Crystal structure of transcription factor E47: E-box recognition by a basic region helix-loop-helix dimer.

A large group of transcription factors regulating cell growth and differentiation share a dimeric alpha-helical DNA-binding domain termed the basic region helix-loop-helix (bHLH). bHLH proteins associate as homodimers and heterodimers having distinctive DNA-binding activities and transcriptional activities that are central to the regulated differentiation of a number of tissues. Some of the bHLH residues specifying these activities have been identified, but a full understanding of their function has awaited further structural information. We report here the crystal structure of the transcription factor E47 bHLH domain bound to DNA. The bHLH of E47 is a parallel, four-helix bundle with structural features that distinguish it from the bHLH-zipper protein Max. The E47 dimer makes nonequivalent contacts to each half of the -CACCTG- binding site. Sequence discrimination at the center of the E box may result from interaction with both the DNA bases and the phosphodiester backbone.