MECHANISMS UNDERLYING EXPRESSION OF TN10 ENCODED TETRACYCLINE RESISTANCE

Tetracycline-resistance determinants encoding active efflux of the dru g are widely distributed in gram-negative bacteria and unique with res pect to genetic organization and regulation of expression. Each determ inant consists of two genes called tetA and tetR, which are oriented w ith divergent polarity, and between them is a central regulatory regio n with overlapping promoters and operators. The amino acid sequences o f the encoded proteins are 43-78% identical. The resistance protein Te tA is a tetracycline/metal-proton antiporter located in the cytoplasmi c membrane, while the regulatory protein TetR is a tetracycline induci ble repressor. TetR binds via a helix-turn-helix motif to the two tet operators, resulting in repression of both genes. A detailed model of the repressor-operator complex has been proposed on the basis of bioch emical and genetic data. The tet genes are differentially regulated so that repressor synthesis can occur before the resistance protein is e xpressed, This has been demonstrated for the Tn10-encoded tet genes an d may be a common property of all tet determinants, as suggested by th e similar locations of operators with respect to promoters. Induction is mediated by a tetracycline-metal complex and requires only nanomola r concentrations of the drug. This is the most sensitive effector-indu cible system of transcriptional regulation known to date. The crystal structure of the TetR-tetracycline/metal complex shows the Tet repress or in the induced, non-DNA binding conformation. The structural interp retation of many noninducible TetR mutants has offered insight into th e conformational changes associated with the switch between inducing a nd repressing structures of TetR. Tc is buried in the core of TetR, wh ere it is held in place by multiple contacts to the protein.