Thymidine Metabolism in Neurospora crassa

Citation

Williams, Larry Gale (1968) Thymidine Metabolism in Neurospora crassa. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ZQ64-CE98. https://resolver.caltech.edu/CaltechTHESIS:04192018-163310702

Abstract

The metabolism of thymidine by Neurospora crassa was investigated after radioactive labeling experiments showed that thymidine is not specifically incorporated into the DNA of Neurospora; a finding in contrast to the results of labeled thymidine experiments with many other organisms. Labeling experiments in which 2-C¹⁴ deoxyuridine, 2-C¹⁴ thymidine and 2-C¹⁴ thymine were administered to Neurospora revealed that in each case ninety per cent of the nucleic acid label was in the RNA fraction. When thymidine methyl-H³ was administered no label was found in either the RNA or DNA. This and other evidence was taken as proof that Neurospora lacks phosphorylating enzymes for deoxyribose pyrimidine nucleosides but does possess enzymatic reactions by which these same compounds can be converted to RNA precursors.

Three mutants were isolated in which different synthetic steps are blocked in the pathway that converts the pyrimidine ring of thymidine to an RNA precursor. Evidence from genetic studies, nutri­tional tests and accumulation studies with the three mutant strains shows the pathway to proceed as follows: thymidine → thymine → 5-hydroxymethyluracil → 5-formyluracil → uracil → uridylic acid. A mutant strain in which the thymidine to thymine conversion is pre­vented is also unable to utilize deoxyuridine and deoxycytidine as pyrimidine sources and suggests a defective deoxyribose pyrimidine nucleosidase enzyme. A second mutation blocks the pathway at the 5-hydroxymethyluracil to 5-formyluracil step and causes the accumulation of thymine in the growth medium. The third mutation prevents the utilization of uracil and the compounds preceding it in the pathway.

Three mutants were isolated in which the pyrimidine transport system was affected. One of these mutants could utilize the pyrimidine nucleosides (cytidine, uridine, deoxyuridine and thymidine) but could use none of the free bases (thymine, 5-hydroxymethy luracil, 5-formyl­-uracil and uracil). A second mutant could utilize the free bases but not the nucleosides. These results can be most simply interpreted in terms of the first mutation blocking a transport system specific for pyrimidine bases, and the second mutation blocking a system specific for the transport of pyrimidine nucleosides. A third mutation pre­vented the utilization of both pyrimidine bases and nucleosides in a medium containing ammonium salts but permitted their use in a medium containing nitrate salts.

A strain was isolated carrying a mutation which influences the regulation of the thymidine to RNA precursor pathway. This mutation allows the steps of the pathway that convert thymidine to uracil to function in germinating conidia. A second mutation was found which suppresses the action of the first and restores the normal condition, the absence of the thymidine to uracil conversion in germinating conidia.

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