Pyrimidine Nucleotide Metabolism and Pathways of Thymidine Triphosphate Biosynthesis in Salmonella typhimurium

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The nucleoside triphosphate pools of two cytidine auxotrophic mutants of Salmonella typhimurium LT-2 were studied under different conditions of pyrimidine starvation. Both mutants, DP-45 and DP-55, are defective in cytidine deaminase and cytidine triphosphate (CTP) synthase. In addition, DP-55 has a requirement for uracil (uridine). Cytidine starvation of the mutants results in accumulation of high concentrations of uridine triphosphate (UTP) in the cells, while the pools of CTP and deoxy-CTP drop to undetectable levels within a few minutes. Addition of deoxycytidine to such cells does not restore the dCTP pool, indicating that S. typhimurium has no deoxycytidine kinase. From the kinetics of UTP accumulation during cytidine starvation, it is concluded that only cytidine nucleotides participate in the feedback regulation of de novo synthesis of UTP; both uridine and cytidine nucleotides participate in the regulation of UTP synthesis from exogenously supplied uracil or uridine. Uracil starvation of DP-55 in presence of cytidine results in extensive accumulation of CTP, suggesting that CTP does not regulate its own synthesis from exogenous cytidine. Analysis of the thymidine triphosphate (dTTP) pool of DP-55 labeled for several generations with 32P-orthophosphate and 3H-uracil in presence of 12C-cytidine shows that only 20% of the dTTP pool is derived from uracil (via the methylation of deoxyuridine monophosphate); 80% is apparently synthesized from a cytidine nucleotide.

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