DNA binding domains:

This  motif  consists of two -helices  divided  through  a  short  (four-amino   acid) peptide  sequence which forms a -turn that shown in above figure.  When the transcription factor binds to the DNA one of the helices, known as the recognition helix, lies in the main groove of the DNA double helix that also shown in the figure. The helix-turn-helix motif was originally exposed in certain transcription factors which play main roles in Drosophila early development. These  proteins every contain a  60-amino  acid DNA-binding region  known as  a  homeodomain  (encoded  through a DNA  sequence known as a homeobox).  The homeodomain has four -helices in those helices II and III are the classic helix-turn-helix motif.  Because the real discovery, the helix-turn-helix motif has been found in a wide range of transcription factors, involving several which have no role in development.