Target-Site Selection by Transcription Factors: Roles of DNA, Chromatin, and Cofactor-Mediated Regulation.
Transcription factors (TFs) are sequence-specific DNA-binding proteins that regulate gene-expression programs and cell fate. The ability of a defined combination of four TFs to reprogram differentiated cells into induced pluripotent stem cells illustrates the powerful role of TFs in determining cellular identity. However, TFs usually recognize short and degenerate DNA motifs of approximately 6-12 base pairs, generating thousands to millions of potential motif matches in mammalian genomes. In living cells, TFs occupy only a restricted subset of these sites, indicating that motif presence alone is insufficient for functional target selection. Several layers of regulation contribute to this selective occupancy, including DNA methylation, nucleosome organization, histone modifications, chromatin remodeling, TF oligomerization, TF availability and localization, and cofactors that regulate DNA-binding domains. This review outlines how DNA/chromatin features and TF-centered mechanisms contribute to target-site selection. The principal aim is to highlight DNA-binding domain-directed cofactor regulation as an underappreciated mechanism that modulates TF-DNA binding and may help explain selective genomic occupancy.