Tetracycline resistance: Efflux, Mutations and, Other Mechanisms

Four mechanisms have evolved to counteract tetracycline. They are active efflux (keeping tetracycline out of the cytoplasm), inactivation of the tetracycline molecule, rRNA mutations (preventing tetracycline from binding to the ribosome), and ribosomal protection (preventing tetracycline from binding to the ribosome). Most of the genes that exclusively encode tetracycline efflux are positioned on transferable plasmids and/or transposons. Minicells are formed by abberant cell division in certain Escherichia coli mutants, contain only plasmid DNA, and synthesize only (radiolabelled) plasmid-encoded proteins. Two-dimensional (2D) crystallization was done with the Tet-6H fusion protein, which was over expressed in E. coli and purified as described, with a subsequent strong anion exchange chromatography step. Tet-6H was then reconstituted into a lipid bilayer to form protein arrays in two dimensions, which were negatively stained and examined by electron microscopy. Protection by tetracycline of (mutant) Cys residues from attack by N-ethylmaleimide (NEM) is another way of identifying the substrate binding site. Mutations of His257 in TM8 of TetA(B) permit downhill tetracycline transport in vesicles but no proton antiport, suggesting a role for this residue in proton exchange. Tetracycline resistance is inducible upon addition of a subinhibitory amount of tetracycline. The tet(K) and tet(L) genes are each inducible by tetracycline and are probably regulated by translation attenuation and translation reinitiation, respectively.