Identification of genetic alterations induced by ethyl methanesulfonate in Neurospora crassa.

Abstract

Abstract The genetic alterations in ethyl methanesulfonate (EMS)-induced ad- 3 B mutants of Neurospora crassa have been identified by tests for specific revertibility after treatment with O -methyl-hydroxylamine (OMHA), nitrous acid (NA), ethyl methanesulfonate (EMS), and a monofunctional acridine mustard (ICR-170). Mutants classified as base-pair substitutions reverted after treatment with at least 1 of the first 3 compounds, whereas mutants classified as base-pair insertions or deletions reverted only after treatment with ICR-170. A random sample of 82 EMS-induced ad-3B mutants were analyzed; 76 were characterized by their reversion response, and 6 were excluded because of their high rate of spontaneous reversion or leaky growth. The mutants characterized as to reversion mechanism and type of unidentified genetic alteration consisted of (1) base-pair transitions: AT to GC, 41%, and GC to AT, 17%; (2) base-pair insertions or deletions, 9%; (3) nonrevertible, 7%; (4) mutants which only revert spontaneously and not after treatment with any of the four mutagens, 18%. It is likely that a part of the mutants in the latter class result from a base-pair substitution. Of the mutants which only reverted spontaneously, 13% out of the total of 18% had a nonpolarized complementation pattern and are therefore likely to result from a base-pair substitution. As a result of these considerations, a range from 58% to 79% of the EMS-induced mutants may result from single base-pair substitutions. The correlation between complementation pattern and genetic alteration at the molecular level found previously by us among NA-induced ad-3B mutants was also found within EMS-induced mutants. Mutants with nonpolarized complementation patterns resulted mainly from base-pair substitutions, whereas mutants with polarized complementation patterns and noncomplementing mutants are derived from a variety of genetic alterations. In addition, the data on the EMS-induced mutants provide further support for the hypothesis that an AT pair at the mutant site gives rise to polarized or noncomplementing mutants more often than a GC pair at the mutant site.