The R-factor R388 mediates the production of a trimethoprim-resistant dihydrofolate reductase. This enzyme has a different molecular weight and pH profile to the trimethoprim-sensitive enzyme of the Escherichia coli host. The R-factor mediated enzyme was separated completely from the host E. coli enzyme by DEAE-cellulose ion-exchange chromatography. The purified R-factor enzyme was about 20 000 times less susceptible to trimethoprim than the E. coli enzyme and although it was inhibited competitively by trimethoprim, its inhibitor constant (Ki) was 20 000 times greater than that of the host enzyme. The R388 and E. coli enzymes also differed in their substrate specificity requirements. In addition, the R388 enzyme suprisingly conferred high level resistance to the broad spectrum dihydrofolate reductase inhibitor, amethopterin. The possible origins of the R388 enzyme are discussed.
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