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The effect of trimethoprim [2,4-diamino-5(2′,4′,5′trimethoxybenzyl)-pyrimidine] in the presence of thymine onEscherichia coli B temperature-sensitive and non-temperature-sensitive Thy′ strains and a phosphodeoxyribomutase-negative mutant was studied. The inhibitory effect of 5 μg of trimethoprim per ml on the growth ofE. coli B was not overcome by thymine, thymidine, or thymidylate even in the presence of one-carbon metabolites and related metabolites. Deoxyribonucleic acid (DNA) and protein synthesis were more severely inhibited than ribonucleic acid (RNA) synthesis. The inhibition of DNA synthesis was partially reversed by addition of deoxyadenosine to increase the incorporation of exogenous thymine. By contrast, the inhibition of protein was not reversed even with one-carbon metabolites present, in keeping with the requirement for formylmethionyl-transfer RNAF for initiation. However, the inhibition of both DNA and protein synthesis in a phosphodeoxyribomutase-negative strain by 1 μg of trimethoprim per ml with thymine present was partially reversed by deoxyadenosine and one-carbon metabolites, and nearly normal growth occurred. 5-Fluorodeoxyuridine added at the time of addition of trimethoprim prevented the inhibition. Sulfadiazine in the presence of thymine inhibited both Thy+ and Thy− strains whereas trimethoprim (with thymine) did not inhibit Thy− organisms. The effect of trimethoprim on the incorporation of labeled thymine into DNA was also studied. These experiments support the concept that trimethoprim in conjunction with the action of thymidylate synthetase inhibits the growth of Thy+ cells because of a depletion of tetrahydrofolate. DNA synthesis is inhibited initially by a limitation of thymine nucleotide precursor, resulting from the indirect inhibition of thymidylate synthetase and the poor incorporation of exogenous thymine.

Effect of the Folic Acid Analogue, Trimethoprim, on Growth, Macromolecular Synthesis, and Incorporation of Exogenous Thymine inEscherichia coli