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The Helix-Turn-Helix motif consists of two a helices and a short extended amino acid chain between them. The more carboxyl-terminal helix can fit into the major groove of DNA. This motif is found in hundreds of DNA-binding proteins, including l repressor, tryptophan repressor, catabolite activator protein (CAP), octamer transcription factor 1 (Oct-1) and heat shock factor (HSF), Homeotic genes are known to play a critical role in fly development. Their products contain a homeodomain which is a special form of the helix-turn-helix motif.
Figure 4-F-4. Interaction between the l repressor dimer and DNA. Each l repressor contains a helix-turn-helix motif. One of helices fits into the major groove of DNA. PDB ID = 1LMB.
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