Williams L G, Mitchell H K
J Bacteriol. 1969 Oct;100(1):383-9. doi: 10.1128/jb.100.1.383-389.1969.
When (14)C-thymidine labeled only in the ring is administered to Neurospora crassa, the majority of the recovered label is found in the ribonucleic acid (RNA). Three mutants were isolated in which different steps are blocked in the pathway that converts the pyrimidine ring of thymidine to an RNA precursor. Evidence from genetic, nutritional, and accumulation studies with the three mutants 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 blocked is unable to metabolize thymidine appreciably by any route, including entry into nucleic acids. This suggests that Neurospora lacks a thymidine phosphorylating enzyme. A second mutation blocks the pathway at the 5-hydroxymethyluracil to 5-formyluracil step, whereas a third prevents utilization of uracil and all compounds preceding it in the pathway. The mutant isolation procedures yielded three other classes of mutations which are proposed to be affecting, respectively, regulation of the thymidine degradative pathway, transport of pyrimidine free bases, and transport of pyrimidine nucleosides.
当仅在环上标记的¹⁴C - 胸腺嘧啶核苷施用于粗糙脉孢菌时，回收的大部分标记物存在于核糖核酸（RNA）中。分离出了三个突变体，在将胸腺嘧啶核苷的嘧啶环转化为RNA前体的途径中，不同步骤被阻断。对这三个突变体进行的遗传学、营养和积累研究的证据表明，该途径如下进行：胸腺嘧啶核苷→胸腺嘧啶→5 - 羟甲基尿嘧啶→5 - 甲酰基尿嘧啶→尿嘧啶→尿苷酸。胸腺嘧啶核苷到胸腺嘧啶转化被阻断的突变菌株无法通过任何途径（包括进入核酸）明显代谢胸腺嘧啶核苷。这表明粗糙脉孢菌缺乏胸腺嘧啶核苷磷酸化酶。第二个突变在5 - 羟甲基尿嘧啶到5 - 甲酰基尿嘧啶步骤阻断了该途径，而第三个突变阻止了尿嘧啶及其在途径中位于其之前的所有化合物的利用。突变体分离程序产生了另外三类突变，分别被认为影响胸腺嘧啶核苷降解途径的调节、嘧啶游离碱的转运以及嘧啶核苷的转运。
