Hydroxylamine-induced purple mutants (ad-3) in Neurospora crassa. II. Identification of genetic alteration at the molecular level.

Abstract

There is evidence that most hydroxylamine (HA)-induced mutants in Neurospora arise from a GC-to-AT base-pair transition. Evidence supporting this hypothesis was obtained by a study of the revertibility of HA-induced mutants after treatment with specific chemical mutagens. Hydroxylamine-induced ad-3 mutants of Neurospora crassa were tested for revertibility after treatment with nitrous acid (NA), ethyl methanesulfonate (EMS), O-methyl-hydroxylamine (OMHA), and 2-methoxy-6-chloro-9- [3-(ethyl-2-chloraethyl) amino propyl-amino] acridine dihydrochloride (ICR-170). Mutants not reverting after treatment with these mutagens were tested for revertibility after treatment with N-methyl-N'-nitro-N-nitroso-guanidine (MNNG). An analysis was made of mutants induced in a 2-component heterokaryon. Of this material 78.2% reverted by an AT-to-GC transition; 10% by a GC-to-AT transition, 4.5% by base-pair insertion or deletion; and 7.3% reverted spontaneously but not after treatment with any of the 5 mutagens. Thus, HA produces mainly GC-to-AT transitions when the treatment is carried out under conditions promoting this specificity. This means that HA has a mutagenic specificity in higher organisms similar to that observed in phage and transforming DNA from B. subtilis. HA can be used as an analytical tool for mutagenesis in higher organisms to characterize the mutagenic activity of agents with an unknown mutation mechanism. HA can also be used to produce mutants with AT at the mutant site. The spectra of genetic alterations among the ad-3B mutants with nonpolarized or polarized complementation patterns and among noncomplementing mutants do not differ from each other, because the majority of HA-induced mutants result from the same genetic alteration.