AN EFFECT OF GENETIC BACKGROUND ON DOSE-RESPONSE CURVES IN NEUROSPORA CRASSA

1. 1. Two inositol and three adenine-3A alleles of Neurospora crassa were combined into diauxotrophic strains, and these were examined for their survival and reversion kinetics after treatment with two alkylating agents, diepoxybutane and ethyl methanesulphonate. The former produced very few inositol mutations, so that only survival and adenine reversions could be studied. Ethyl methanesulphonate produced both type of reversion in sufficient frequencies for analysis. 2. 2. The main result was a marked effect of genetic background on the kinetics of survival and reversion. In all strains with the inositol allele 37102, the survival curve had a long shoulder and the reversion curves were best fitted by equations of the form m = b+fdn, with n usually near 2. In all strains but one with the inositol allele 37401, the survival curve had a short shoulder and the reversion curves were best fitted by exponential equations of the form m = aecd. The one exception was a strain in which diepoxybutane produced one type of kinetic response, while ethyl methanesulphonate produced the other. Although ethyl methanesulphonate in any given strain gave the same type of reversion curve for the adenine and inositol allele, there were systematic, though minor, differences in shape between the two curves. 3. 3. These findings show that the dose-response curves cannot be entirely controlled by the reactions between mutagen and DNA but must include components that represent the effect of the genetic background on secondary steps in mutagenesis. The possible nature of these components is discussed, differences between strains in the efficiency of repair processes may play a role. © 1969.