The N-terminal domain of the bacterial purine repressor PuR is a winged-helix domain, a subdivision of the HTH structural family. It consists of a canonical arrangement of secondary structures: a1-b1-a2-T-a3-b2-W-b3, where a2-T-a3 is the HTH motif, a3 is the recognition helix, and W is the wing. The domain allows for recognition of a conserved CGAA sequence in the centre of a DNA PurBox, resulting in binding to the major groove of DNA [1].

1: The purine repressor of Bacillus subtilis: a novel combination of domains adapted for transcription regulation. Sinha SC, Krahn J, Shin BS, Tomchick DR, Zalkin H, Smith JL; J Bacteriol. 2003;185:4087-4098. PMID:12837783