TetX Is a Flavin-dependent Monooxygenase Conferring Resistance to Tetracycline Antibiotics

Abstract

The tetracycline antibiotics block microbial translation and constitute an important group of antimicrobial agents that find broad clinical utility. Resistance to this class of antibiotics is primarily the result of active efflux or ribosomal protection; however, a novel mechanism of resistance has been reported to be oxygen-dependent destruction of the drugs catalyzed by the enzyme TetX. Paradoxically, the tetX genes have been identified on transposable elements found in anaerobic bacteria of the genus Bacteroides. Overexpression of recombinant TetX in Escherichia coli followed by protein purification revealed a stoichiometric complex with flavin adenine dinucleotide. Reconstitution of in vitro enzyme activity demonstrated a broad tetracycline antibiotic spectrum and a requirement for molecular oxygen and NADPH in antibiotic degradation. The tetracycline products of TetX activity were unstable at neutral pH, but mass spectral and NMR characterization under acidic conditions supported initial monohydroxylation at position 11a followed by intramolecular cyclization and non-enzymatic breakdown to other undefined products. TetX is therefore a FAD-dependent monooxygenase. The enzyme not only catalyzed efficient degradation of a broad range of tetracycline analogues but also conferred resistance to these antibiotics in vivo. This is the first molecular characterization of an antibiotic-inactivating monooxygenase, the origins of which may lie in environmental bacteria.

The effects of subtherapeutic antibiotic use in farm animals on the proliferation and persistence of antibiotic resistance among soil bacteria

“…Both tet(A) and tet(L) genes encode for efflux pumps that are predominantly associated with gram-negative and gram-positive bacteria, respectively (Chopra and Roberts, 2001;Aminov et al, 2002). The detection of tet(X) in a Sphingobacteria spp., which encodes for a tetracycline-degrading mono-oxygenase (Yang et al, 2004), is also interesting because this gene, thus far, has been found only in Bacteroides spp. (Whittle et al, 2001), which are obligate anaerobes, perhaps suggesting that we have found the natural host of this gene.…”

Prevalence and molecular characterization of chloramphenicol resistance inRiemerella anatipestiferisolated from ducks and geese in Taiwan

“…TetX is a FAD-dependent mono-oxygenase. The enzyme not only catalyses efficient degradation of a broad range of tetracycline analogues, but also confers resistance to these antibiotics in vivo (Yang et al, 2004). Bacterial ABC transporters involving drug resistance are mainly drug-specific transporters, and a number of them are found in antibiotic-producing organisms, mainly Streptomyces spp., to ensure self-resistance to drugs they produce.…”

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“…Redox process is a less common mechanism which involves flavin-dependent monoxygenase enzyme TetX. This enzyme transfers a single hydroxyl group to tetracycline and makes it less able to sequester Mg+ ions which are critical for binding of tetracycline to its bacterial target (Yang et al, 2004).…”