The incorporation of the carbon skeleton of cytidine into the pyrimidine nucleosides of ribonucleic acid and deoxyribonucleic acid by Neurospora.

Experiments have been carried out with Neurospora which demonstrate that the quantitatively important route for the biosynthesis of the pyrimidine deoxyribonucleosides in the DNA* of this organism involves the entire carbon skeleton of a pyrimidine ribonucleoside. A very high labeling of the pyrimidine nucleosides of the RNA and DNA was brought about by feeding uniformly labeled CY4-cytidine to a pyrimidine-requiring mutant of this organism, grown in the presence of sucrose. The specific activities of the deoxyribose in the deoxycytidine and thymidine of the DNA and the ribose in the cytidine and uridine of the RNA were quite similar, nor were they very different, carbon atom for carbon atom, from that of the corresponding pyrimidine ring. It is clear, therefore, that the carbon skeleton of the pyrimidine nucleosides of the DNA has arisen as such from an intact pyrimidine ribonucleosyl intermediate. The likelihood of this direct conversion was fully appreciated by Hammarsten et al. (l), and Rose and Schweigert (2) demonstrated that such a conversion can take place in the rat. It is the purpose of this article to show that in Neurospora such a conversion is the important route for the formation of the pyrimidine nucleosides of DNA. In a subsequent article evidence is presented to the effect that the purine deoxyribonucleosides also arise from an intact purine ribonucleosyl intermediate (3).