title: Tetracycline repressor allostery does not depend on divalent metal recognition.
abstract: Genes that render bacteria resistant to tetracycline-derived antibiotics are tightly regulated by repressors of the TetR family. In their physiologically relevant, magnesium-complexed form, tetracyclines induce allosteric rearrangements in the TetR homodimer, leading to its release from the promoter and derepression of transcription. According to earlier crystallographic work, recognition of the tetracycline-associated magnesium ion by TetR is crucial and triggers the allosteric cascade. Nevertheless, the derivative 5a,6-anhydrotetracycline, which shows an increased affinity for TetR, causes promoter release even in the absence of magnesium. To resolve this paradox, it has been proposed that metal-free 5a,6-anhydrotetracycline acts via an exceptional, conformationally different induction mode that circumvents the normal magnesium requirement. We have tested this hypothesis by determining crystal structures of TetR-5a,6-anhydrotetracycline complexes in the presence of magnesium, ethylenediaminetetraacetic acid, or high concentrations of potassium. Analysis of these three structures reveals that, irrespective of the metal, the effects of 5a,6-anhydrotetracycline binding are indistinguishable from those of canonical induction by other tetracyclines. Together with a close scrutiny of the earlier evidence of a metal-triggered mechanism, these results demonstrate that magnesium recognition per se is not a prerequisite for tetracycline repressor allostery.
authors.affiliation: Department of Molecular Structural Biology, Institute for Biochemistry, University of Greifswald , Felix-Hausdorff-Strasse 4, D-17487 Greifswald, Germany.
authors.forename: Sebastiaan
authors.initials: S
authors.lastname: Werten
authors.forename: Daniela
authors.initials: D
authors.lastname: Dalm
authors.forename: Gottfried Julius
authors.initials: GJ
authors.lastname: Palm
authors.forename: Christopher Cornelius
authors.initials: CC
authors.lastname: Grimm
authors.forename: Winfried
authors.initials: W
authors.lastname: Hinrichs
mesh_terms: D000494:Allosteric Regulation / Q000502:physiology; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D008274:Magnesium / Q000737:chemistry; D011485:Protein Binding; D055503:Protein Multimerization; D017434:Protein Structure, Tertiary; D012097:Repressor Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D013754:Tetracyclines / Q000737:chemistry
chemical_list: D029968:Escherichia coli Proteins; D012097:Repressor Proteins; D013754:Tetracyclines; C016229:4-epianhydrotetracycline; D008274:Magnesium