Effect of trimethoprim on macromolecular synthesis in Escherichia coli.

Trimethoprim is a specific inhibitor of dihydrofolate reductase. When added to amino acid supplemented cultures of Escherichia coli, protein, ribonucleic acid (RNA), and deoxyribonucleic acid synthesis are rapidly inhibited. It is shown that thymidylate and purine synthesis must continuously occur if the inhibition is to be maintained. These results are interpreted as demonstrating that in vivo trimethoprim can reduce but not totally inhibit the action of dihydrofolate reductase and therefore reduce the amount of tetrahydrofolate derivatives available for formylmethionyl transfer RNA formylation, purine, and thymidylate synthesis.