We inserted the Tn10 tetracycline resistance determinant (tet) into the multicopy plasmid pACYC177, and we examined the phenotype of Escherichia coli K-12 strains harboring these plasmids. In agreement with others, we find that Tn10 tet exhibits a negative gene dosage effect. Strains carrying multicopy Tn10 tet plasmids are 4- to 12-fold less resistant to
tetracycline than are strains with a single copy of Tn10 in the bacterial chromosome. In addition, we find that multicopy tet strains are 30- to 100-fold less resistant to the
tetracycline derivative
5a,6-anhydrotetracycline than are single-copy tet strains. Multicopy tet strains are, in fact, 10- to 25-fold more sensitive to
anhydrotetracycline than are strains that lack tet altogether. The
hypersensitivity of multi-copy strains to
anhydrotetracycline is correlated with the effectiveness of
anhydrotetracycline as an inducer of tet gene expression, rather than its effectiveness as an inhibitor of
protein synthesis.
Anhydrotetracycline is 50- to 100-fold more effective than
tetracycline as an inducer of tetracycline resistance and as an inducer of
beta-galactosidase in strains that harbor tet-lac gene fusions. In contrast,
anhydrotetracycline appears to be two- to fourfold less effective than
tetracycline as an inhibitor of
protein synthesis. Both
anhydrotetracycline and
tetracycline induce synthesis of tet
polypeptides in minicells harboring multicopy tet plasmids. Differences between E. coli K-12 backgrounds influence the
tetracycline and
anhydrotetracycline sensitivity of multicopy strains; ZnCl2 enhances the
tetracycline and
anhydrotetracycline sensitivity of these strains two- to threefold. We propose that the overexpression of one or more Tn10 tet gene products inhibits the growth of multicopy tet strains and accounts for their relative sensitivity to inducers of tet gene expression.