Transcription regulator HTH, LacI Numerous bacterial transcription regulatory proteins bind DNA via a helix-turn-helix (HTH) motif. These proteins are very diverse, but for convenience may be grouped into subfamilies on the basis of sequence similarity. One such family groups together a range of proteins, including ascG, ccpA, cytR, ebgR, fruR, galR, galS, lacI, malI, opnR, purF, rafR, rbtR and scrR [<cite idref="PUB00002732"/>, <cite idref="PUB00005082"/>]. Within this family, the HTH motif is situated towards the N terminus.