| - | The homodimeric tetracycline repressor (TetR) regulates resistance to the, antibiotic tetracycline at the transcriptional level. TetR binds in the, absence of Tc to palindromic operator sequences utilizing two, helix-turn-helix (HTH) motifs. If the tetracycline-Mg2+ complex [MgTc]+, enters two identical binding tunnels buried within the TetR homodimer, a, conformational change takes place, and the induced [TetR/[MgTc]+]2 complex, releases operator DNA. To demonstrate the contribution of Mg2+ to [MgTc]+, binding and TetR induction, the Mg2+ concentration in the induced TetR, homodimer was progressively reduced by addition of EDTA, resulting in two, X-ray crystal structures of Mg2+-free and half-occupied TetR(D). Tc, remains bound to the [MgTc]+-binding sites, despite the complete or, partial absence of Mg2+. Together with inducer-free TetR(D), the, structures were refined to between 2.2 and 2.7 A resolution and compared, with fully induced TetR(D) in complex with two [MgTc]+. Each inducer, binding tunnel has three constituent parts, one hydrophobic and two, hydrophilic ones. One of the hydrophilic contact areas binds Tc by, hydrogen bonding; the hydrophobic region correctly positions Tc and, partially closes the entrance to the binding tunnel; the second, hydrophilic region coordinates Mg2+, transduces the induction signal, and, completes the process of closing the tunnel entrance. Tc confers binding, specificity to TetR while Mg2+ is primarily responsible for induction:, After binding to the imidazole Nepsilon of His100, Mg2+ is octahedrally, coordinated to the 1,3-ketoenolate group of Tc and to three water, molecules. One of these waters forms a hydrogen bond to the hydroxyl group, Ogamma of Thr103. The induced 2.5 A movement of Thr103 results in the, partial unwinding of helix alpha6, associated with a lateral shift of, helices alpha4 and alpha9. They simultaneously close the tunnel entrance, and cause the DNA-binding domains to adopt a nonbinding conformation, leading to release of operator DNA and expression of the genes responsible, for resistance.
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