Attempts were made to identify at the molecular level the genetic alterations in x-ray-induced ad-3B mutants of Neurospora crassa obtained in three separate experiments after doses of 710, 850, 1420, and 1700 R. Revertibility after treatment with nitrous acid (NA), N-methyl-Nā€²-nitro-N-nitrosoguanidine (MNNG), methoxylamine (OMHA), and a monofunctional acridine mustard (ICR-170) was used to identify genetic alterations in the individual mutants. Base-pair transition mutants revert after treatment with NA and/or MNNG and/or OMHA. Base-pair insertion and deletion mutants revert after treatment with ICR-170 and are not usually revertible with the other three mutagens. A total of 135 ad-3B mutants were obtained after irradiation of heterokaryotic conidia from a biochemically marked, balanced heterokaryon. A total of 101 of these mutants could be tested for induction of revertants; the rest were too leaky or were aconidial. Among the mutants tested, the spectrum of genetic alterations induced by x-rays in the wild-type strain was as follows: base-pair transitions, 42% (10% GC to AT, 32% AT to GC); base-pair insertion or deletions, 37%; mutants reverting only spontaneously, 15%; and nonrevertible mutants, 6%.

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