Mutations in multicopy Tn10 tet plasmids that confer resistance to inhibitory effects of inducers of tet gene expression.

Escherichia coli K-12 strains that carry the Tn10 tetracycline resistance determinant (tet) on multicopy plasmids are hypersensitive to 5a,6-anhydrotetracycline and heated chlortetracycline, two tetracycline derivatives that are relatively more effective as inducers of tet gene expression than as inhibitors of bacterial growth. Twenty spontaneous mutations that confer resistance to anhydrotetracycline (Atr) and resistance to heated chlortetracycline (Ctr) were isolated and characterized. All of these Atr mutations are located in the Tn10 tet region; the majority (18 of 20) have no effect on tetR repressor function. Atr mutations can increase, reduce, or eliminate the phenotypic expression of plasmid tetracycline resistance (Tcr). IS insertions that result in an Atr Tcs phenotype are clustered in a 150-base-pair promoter-proximal region of the tetA resistance gene. Some Atr mutations reduce expression of the tetA gene by altering either the tetR repressor or the tetA promoter. In addition, it appears that E. coli cannot tolerate constitutive expression of the wild-type tetA gene from a multicopy plasmid containing a tetR deletion. These observations support the proposal that high level expression of the 36-kilodalton tetA gene product inhibits the growth of E. coli. We speculate that this inhibition is related to the interaction of the tetA gene product with the cytoplasmic membrane.