The chromosomal tetracycline resistance locus of Bacillus subtilis encodes a Na⁺/H⁺ antiporter that is physiologically important at elevated pH

The chromosomal tetB(L) gene of Bacillus subtilis encodes a transporter that catalyzes Na⁺/H⁺ antiport even more actively than tetracycline/H+ antiport, as shown by assays of membrane antiporter activity upon transformation of Na⁺/H⁺ antiporter-deficient Escherichia coli with the cloned gene; the transformation results in a substantial increase in Na⁺ resistance as well as detectable resistance to low tetracycline concentrations. Transpositional disruption of the chromosomal tetB(L) locus of B. subtilis led to reduced rates of electrogenic Na⁺ efflux and revealed a physiological role for this locus in Na⁺ resistance and Na⁺-dependent pH homeostasis at pH 8.5. The mutant phenotype was reversed by transformation with a plasmid expressing the cloned tetB(L) gene. Energy-dependent tetracycline efflux rates in the wild type were greater than in the transposition mutant but were not sufficient to confer resistance to the antibiotic. TetB(L) is also inferred to have a modest capacity for K⁺ efflux, since the transposition mutant is slightly impaired in K⁺-dependent pH homeostasis at pH 8.5 and grew better than the wild type at pH 7 on limiting K⁺ concentrations.