The first DNA-binding protein motif to be recognized. helix-turn-helix motifs were originally identified in bacterial proteins but have since been found in hundreds of dna-binding proteins from both eukaryotes and prokaryotes. They are constructed from two alpha helices connected by a short extended chain of amino acids, which constitute the "turn." The two helices are held at a fixed angle, primarily through interactions between the two helices. (From Alberts et al., molecular biology of the Cell, 3d ed, p408-9)