Incorporation of Uracil-2-C14 into the Nucleic Acids of Neurospora crassa

The pyrimidine-deficient mutant (1298) of Neurospora crassa, utilizes erotic acid, uracil, uridine, or cytidine for growth, but neither cytosine nor thymine (1). It seems likely, therefore, that this mutant can convert erotic acid and uracil to the pyrimidine bases of ribonucleic acid and deoxyribonucleic acid, and can utilize the nucleoside directly for ribonucleic acid biosynthesis and for conversion to deoxycytidine and thymidine of deoxyribonucleic acid. As the latter two nucleosides have not been studied before as possible growth supplements in pyrimidine-deficient Neurospora, it has not been determined previously whether these compounds might also function as sources of pyrimidine components for RNA, or for DNA, or for both types of nucleic acids. Recently, from experiments with uniformly labeled C14-cytidine in the pyrimidine-deficient Neurospora mutant 36601, McNutt (2) has demonstrated that this nucleoside is incorporated into RNA and is also converted to uridine, deoxycytidine, and thymidine. A surprising result, however, was that the specific activities of the nucleosides isolated from the recovered RNA and DNA were only about 70% of that of the starting cytidine. It was concluded that this mutant has a considerable capacity for synthesizing the pyrimidine nucleosides, even though provided with an outside pyrimidine source. By studying the incorporation of uracil-2-W into the nucleic acids of the mutant 1298, it seemed possible to determine whether uracil utilization was the only pathway of pyrimidine biosynthesis in this organism, or whether some other pathway might also be involved. In the latter event the pyrimidine components of RNA and DNA would be expected to show decreased specific activities as compared to that of the initial uracil-2-C14. Similarly, if deoxycytidine or thymidine were utilized for DNA synthesis only, it should be possible to label RNA specifically with uracil-2-C14. In the present paper we have studied the availability of deoxyuridine, deoxycytidine, and thymidine for growth of mutant 1298, as well as the supplementary growth effects of the above nucleosides and of uridine and cytidine, in the presence of a minimal amount of uracil. The utilization of uracil-2-Cl4 for the formation of uracil, cytosine, thymine, adenine, and guanine,