Na⁺/H⁺ antiport activity conferred by Bacillus subtilis tetA(L), a 5′ truncation product of tetA(L), and related plasmid genes upon Escherichia coli

An Escherichia coli transformant expressing the Bacillus subtilis tetA(L) gene from a weak promoter was challenged by growth on medium with low, increasing tetracycline concentrations. Changes in the substrate preference ratios of the TetA(L)-mediated resistances and antiports were examined in view of recent findings suggesting that TetA(L) catalyzes efflux of Na⁺ in exchange for protons in addition to having the ability to catalyze metal-tetracycline/H⁺ antiport. After growth of the transformant on 1 μg or more of tetracycline per ml for 12 to 15 h, the tetA(L) gene in the plasmid was found to be disrupted by an IS10 element 50 bp from the 5′ end of the coding sequence. This disrupted recombinant plasmid, pKB1, conferred greater tetracycline resistance and higher levels of membrane metal-tetracycline/proton antiport than the original plasmid, pJTA1, but conferred lower Na⁺ resistance and Na⁺/H⁺ antiport levels than the original plasmid. The results indicate that the 5′ end of the gene is necessary for optimal Na⁺/H⁺ antiport but that some such activity as well as robust tetracycline/H⁺ antiport persists in its absence. Two plasmid genes, tet(K) and qacA, were compared with tetA(L) vis-à-vis their abilities to enhance the Na⁺/H⁺ antiporter activity of everted vesicles from E. coli transformants. tet(K), which is more closely related to tetA(L), catalyzed ²²Na⁺ uptake by energized vesicles, whereas the less closely related qacA gene did not.