Thymidine-requiring mutants of Salmonella typhimurium that are defective in deoxyuridine 5'-phosphate synthesis

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Abstract
In a S. typhimurium strain made diploid for the thy region by introduction of the Escherichia coli episome, F''15, mutants resistant to trimethoprim in the presence of thymidine were selected. One was defective in dUMP synthesis; it requires deoxyuridine or thymidine for growth and is sensitive to trimethoprim in the presence of deoxyuridine. Genetic studies showed that the mutant is mutated in 2 genes, dcd and dum, located at 70 and 18 min, respectively, on the Salmonella linkage map. The dcd gene cotransduces 95% with udk, the structural gene for uridine kinase. Both mutations are necessary to create a deoxyuridine requirement, providing evidence for the existence of 2 independent pathways for dUMP synthesis. Pool studies showed that a dum mutation by itself causes a small decrease in the dTTP pool of the cells, but a dcd mutation results in a much more marked decrease. The double mutant dcd dum, when incubated in the absence of deoxyuridine, contains barely detectable levels of dTTP. Enzyme analysis revealed that dcd encodes dCTP deaminase [EC 3.5.4.13]. The gene product of the dum gene was not yet identified; it does not encode subunit of ribonucleoside diphosphate reductase or dUTP pyrophosphatase. Mutants deleted for the dcd-udk region of the S. typhimurium chromosome were isolated.