The helix-turn- helix motif ( HTH ) is a secondary structural element in proteins. It is a component of DNA-binding proteins with sequence-specific DNA binding domain and is composed of two α - helices ( β -turn as well) are connected by a β - loop. Transcriptional regulators in bacteria often contain an HTH motif. Homeodomain contains a helix-turn- helix motif.
A helix binds as a recognition helix sequence-specifically in the major groove of DNA and is a multiple molecular interactions ( zBWasserstoffbrücken, ionic bonds and hydrophobic interactions ) between amino acids and the bases of DNA, the other helix positioned at right angles to it, and thus strengthens the labile bond with the DNA. Thus, such an arrangement is used on the one hand the stability of the protein -DNA complex, on the other hand, the specificity of the reaction is increased, since both binding partner should have a specific spatial structure.
DNA-binding proteins of this type are homodimers or tetramers with mirror-image symmetrical arrangement of the DNA - binding domain. The centers of the binding domains are at a distance of 34 Å. This length corresponds to the pitch of the DNA, so that the two recognition sequences to fit in two successive grooves of the DNA. The binding sequences are palindromes, that is, they are made up of two inverted, symmetrically arranged sequences with a distance of 11 base pairs, which also corresponds to the pitch of the DNA. Thus, the two recognition sequences are precisely in successive furrows of the DNA and can thus be detected by the two recognition helices of a dimeric binding protein.
- Georg Fuchs: General Microbiology. 8th edition, Thieme Verlag, ISBN 978-3-13-444608-1, p 500