Intrinsically disordered regions facilitate target search to drive promoter selectivity by a yeast transcription factor.

Transcription factors regulate gene expression by binding specific DNA motifs, yet only a fraction of putative sites is occupied in vivo. Intrinsically disordered regions have emerged as key contributors to promoter selectivity, but the underlying mechanisms remain incompletely understood. Here, we use single-molecule optical tweezers to dissect how disordered regions influence DNA binding by Msn2, a yeast stress-response regulator. We show that these regions power a search mechanism, facilitating initial non-specific association with DNA and promoting one-dimensional scanning toward target motifs, supported by charge-mediated interactions. Remarkably, this mechanism displays sequence sensitivity, with promoter-derived sequences enhancing both initial binding and scanning rates, demonstrating that Msn2-DNA interactions alone are sufficient to confer promoter selectivity in the absence of chromatin or cofactors. Our findings provide direct mechanistic evidence for how intrinsically disordered regions tune transcription factor search dynamics for Msn2 and expand sequence recognition beyond canonical motifs, supporting promoter selectivity in complex genomic contexts.