Genome organization by SATB1 binding to base-unpairing regions (BURs) provides scaffold for SATB1-regulated gene expression

Abstract

Mammalian genomes are organized by multi-level folding, yet how this organization contributes to cell type-specific transcription remain unclear. A nuclear protein SATB1 forms a SATB1-rich subnuclear structure that resists high-salt extraction. SATB1 binds in the minor groove of double-stranded base-unpairing regions (BURs), genomic elements with high unwinding propensities. We uncovered that SATB1 establishes a two-tiered chromatin organization, one through indirect binding and another by direct binding of BURs. Published ChIP-seq datasets show SATB1 binding to highly accessible chromatin at enhancers and CTCF sites, but not to BURs. By employing urea ChIP-seq, which retains only directly bound protein:DNA complexes, we found that BURs, but not CTCF sites, are direct SATB1 binding targets genome-wide. BURs bound to the SATB1 nuclear substructure interact with accessible chromatin crossing multiple topologically associated domains (TADs). SATB1 is required for these megabase-scale interactions linked to cell type-specific gene expression. BURs are highly enriched within lamina associated domains (LADs), but some ([~]10%) are found in gene-rich accessible chromatin outside LADs as well. Only a subset of BURs is bound to SATB1 depending on cell type. Notably, despite the mutually exclusive SATB1-binding profiles uncovered by the two ChIP-seq methods, we found most peaks in both profiles are valid and require SATB1. Based on these and previous data, we propose that the SATB1 protein network forms a chromatin scaffold, providing an interface that connects accessible chromatin to a subnuclear architectural structure, thereby facilitating the three-dimensional organization linked to cell type-specific gene expression.