Effects of Irradiated and Reconstituted Uridine on Growth of a Neurospora Crassa Mutant and Synergism Between Uridine and Uracil

¹ From the Department of Physiology, Tufts University School of Medicine, Boston, Massachusetts

Solutions of uridine were irradiated with ultraviolet light for varying periods, resulting in a progressive loss of the selective ultraviolet absorption spectrum. Such absorption can be restored (‘reconstitution’) by treatment of the irradiated material with heat, acid, a combination of the two, and alkali, though the restored spectrum is not identical with spectrum of uridine. Further study of the ‘reconstituted’ material shows that both uridine and uracil were present in varying percentages, depending on the duration of irradiation, as was a small amount of ribose, and small amounts of two other unidentified U.V.-absorbing substances. The growth of a uridine-requiring mutant of Neurospora crassa is well supported by the ‘reconstituted’ material. Uracil will also support growth of the mutant, but only in much larger concentrations than is the case with uridine and after a latent period of several days. Uracil, in concentrations which alone are not adequate for immediate growth, will support growth if ‘catalytic’ amounts of uridine are added either simultaneously or previously. The possible mechanisms of this synergism are discussed.