THE METABOLISM OF PURINES AND PYRIMIDINES BY GROWING YEAST

In order to extend previous studies carried out in this laboratory on the metabolism of purines and pyrimidines (l), bakers’ yeast was selected as a suitable material for metabolic tracer experiments because of its rapid growth and high ribose nucleic acid content. When these experiments were begun, the nature of the carbon precursors of the uric acid excreted by the pigeon (2, 3) and of the purines of the nucleic acids of the rat (1) had been established. In addition, the nitrogen of glycine had been shown to be a precursor of nitrogen 7 of uric acid in man (4), and of the yeast, Todopsis utilis (5). Recently bacteria have been shown to utilize glycine for purine synthesis (6). The role of preformed purines in the biosynthesis of the nucleic acid of the rat has been studied extensively by Brown and coworkers (7-9). Studies of this type have now been extended to yeast (10) by Kerr et al., who have shown both adenine and guanine to be utilized by Saccharomyces wrevisiae for nucleic acid synthesis. Knowledge of the biosynthesis of pyrimidines remains more limited. Heinrich and Wilson (1) showed that CO2 is the precursor of the ureide carbon of uracil and thymine. Neither formate nor glycine was observed to be incorporated into the pyrimidines of the nucleic acids of the rat in their experiments. LePage and Heidelberger (11) have also failed t,o find significant incorporation of glycine-2-Cl4 into pyrimidines. The methyl group of thymine has been shown to arise from the P-carbon of serine and the a-carbon of glycine by Elwyn and Sprinson (12). Totter et al. (13) have reported the incorporation of radioformate into thymine, and a partial degradation indicated that the radioactivity was very likely in the methyl carbon. That formate is necessarily the intermediate in the transfer of the P-carbon of serinc to the methyl group of thymine has been * E’rom the dissertation