Relative retention of H3 and C14 labels of nucleosides incorporated into nucleic acids of Neurospora.

Reichard and E&born (1) demonstrated that rats selectively incorporated thymidine-N16 as deoxyribonucleic acid thymidine, and their observation has been extended by numerous investigators to include many organisms and several isotopic labels. Tritiated thymidine has been particularly useful as a means of marking DNA for study by high-resolution radioautography (Z), and the histological distribution of the radioactivity has followed the pattern expected for selective DNA labeling in all but a few experiments, such as those of Stocking and Gifford (3) and Plaut and Sagan (4). The latter workers deprived their organisms of food and employed DNAse and RNAse in confirmatory tests to obtain evidence that the cytoplasmic radioactivity was associated with DNA and probably did not represent food particles. When it was found that Neurospora crassa utilized thymidine2-cl4 and thymidine-5, g-Cl4 primarily for RNA synthesis (5, 6), it seemed of particular interest to determine the metabolic fate of tritiated thymidine in this organism. The present paper reports that tritium attached to carbon 6 of various pyrimidines was retained during their transformation to DNA thymine, and that Neurospora produced highly radioactive RNA and DNA from biologically synthesized thymidine-6-H” but not from a commercial preparation of thymidine-methyl-H3 that had been purchased as “thymidine (H3 on C-6).” These results, together with studies of possible intermediates in the utilization of thymidine for RNA synthesis, have been summarized in preliminary papers (7,s).