Reduced Expression of Tn10-mediated Tetracycline Resistance in Escherichia coli Containing More Than One Copy of the Transposon

Abstract

In Escherichia coli a single copy of Tn10 confers high-level resistance to tetracycline. Resistance itself results from expression of three distinct mechanisms which normally act together (Shales et al., 1980). In cells containing two copies of Tn10, the level of resistance to tetracycline was reduced. This was not due to overproduction of the repressor which controls the resistance genes, because strains diploid for an operator-constitutive allele of Tn10 also exhibited reduced expression of resistance. The negative gene dosage effect resulted from decreased expression of two mechanisms (1 and 2) consequent on enhanced expression of the third mechanism. The net result of increasing the copy number was a decrease in resistance because mechanism 3 was less efficient than mechanisms 1 and 2 in protecting the cell against tetracycline. The DNA sequence responsible for the reduced expression of resistance was contained in the internal BglII fragment of Tn10. This sequence, which is probably unique to Tn10, may encode the protein which mediates mechanism 3. Elevated levels of this protein probably cause decreased expression of mechanisms 1 and 2.

Differential mRNA stability controls relative gene expression within the plasmid-encoded arsenical resistance operon

“…This revealed a clear difference in the rate of degradation of the transcripts ( mRNA degradation (1, 3, 4, 12, 13, 21). Differential expression is especially prevalent when one or more of the gene products is an integral membrane protein (2,8,10,20), as is the case with arsB (26). The molecular mechanism responsible for the decreased expression of these proteins is obscure.…”

Amplification of lac Cannot Account for Adaptive Mutation to Lac + in Escherichia coli

“…To test this prediction, Hendrickson et al (28) exploited the fact that cells with one copy of the transposon Tn10 are Tc r , but cells with multiple copies of Tn10 become Tc s . This is because Tc induces the tetA gene that is carried on the transposon, and overproduction of the efflux transporter encoded by tetA is toxic (11,15).…”

Overproduction of transposon Tn10-encoded tetracycline resistance protein results in cell death and loss of membrane potential

“…Its localization within the bacterial cell inner membrane (13) is in agreement with such a function. A surprising finding was that strains with transposon TnWO located on a high-copy-number plasmid exhibited lower levels of resistance to tetracycline than did strains which carry the tet genes in a low-copy state (5,6). Moreover, this negative gene dosage effect was correlated with overexpression of the tetA gene (16,17).…”