Tuning Transcription Factor Availability through Acetylation-Mediated Genomic Redistribution

It is widely assumed that decreasing transcription factor DNA-binding affinity reduces transcription initiation by diminishing occupancy of sequence-specific regulatory elements. However in vivo transcription factors find their binding sites while confronted with a large excess of low affinity degenerate motifs. Here we show, using the melanoma lineage survival oncogene MITF as a model, that low affinity binding sites act as a competitive reservoir in vivo from which TFs are released by MAPK-stimulated acetylation to promote increased occupancy of their regulatory elements. Consequently a low DNA-binding affinity acetylation-mimetic MITF mutation supports melanocyte development and drives tumorigenesis, whereas a high affinity, non-acetylatable mutant does neither. The results reveal a paradoxical acetylation-mediated molecular clutch that tunes transcription factor availability via genome-wide titration and couples BRAF to tumorigenesis. Our results further suggest that genomic titration regulated by p300/CBP-mediated acetylation will represent a common mechanism to control TF availability. Overall design: 501mel melanoma cell lines engineered to express ectopic-doxycycline-inducible-HA-epitope-tag-murine Mitf-WT or K243Q or K243R were treated with 20ng Dox prior to fixation and ChIP-sequencing