Röntgenkristallographische Untersuchungen zum enzymatischen Abbau von Tetracyclinen durch die Monooxygenase TetX sowie zur Erkennung natürlicher Tetracyclin Abbauprodukte durch den Tet Repressor Tetracyclin Tetracyclinrepressor Resistenz Monooxygenase tetracycline antibiotic resistance degradation monooxygenase The monooxygenase TetX was identified in Bacteroides sp. TetX hydroxylates tetracyclines which leads to loss of antibiotic properties and to tetracycline resistance of aerobic bacteria. The crystal structure of TetX was solved by anomalous dispersion on a selenomethionine derivative. Complexes of TetX with different tetracyclines were solved and show tetracycline recognition by TetX in detail. The binding of non enzymatic tetracycline degradation products (isotetracyclines) to the Tet repressor was investigated by surface plasmon resonance and crystal structure analyses.