Solution structure of a DNA-binding unit of Myb: a helix-turn-helix-related motif with conserved tryptophans forming a hydrophobic core.

The DNA-binding domain of the c-myb protooncogene product consists of three imperfect tandem repeats of 51 or 52 amino acids, each of which contains three conserved tryptophans, spaced 18 or 19 amino acids apart. The structure of the third repeat, which is essential for sequence-specific DNA binding, has been determined by NMR with distance geometry calculation. It includes three well-defined helices (residues 149-162, 166-172, and 178-187) maintained by a hydrophobic core that includes the three conserved tryptophans, together with two histidines. Helices 2 and 3 form a structure related to but distinct from a canonical helix-turn-helix motif. In particular, the turn between these helices is one amino acid longer than the corresponding turn in bacterial repressors and homeodomains and contains a proline residue. In addition, the architecture of the three helices is different from those of homeodomains and DNA-binding domains of bacterial repressors. Based on the present structure, the binding mode of Myb repeat 3 with a specific DNA is also discussed.