Deeply hidden genome organization mediated by base-unpairing regions (BURs) directly bound by SATB1 linked to transcription

Mammalian genomes are organized by multi-layered chromatin folding. How three-dimensional genome organization contributes to cell-type specific transcription remains unclear. We uncover genomic elements termed base-unpairing regions (BURs), distributed genome-wide, as the sole and direct targets of cell-type specific SATB1 protein in vivo. The SATB1 direct-binding profile was generated by analyzing stringently-purified genomic DNA crosslinked to its directly-bound proteins only (ureaChIP-seq). Furthermore, a SATB1-bound BUR interacts extensively and frequently over the entire 5.7 megabase gene-rich region within many regulatory regions, including those near SATB1-dependent Rag1/Rag2 genes. SATB1 depletion leads to major loss of these interactions with greatly reduced Rag1/Rag2 expression. Most BURs reside within lamina associated domains (LADs), among which SATB1 binds to cell-type specific groups of BURs. Genome organization mediated by CTCF and SATB1 are distinct as these proteins do not co-bind chromatin in vivo and their direct binding sites are mutually exclusive genome-wide. These results revealed a previously undetected chromatin organization mediated by SATB1 direct binding to selected BURs genome-wide and suggest that chromatin interactions from some of these BURs provide a regulatory network underlying cell-type specific gene expression. Overall design: Studying genome-wide binding and chromatin interaction profiles of SATB1 using urea-purified crosslinked chromatin