<p>This entry represents a DNA-binding domain with a helix-turn-helix (HTH) structure that is found in several bacterial and archaeal transcriptional regulators, such as TetR, the tetracycline resistance repressor. Numerous other transcriptional regulatory proteins also contain HTH-type DNA-binding domains, and can be grouped into subfamiles based on sequence similarity. The domain represented by this entry is found in a subfamily of proteins that includes the transcriptional regulators TetR, TetC, AcrR, BetI, Bm3R1, EnvR, QacR, MtrR, TcmR, Ttk, YbiH, and YhgD [<cite idref="PUB00000126"/>, <cite idref="PUB00003860"/>, <cite idref="PUB00002819"/>]. Many of these proteins function as repressors that control the level of susceptibility to hydrophobic antibiotics and detergents. They all have similar molecular weights, ranging from 21 to 25 kDa. The helix-turn-helix motif is located in the initial third of the protein. The 3D structure of the homodimeric TetR protein complexed with 7-chloro-tetracycline-magnesium has been determined to 2.1 A resolution [<cite idref="PUB00003347"/>]. TetR folds into ten alpha-helices with connecting turns and loops. The three N-terminal alpha-helices of the repressor form the DNA-binding domain: this structural motif encompasses an HTH fold with an inverse orientation compared with that of other DNA-binding proteins.</p>