Nucleosomes specify co-factor access to p53.

Pioneer transcription factors (TFs) engage chromatinized DNA motifs. However, it is unclear how the resultant TF-nucleosome complexes are decoded by co-factors. In humans, the TF p53 regulates cell-cycle progression, apoptosis, and the DNA damage response, with a large fraction of p53-bound sites residing in nucleosome-harboring inaccessible chromatin. We examined the interaction of chromatin-bound p53 with co-factors belonging to the ubiquitin proteasome system (UPS). At two distinct motif locations on the nucleosome (super-helical location [SHL]-5.7 and SHL+5.9), the E3 ubiquitin ligase E6-E6AP was unable to bind nucleosome-engaged p53. The deubiquitinase USP7, on the other hand, readily engages nucleosome-bound p53 in vitro and in cells. A corresponding cryo-electron microscopy (cryo-EM) structure shows USP7 engaged with p53 and nucleosomes. Our work illustrates how chromatin imposes a co-factor-selective barrier for p53 interactors, whereby flexibly tethered interaction domains of co-factors and TFs govern compatibility between co-factors, TFs, and chromatin.