Preferred binding of gain-of-function mutant p53 to bidirectional promoters with coordinated binding of ETS1 and GABPA to multiple binding sites.

Gain-of-function mutant p53 is thought to induce gene expression in part by binding transcription factors bound to promoters for genes that mediate oncogenesis. We investigated the mechanism of mutant p53 binding by mapping the human genomic binding sites for p53 R273H using ChIP-Seq and showed them to localize to ETS DNA sequence motifs and locations with ETS1 and GABPA binding, both within promoters and distal to promoters. Strikingly, p53 R273H showed statistically significant and substantial binding to bidirectional promoters, which are enriched for inverted repeated ETS DNA sequence motifs. p53 R273H exhibited an exponential increase in probability of binding promoters with a higher number of ETS motifs. Both ETS1 and GABPA also showed an increase in the probability of binding to promoters with a higher number of ETS motifs. However, despite this increase in probability of binding by p53 R273H and ETS1, there was no increase in the binding signal, suggesting that the number of ETS1 and p53 R273H proteins bound per promoter is being limited. In contrast, GABPA did exhibit an increase in binding signal with higher numbers of ETS motifs per promoter. Analysis of the distance between inverted pairs of ETS motifs within promoters and binding by p53 R273H, ETS1 and GABPA, showed a novel coordination of binding for the three proteins. Both ETS1 and p53 R273H exhibited preference for binding promoters with distantly spaced ETS motifs in face-to-face and back-to-back orientations, and low binding preference to promoters with closely spaced ETS motifs. GABPA exhibited the inverse pattern of binding by preferring to bind promoters with closely spaced ETS motifs. Analysis of the helical phase between ETS motifs showed that ETS1 and p53 R273H exhibited a low preference for binding promoters with ETS motifs on the same face of the DNA helix. We propose a model for the binding of ETS1 and p53 R273H in which two inverted ETS motifs on a looped DNA helix are juxtaposed for ETS1 binding as a homodimer, with p53 R273H bound to ETS1. We propose that the formation of this DNA loop and protein-bound complex prevents additional binding of ETS1 and p53 R273H proteins to other proximal binding sites.