Characterization of a novel tetracycline resistance that functions only in aerobically grown Escherichia coli

Abstract

A tetracycline resistance (*Tc(r)) gene that was found orginally on two Bacteroides plasmids (pBF4 and pCP1) confers tetracycline resistance on Escherichia coli, but only when it is grown aerobically. Using maxicells, we have identified a 44-kilodalton protein which is encoded by the region that carries the *Tc(r) gene and which may be the *Tc(r) gene product. Localization experiments indicate that this 44-kilodalton protein is cytoplasmic. To determine whether the tetracycline resistance gene is expressed under anaerobic conditions, we have constructed a protein fusion between the *Tc(r) gene and LacZ. In strains of E. coli carrying the fusion, β-galactosidase activity was the same when the cells were grown under anaerobic conditions as when the cells were grown under aerobic conditions. This indicates that the tetracycline resistance gene product is made under anaerobic conditions but does not work. The failure of the *Tc(r) protein to function under anaerobic conditions was not due to a requirement for function of the anaerobic electron transport system, because neither nitrate nor fumarate added to anaerobic media restored tetracycline resistance. Inhibition of the aerobic electron transport system with potassium cyanide did not prevent growth on tetracycline of cells containing the *Tc(r) gene. A heme-deficient mutant, E. coli SHSP19, which carries the *Tc(r) gene, was still resistant to tetracycline even when grown in heme-free medium. These results indicate that functioning of the *Tc(r) gene product is not dependent on the aerobic electron transport system. Thus the requirement for aerobic conditions appears to reflect a requirement for oxygen. Spent medium from an E. coli strain carrying the *Tc(r) gene, which was grown in medium containing tetracycline (50 μg/ml), did not inhibit growth of a tetracycline-susceptible strain of E. coli. Thus, the *Tc(r) gene product may be detoxifying tetracycline.