A dynamic folded hairpin conformation is associated with ??-globin activation in erythroid cells

We investigate the three-dimensional conformations of the ??-globin locus at the single-allele level in murine ES and erythroid cells, combining polymer physics models and high-resolution Capture-C data. Model predictions are validated against independent FISH data measuring pairwise distances, and Tri-C data identifying three-way contacts. The architecture is rearranged during the transition from ES cells to erythroid cells, associated with the activation of the globin genes. We find that in ES cells, the spatial organization conforms to a highly intermingled 3D structure involving non-specific contacts, while in erythroid cells the ??-globin genes and their enhancers form a self-contained domain, arranged in a folded hairpin conformation, separated from intermingling flanking regions by a thermodynamic mechanism of micro-phase separation. The flanking regions are rich in convergent CTCF sites, which only marginally participate in the erythroid-specific gene-enhancer contacts, suggesting that beyond the interaction of CTCF sites multiple molecular mechanisms cooperate to form an interacting domain. Overall design: Embryonic stem cells were grown, and used for ChIP-seq using H3K27me3 antibody.