Expression in Escherichia coli of a TetK determinant from Staphylococcus aureus

The expression in Escherichia coli of a tet(K) gene, which originated in Staphylococcus aureus, was studied. The minimum inhibitory concentration (MIC) of tetracycline (TC) for E. coli cells that carried the tet(K) gene was only slightly higher than that for the recipient cells. This result indicated that the level of expression of the tet(K) gene in E. coli was very low. Insertion of a lac promoter into the upstream region of the tet(K) gene resulted in a slight increase in the MIC, from 12.5 to 50 micrograms/ml in the presence of isopropyl-beta-D-thiogalactopyronoside. An altered tet(K) gene, in which the initiation codon and the ribosome-binding site (RBS) were changed from TTG to ATG and from GAGG to GGAGG, respectively, and in which the distance between the RBS and the initiation codon was increased from 4 to 11 bases, was associated with high-level resistance to TC, with a MIC of 200 micrograms/ml. The MIC resembles that associated with expression of the tetA(B) gene of E. coli. These results indicate that the barrier to expression of the tet(K) gene in E. coli is located at the initiation of translation.