Experimental Data Snapshot

  • Resolution: 2.20 Å
  • R-Value Free: 0.296 
  • R-Value Work: 0.232 
  • R-Value Observed: 0.232 

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Tetracycline-chelated Mg2+ ion initiates helix unwinding in Tet repressor induction.

Orth, P.Saenger, W.Hinrichs, W.

(1999) Biochemistry 38: 191-198

  • DOI: https://doi.org/10.1021/bi9816610
  • Primary Citation of Related Structures:  

  • PubMed Abstract: 

    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.

  • Organizational Affiliation

    Institut für Kristallographie, Freie Universität Berlin, Germany.

Find similar proteins by:  (by identity cutoff)  |  3D Structure
Entity ID: 1
MoleculeChains Sequence LengthOrganismDetailsImage
TETRACYCLINE REPRESSOR207Escherichia coliMutation(s): 0 
Find proteins for P0ACT4 (Escherichia coli)
Explore P0ACT4 
Go to UniProtKB:  P0ACT4
Entity Groups  
Sequence Clusters30% Identity50% Identity70% Identity90% Identity95% Identity100% Identity
UniProt GroupP0ACT4
Sequence Annotations
  • Reference Sequence
Experimental Data & Validation

Experimental Data

  • Resolution: 2.20 Å
  • R-Value Free: 0.296 
  • R-Value Work: 0.232 
  • R-Value Observed: 0.232 
  • Space Group: I 41 2 2
Unit Cell:
Length ( Å )Angle ( ˚ )
a = 69.1α = 90
b = 69.1β = 90
c = 182.93γ = 90
Software Package:
Software NamePurpose
X-PLORmodel building
DENZOdata reduction
SCALEPACKdata scaling

Structure Validation

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Entry History 

Deposition Data

Revision History  (Full details and data files)

  • Version 1.0: 1999-01-13
    Type: Initial release
  • Version 1.1: 2008-03-24
    Changes: Version format compliance
  • Version 1.2: 2011-07-13
    Changes: Derived calculations, Version format compliance
  • Version 1.3: 2023-08-02
    Changes: Database references, Other, Refinement description