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Tetracycline-Controlled Transcriptional Activation is a method of inducible gene expression where transcription is reversibly turned on or off in the presence of the antibiotic tetracycline or one of its derivatives (e.g. doxycycline). In nature, the Ptet promoter expresses TetR, the repressor, and TetA, the protein that pumps tetracycline antibiotic out of the cell.〔(【引用サイトリンク】title=TetR family )〕 The difference between Tet-On and Tet-Off is not whether the transactivator turns a gene on or off, as the name might suggest; rather, both proteins activate expression. The difference relates to their respective response to doxycycline (Dox, a more stable tetracycline analogue); Tet-Off activates expression in the ''absence'' of Dox, whereas Tet-On activates in the ''presence'' of Dox. ==Tet-Off and Tet-On== The two most commonly used inducible expression systems for research of eukaryote cell biology are named Tet-Off and Tet-On.〔Tet-On and Tet-Off are registered trademarks of Clontech Laboratories, Inc. in the United States.〕 The Tet-Off system for controlling expression of genes of interest in mammalian cells was developed by Professors Hermann Bujard and Manfred Gossen at the University of Heidelberg and first published in 1992. The Tet-Off system makes use of the ''tetracycline transactivator (tTA)'' protein, which is created by fusing one protein, TetR (tetracycline repressor), found in ''Escherichia coli'' bacteria, with the activation domain of another protein, VP16, found in the Herpes Simplex Virus. The resulting tTA protein is able to bind to DNA at specific ''TetO'' operator sequences. In most Tet-Off systems, several repeats of such TetO sequences are placed upstream of a minimal promoter such as the CMV promoter. The entirety of several TetO sequences with a minimal promoter is called a ''tetracycline response element (TRE)'', because it responds to binding of the tetracycline transactivator protein tTA by increased expression of the gene or genes downstream of its promoter. In a Tet-Off system, expression of TRE-controlled genes can be repressed by tetracycline and its derivatives. They bind tTA and render it incapable of binding to TRE sequences, thereby preventing transactivation of TRE-controlled genes. A Tet-On system works similarly, but in the opposite fashion. While in a Tet-Off system, tTA is capable of binding the operator ''only if not'' bound to tetracycline or one of its derivatives, such as doxycycline, in a Tet-On system, the rtTA protein is capable of binding the operator ''only if'' bound by a tetracycline. Thus the introduction of doxycycline to the system initiates the transcription of the genetic product. The Tet-On system is sometimes preferred over Tet-Off for its faster responsiveness. Tet-Off expression systems are also used in generating transgenic mice, which conditionally express gene of interest. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Tetracycline-controlled transcriptional activation」の詳細全文を読む スポンサード リンク
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