The Effect of Irradiated Medium, Cyanide and Peroxide on the Mutation Rate in Neurospora

Abstract

S have demonstrated that nutrient broth irradiated with ultra-violet light prior to inoculation of Staphylococcus aureus induces mutation in this organism. The rate of mutation of S. aureus to penicillin and streptomycin resistance and to inability to ferment mannitol can be increased by this method of indirect treatment of the bacterium with ultra-violet light.The discovery of the effect of irradiated medium upon bacteria has opened up a new avenue of attack upon the problem of induction of mutation and possibly mutation itself. It would be of some interest therefore, to extend this method of treatment to an organism other than Staphylococcus, preferably one in which a sexual cycle is easily demonstrated so that the mutations which are derived can be shown to fulfill the more conservative definitions of a mutation. The organism should also be one in which the gametes are easily exposed to a medium containing chemical compounds with a relatively short lifetime, and which lends itself to the analysis of mutation rates. Neurospora was chosen as the test organism because of its easily controlled sex cycle, and the fact that the gametic nuclei are easily exposed to treatment with chemicals just as are the cells of a bacterium.I n addition to testing the effects of irradiated medium on Neurospora, experiments were also performed using hydrogen peroxide and potassium cyanide. WYSS, have reported results which indicate that hydrogen peroxide may play an important role in the effect of the irradiated medium on the mutation rate. If this is so, then one might expect cyanide to be effective, since it is a known inhibitor of the hydrogen peroxide decomposing enzyme, catalase. Theoretically, poisoning the catalase should permit the peroxide concentration in the cell to build up to a higb enough level to have a mutagenic effect. GENERAL METHODS AND MATERIALSThe wild type strains of Neurospora crassa used in all of these experiments were derived from single ascospore cultures obtained from a cross between Emerson wild types 5297a and 5256A. All irradiation was done with a Hanovia double-U, SC-2537 ultra-violet mercury vapor lamp operating a t 120 milliamperes. The material was irradiated a t a distance of 15 cm from the lamp.Difco nutrient broth such as used in experiments described by STONE