R-factor Mediated Dihydrofolate Reductases which Confer Trimethoprim Resistance

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Abstract
Six different R-factors conferring trimethoprim resistance were isolated from a variety of sources. The trimethoprim-resistant dihydrofolate reductases (EC 1.5.1.3) from strains containing these R-factors were purified by (NH4)2 SO4 precipitation and DEAE-cellulose ion-exchange chromatography. The enzymes showed no significant differences in MW, pH profile, substrate profile, heat sensitivity, inhibition profile and Michaelis-Menten kinetics. There was considerable variation in the specific activity of these enzymes in the same bacterial host. When 2 Escherichia coli trimethoprim-sensitive dihydrofolate reductases were examined as controls, considerable differences between their properties and those of the enzymes mediated by R-factors were detected. One trimethoprim resistance gene could probably be spreading through the bacterial population, possibly situated on a transposon.