Transcriptional activation of the interferon-beta (IFN-beta) gene requires assembly of an enhanceosome containing ATF-2/c-Jun, IRF-3/IRF-7, and NFkappaB. These factors bind cooperatively to the IFN-beta enhancer and recruit coactivators and chromatin-remodeling proteins to the IFN-beta promoter. We describe here a crystal structure of the DNA-binding domains of IRF-3, IRF-7, and NFkappaB, bound to one half of the enhancer, and use a previously described structure of the remaining half to assemble a complete picture of enhanceosome architecture in the vicinity of the DNA. Association of eight proteins with the enhancer creates a continuous surface for recognizing a composite DNA-binding element. Paucity of local protein-protein contacts suggests that cooperative occupancy of the enhancer comes from both binding-induced changes in DNA conformation and interactions with additional components such as CBP. Contacts with virtually every nucleotide pair account for the evolutionary invariance of the enhancer sequence.

Cohesin catalyzes folding of the genome into loops that are anchored by CTCF 1. The molecular mechanism of how cohesin and CTCF structure the 3D genome has remained unclear. Here we show that a segment within the CTCF N-terminus interacts with the SA2-SCC1 subunits of cohesin. A 2.6Å crystal structure of SA2-SCC1 in complex with CTCF reveals the molecular basis of the interaction. We demonstrate that this interaction is specifically required for CTCF-anchored loops and contributes to the positioning of cohesin at CTCF binding sites. A similar motif is present in a number of established and novel cohesin ligands, including the cohesin release factor WAPL 2,3. Our data suggest that CTCF enables chromatin loop formation by protecting cohesin against loop release. These results provide fundamental insights into the molecular mechanism that enables dynamic regulation of chromatin folding by cohesin and CTCF.

CBP and p300 are histone acetyltransferases (HATs) that associate with and acetylate transcriptional regulators and chromatin. Mutations in their catalytic 'cores' are linked to genetic disorders, including cancer. Here we present the 2.8-Å crystal structure of the catalytic core of human p300 containing its bromodomain, CH2 region and HAT domain. The structure reveals that the CH2 region contains a discontinuous PHD domain interrupted by a RING domain. The bromodomain, PHD, RING and HAT domains adopt an assembled configuration with the RING domain positioned over the HAT substrate-binding pocket. Disease mutations that disrupt RING attachment led to upregulation of HAT activity, thus revealing an inhibitory role for this domain. The structure provides a starting point for understanding how chromatin-substrate targeting and HAT regulation are coupled and why mutations in the p300 core lead to dysregulation.