Helix-turn-helix motif. These structures show three sequence-specific DNA-binding proteins that interact with DNA through a helix-turn-helix motif (highlighted in yellow). Notice that, in each case, the helix-turn-helix units within a protein dimer are approximately 34 Å apart, corresponding to one full turn of DNA.
[Drawn from 1EFA, 1RUN, and 1TRO.pdb.]