The effect of photoreactivation on mutation frequency in Neurospora.

Kelner (1949a) first reported that mutations to phage resistance induced in Escherichia coli by ultraviolet light could be reversed by immediate irradiation with white light. In addition he noted that end-point mutations in contrast to zero-point mutations were not reversed by visible light (1949b). At the same time Novick and Szilard (1949) reported that light-reactivation of E. coli reduced the number of phage resistant mutants found after treatment with ultraviolet light. Recently Newcombe (1950) has tested for streptomycin resistant mutants of E. coli after photoreactivation. His results are essentially similar to the studies on phage resistance. These results indicated that the killing of the bacteria and the induction of mutations might be due to the same chemical effect produced by the ultraviolet irradiation. In view of the importance of this finding to our understanding of the mechanism of action of ultraviolet radiation, it was desired to test the effect of photoreactivation on another entirely different mutation. The mutation selected was the reversion of the inositol locus in Neurospora crassa. Giles and Lederberg (1948) studied the adaption from inositol dependence to independence using macroconidia of a Neurospora, strain no. 37401, and found that reverse mutation occurred at the inositol locus, and that the frequency of this mutation could be increased by ultraviolet or X-radiation. This reverse mutation was followed in our experiments by plating treated uninucleate microconidia.