Activator recruitment by the general transcription machinery: X-ray structural analysis of the Oct-1 POU domain/human U1 octamer/SNAP190 peptide ternary complex.

Transcriptional activation of the human U1 snRNA genes is dependent on a noncanonical octamer element contained within an upstream enhancer. The U1 octamer only weakly recruits the Oct-1 POU domain, although recruitment is stimulated by a peptide containing the Oct-1-binding domain of SNAP190. Structural analysis of the Oct-1 POU domain/U1 octamer/SNAP190 peptide complex revealed that SNAP190 makes extensive protein contacts with the Oct-1 POU-specific domain and with the DNA phosphate backbone within the enhancer. Although SNAP190 and OCA-B both interact with the Oct-1 POU domain through the same Oct-1 interface, a single nucleotide within the U1 octamer ablates OCA-B recruitment without compromising activator recruitment by SNAP190.