Missense variants in human forkhead transcription factors reveal determinants of forkhead DNA bispecificity

Recognition of specific DNA sequences by transcription factors (TFs) is a key step in transcriptional control of gene expression. While most forkhead (FH) TFs bind either an FKH (RYAAAYA) or an FHL (GACGC) recognition motif, some FHs can bind both motifs. Mechanisms that control whether an FH is monospecific vs. bispecific have remained unknown. Screening a library of 12 reference FH proteins, 61 naturally occurring missense variants including clinical variants, and 22 designed mutant FHs for DNA-binding activity using universal ("all 10-mer") protein-binding microarrays revealed non-DNA-contacting residues that control mono- vs. bispecificity. Variation in non-DNA-contacting amino acid residues of TFs is associated with human traits and may play a role in the evolution of TF DNA-binding activities and gene regulatory networks.