Nucleosome-mediated competition of pioneer transcription factors

Pioneer transcription factors Nanog, Pou5f3 and Sox19b promote chromatin accessibility on the enhancers of the early zebrafish embryos. It is poorly understood how they act together; it is commonly assumed that they act additively or cooperatively to clear chromatin from the nucleosomes. We investigated this question by comparing chromatin accessibility (Omni ATAC-seq) in seven mutant genotypes: single, double and triple maternal-zygotic mutants by Pou5f3, Sox19b and Nanog, and the wild-type embryos. We found, that at most of the directly bound sites, either Pou or Nanog is required for chromatin accessibility; while at the minority of the sites two factors act additively. Further, on the sites, where Pou5f3 promotes chromatin accessibility, Nanog often suppresses it, and vice versa. To explain this phenomenon, we suggest a model of nucleosome-mediated competition between pioneer transcription factors: Pou5f3 and Nanog compete for the same binding motif on the DNA, but only one of the factors has a pioneer activity on this site. This entry contains only part fo the Omni-ATAC-seq experiments, the other part is in the previously released entry GSE188364. Overall design: Omni-ATAC-seq on whole embryos