Effect of N2 on the mutagenic and lethal activities of ICR-170 in Neurospora crassa

The nature of the N₂ effect for ICR-170, i.e., the greater mutagenic and lethal activities of this agent in the presence of N₂ than O₂, has been studied at the ad-3 region of Neurospora crassa. The characteristics of the N₂ effect for ICR-170 were that (1) the N₂ effect with ICR-170 was displayed in conidia when N₂ was administered during, but not before or after, ICR-170 treatment, (2) the highly increased mutagenic and lethal activities of ICR-170 in the presence of N₂ were due to an anoxic condition rather than to the presence of N₂ per se, (3) the high killing activity of ICR-170 under N₂ was due largely to increased cytoplasmic inactivation, (4) the N₂ effect was a general phenomenon at the ad-3 region of N. crassa, and (5) the highly ICR-170-induced mutation in conidia under N₂ was attributable to an actual enhancement in the mutagenic activity of ICR-170 rather than to selective killing. With regard to the mechanisms of the N₂ effect with ICR-170, results indicate that this effect (1) was not due to more extracellular oxidative degradation of ICR-170 molecules in the presence of O₂, or to a greater uptake of ICR-170 by conidia under N₂, but (2) was due to the inhibition of conidial respiration under an anoxic environment.