Abstract
The transposon-like structure Tn4003 and related elements were found to encode high- and low-level trimethoprim resistance (Tpr) in Staphylococcus aureus and coagulase-negative staphylococci. By using transcriptional fusions in Escherichia coli, the variation in resistance levels was found to correlate with the transcriptional activity of the region presumed to carry the promoter for the operon containing the Tpr dihydrofolate reductase gene, dfrA, encoded by these elements. The reduced transcriptional activities exhibited by elements encoding low-level Tpr appear to be a consequence of deletions adjacent to the copy of IS257 which normally encodes the -35 sequences of these promoters. The data obtained not only support the involvement of IS257 in the transcription of the proposed thyE-dfrA-orf-140 operon of Tn4003 but may also implicate this insertion sequence in the mechanisms resulting in the variation in Tpr levels observed in staphylococci.
Highlights
Possible Role of Insertion Sequence IS257 in Dissemination and Expression of High- and Low-Level Trimethoprim Resistance in Staphylococci
In contrast to the TprH phenotype encoded by pSK1, the trimethoprim resistance (Tpr) determinant on the multiresistant conjugative plasmid pJE1, detected in an S. aureus strain isolated in Germany [12], confers resistance to only a low level of Tpr (TprL; MICs, >50 and .300,ug/ml) [7]
To gain further insights into the relationships between the plasmid-encoded Tpr determinants found in S. aureus and coagulase-negative staphylococci and to establish the basis of the variation in expression levels exhibited by different isolates, we undertook a molecular analysis of selected Tpr determinants found on plasmids from these organisms
Summary
Possible Role of Insertion Sequence IS257 in Dissemination and Expression of High- and Low-Level Trimethoprim Resistance in Staphylococci. The transposon-like structure Tn4003 and related elements were found to encode high- and low-level trimethoprim resistance (Tpr) in Staphylococcus aureus and coagulase-negative staphylococci. In contrast to the TprH phenotype encoded by pSK1, the Tpr determinant on the multiresistant conjugative plasmid pJE1, detected in an S. aureus strain isolated in Germany [12], confers resistance to only a low level of Tpr (TprL; MICs, >50 and .300 ,ug/ml) [7]. This determinant is flanked by three IS257 elements in an arrangement similar to that in Tn4003 [7]. To gain further insights into the relationships between the plasmid-encoded Tpr determinants found in S. aureus and coagulase-negative staphylococci and to establish the basis of the variation in expression levels exhibited by different isolates, we undertook a molecular analysis of selected Tpr determinants found on plasmids from these organisms
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