFX5. 11 TFs prefer methylated DNA or display alternative methylation-dependent motifs (e.g. PRDM13), while 13 show aversion to methylated sequences (e.g. USF3). Finally, we identify ZHX2 as a putative Z-DNA binder. Altogether, our study significantly expands the human TF codebook, while providing a versatile platform to quantitatively assay the impact of DNA modifications on TF binding.