The most frequently occurring resistance of Gram-negative bacteria against tetracyclines is triggered by drug recognition of the Tet repressor. This causes dissociation of the repressor-operator DNA complex and enables expression of the resistance protein TetA, which is responsible for active efflux of tetracycline. The 2.5 angstrom resolution crystal structure of the homodimeric Tet repressor complexed with tetracycline-magnesium reveals detailed drug recognition. The orientation of the operator-binding helix-turn-helix motifs of the repressor is inverted in comparison with other DNA binding proteins. The repressor-drug complex is unable to interact with DNA because the separation of the DNA binding motifs is 5 angstroms wider than usually observed.

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The excessive use of antibiotics has enabled bacteria to develop resistance through a variety of mechanisms. The most common bacteriostatic action of the broad‐spectrum antibiotic tetracycline (Tc) is by the inactivation of the bacterial ribosome so that the protein biosynthesis is interrupted and the bacteria die. The most common mechanism of resistance in gram‐negative bacteria against Tc is associated with the membrane‐intrinsic protein TetA, which exports invaded Tc out of the bacterial cell before it can attack its target, the ribosome. The expression of TetA is tightly regulated by the homodimeric Tet repressor (TetR)2, which binds specifically with two helix‐turn‐helix motifs of operator DNA (tetO; Kass≈1011 M−1) located upstream from the tetA gene on a plasmid or transposon. When Tc diffuses into the cell it chelates Mg2+ and the complex [MgTc]+ binds to (TetR)2 to form the induced complex (TetR⋅[MgTc]+)2. This process is associated with conformational changes, which sharply reduce the affinity of (TetR)2 to tetO, so that expression of TetA can take place, thus conferring resistance to the bacteria cells against Tc. Crystallographic studies show sequence‐specific protein–nucleic acid interactions in the (TetR)2⋅tetO complex and how the binding of two [MgTc]+ to (TetR)2 enforces conformational changes that are stabilized by cooperative binding of two chains of eight water molecules each so that the formed (TetR⋅[MgTc]+)2 is no longer able to recognize and bind to tetO. Since the switching mechanisms of the TetR/[MgTc]+ system is so tight, it has proven very useful in the regulation of eukaryotic gene expression and may also be applicable in gene therapy.