ABSTRACT
Proteins that regulate transcription of DNA recognize specific DNA sequences through discrete DNA-binding domains within their polypeptide chains. These domains are in general relatively small, less than 100 amino acid residues. Many procaryotic DNA-binding domains contain a helix-turnhelix motif that recognizes and binds specific regulatory regions of DNA. The two a helices have the same orientation relative to each other, and they are connected by a loop region of similar structure in all DNA-binding helixturn-helix motifs. In this chapter we will discuss the functional properties of the helix-turn-helix motif and the way this motif is integrated into structurally different DNA-binding domains of procaryotic repressors and activators. In the following two chapters we discuss the wider range of DNA binding motifs in eucaryotic DNA-binding proteins.
