Collective unstructured interactions drive chromatin binding of transcription factors

Eukaryotic transcription factors (TFs) contain both structured DNA-binding domains (DBDs) and intrinsically disordered regions (IDRs). While the structures and sequence preferences of DBDs have been extensively characterized, the role of IDR-mediated interactions in chromatin binding and nuclear organization remains poorly understood, in part because these interactions have been difficult to measure in living cells. Here, we use a recently developed single-molecule technique, proximity-assisted photoactivation (PAPA), to investigate how IDRs influence TF associations with each other and with chromatin, focusing on the factors Sp1 and Klf1. We find that the number and patterning of aromatic and basic residues within IDRs govern both TF self-association and chromatin binding. Unexpectedly, the isolated DBD of Sp1 binds chromatin very weakly and non-specifically. The isolated IDR, by contrast, interacts poorly with chromatin-bound wild-type Sp1, yet this interaction is enhanced when even minimal DNA-binding capacity is restored. Strikingly, replacing Sp1's native DBD with those of heterologous TFs recovers both IDR-mediated interactions and chromatin association, despite divergent sequence preferences. PAPA measurements also reveal extensive heterotypic interactions between wild-type Sp1 and other TFs. Together, these results establish PAPA as a powerful method for studying unstructured interactions in their native context and suggest that IDRs participate in widespread cooperative associations scaffolded by transient DBD-DNA contacts, which concentrate disordered regions along chromatin. In contrast to classical models, we propose that TF specificity in vivo emerges not solely from DBD sequence preferences, but from a constellation of weak, dynamic, and diverse interactions mediated by IDRs.