Characterization of mutations contributing to sulfathiazole resistance in Escherichia coli

A sulfathiazole-resistant dihydropteroate synthase (DHPS) present in two different laboratory strains of Escherichia coli repeatedly selected for sulfathiazole resistance was mapped to folP by P1 transduction. The folP mutation in each of the strains was shown to be identical by nucleotide sequence analysis. A single C-->T transition resulted in a Pro-->Ser substitution at amino acid position 64. Replacement of the mutant folP alleles with wild-type folP significantly reduced the level of resistance to sulfathiazole but did not abolish it, indicating the presence of an additional mutation(s) that contributes to sulfathiazole resistance in the two strains. Transfer of the mutant folP allele to a wild-type background resulted in a strain with only a low level of resistance to sulfathiazole, suggesting that the presence of the resistant DHPS was not in itself sufficient to account for the overall sulfathiazole resistance in these strains of E. coli. Additional characterization of an amplified secondary resistance determinant, sur, present in one of the strains, identified it as the previously identified bicyclomycin resistance determinant bcr, a member of a family of membrane-bound multidrug resistance antiporters. An additional mutation contributing to sulfathiazole resistance, sux, has also been identified and has been shown to affect the histidine response to adenine sensitivity displayed by these purU strains.

Genetic analysis suggests functional interactions between the N- and C-terminal domains of the TetA(C) efflux pump encoded by pBR322

Genetic analysis of the tetA(C) gene of pBR322 indicates the importance of two-cytoplasmic loops in the TetA(C) protein (P. McNicholas, I. Chopra, and D. M. Rothstein, J. Bacteriol. 174:7926-7933, 1992). In this study, we characterized second-site suppressor mutations that suggest a functional interaction between these two cytoplasmic regions of the protein.

Mutations in the tetA(B) gene that cause a change in substrate specificity of the tetracycline efflux pump

The tetA(B) gene from transposon Tn10 fails to mediate resistance to the novel tetracycline analog 9-(dimethylglycylamido)minocycline (DMG-Mino) (P. E. Sum, V. J. Lee, R. T. Testa, J. J. Hlavka, G. A. Ellestad, J. D. Bloom, Y. Gluzman, and F. P. Tally, J. Med. Chem. 37:184-188, 1994; R. T. Testa, P. Petersen, N. V. Jacobus, P. E. Sum, V. J. Lee, and F. P. Tally, Antimicrob. Agents Chemother. 37:2270-2277, 1993). Mutations in either of two codons of tetA(B) that resulted in increased resistance to DMG-Mino also caused diminished resistance to tetracycline, identifying amino acid residues critical for the recognition of tetracycline.

The tet(K) gene of plasmid pT181 of Staphylococcus aureus encodes an efflux protein that contains 14 transmembrane helices

The corrected DNA sequence of the tet(K) structural gene, encoding the tetracycline efflux protein from Staphylococcus aureus, is reported. The Tet(K) protein is homologous to related tetracycline efflux proteins throughout the protein, including at the C-terminal end. Hydropathy plotting now clearly indicates that the Tet(K) protein contains 14 transmembrane alpha-helices.

The tet(K) gene from Staphylococcus aureus mediates the transport of potassium in Escherichia coli

The tet(K) gene, encoding the tetracycline efflux protein from Staphylococcus aureus, mediates the transport of potassium in an Escherichia coli mutant defective in potassium uptake. Deletion mapping indicates that the first third of the tet(K) gene is sufficient to mediate potassium transport.

Expression of the tetK gene from Staphylococcus aureus in Escherichia coli: comparison of substrate specificities of TetA(B), TetA(C), and TetK efflux proteins

The tetK gene, which encodes a tetracycline efflux pump from Staphylococcus aureus, was expressed in Escherichia coli by using an inducible, low-level expression system. The tetK gene, as well as the tetA(B) gene from the transposon Tn10 and the tetA(C) gene from plasmid pBR322, was subjected to the regulatory control of the lac repressor, and resistance to tetracycline was measured as a function of the isopropyl-beta-D-thiogalactopyranoside concentration. The maximum resistance of the E. coli strain containing the tetK construct was comparable to the maximum resistance of the strain containing the tetA(C) construct but was less than the resistance of the strain containing the tetA(B) construct. Overexpression of the tetK, tetA(B), or tetA(C) genes was toxic. When expression was regulated so that resistance to tetracycline was comparable, then the TetA(B) and TetA(C) proteins conferred very similar levels of resistance to a variety of tetracycline derivatives. In contrast, the TetK protein was less capable of conferring resistance to the tetracycline derivatives minocycline, 6-deoxy-6-demethyltetracycline, and doxycycline. The implications for the recognition of various tetracycline substituents by the TetK protein are discussed.

Gene amplification contributes to sulfonamide resistance in Escherichia coli

A sulfathiazole-resistant strain of Escherichia coli was isolated and shown to contain a fourfold tandemly amplified segment of DNA 18 kilobase pairs in length in addition to a mutationally altered dihydropteroate synthase, the target enzyme for sulfonamide inhibition. The amplified DNA contained a gene designated sur that contributed to sulfathiazole resistance when present in greater amounts than those in the wild type. Sulfathiazole resistance was markedly decreased upon loss of the amplified DNA after nonselective growth. Plasmids that contained sur also conferred only weak sulfathiazole resistance on wild-type strains. Comparison of the restriction maps of the amplified DNA, wild-type DNA, and sur-containing plasmids showed that a DNA rearrangement occurred before or concomitant with the DNA amplification event. The DNA rearrangement resulted from an IS5 insertion, which, in conjunction with an IS5 element residing near sur in the wild-type strain, resulted in an -IS5-sur-IS5- configuration. Homologous recombination could account for duplication and subsequent amplification of the sur region. High-copy-number plasmids containing the sur locus did not express a sulfathiazole-resistant dihydropteroate synthase, nor did they overexpress wild-type dihydropteroate synthase. These data suggest that the high level of sulfathiazole resistance in this strain results from a synergistic effect of two different mutations.