Helix turn helix
(Science: molecular biology) a motif associated with transcription factors, allowing them to bind to and recognise specific dna sequences. Two amphipathic _ helices are separated by a short sequence with a _ sheet. One helix lies across the major groove of the dna, while the recognition helix enters the major groove and interacts with specific bases. An example in drosophila is the homeotic gene fushi tarazu, that binds to the sequence TCAATTAAATGA. Not the same as helix loop helix.