Ligand-induced proteolysis of Notch produces an intracellular effector domain 
that transduces essential signals by regulating the transcription of target 
genes. This function relies on the formation of transcriptional activation 
complexes that include intracellular Notch, a Mastermind co-activator and the 
transcription factor CSL bound to cognate DNA. These complexes form higher-order 
assemblies on paired, head-to-head CSL recognition sites. Here we report the 
X-ray structure of a dimeric human Notch1 transcription complex loaded on the 
paired site from the human HES1 promoter. The small interface between the Notch 
ankyrin domains could accommodate DNA bending and untwisting to allow a range of 
spacer lengths between the two sites. Cooperative dimerization occurred on the 
human and mouse Hes5 promoters at a sequence that diverged from the CSL-binding 
consensus at one of the sites. These studies reveal how promoter organizational 
features control cooperativity and, thus, the responsiveness of different 
promoters to Notch signaling.