Relation between complementation patterns and genetic alterations in nitrous acid-induced ad-3B mutants of Neurospora crassa

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An attempt has been made to determine the association between genetic alterations and complementation patterns among ad-3B mutants of Neurospora crassa The different types of genetic alterations in individual mutants have been identified at the molecular level through tests for specific revertibility after treatment with nitrous acid (NA), ethyl methanesulfonate (EMS), and a monofunctional acridine mustard (ICR-170). Base-pair substitution mutants revert after treatment with NA and/or EMS. Base-pair insertion and deletion mutants revert after treatment with ICR-170 and are not usually revertible with the 2 other mutagens. Genetic alterations induced by NA have been identified in 126 of the 141 NA-induced mutants analyzed. The spectrum of genetic alteration is as follows: base-pair substitution, 74.6%; base-pair insertion and deletion, 8.5%; mutants reverting only spontaneously, 9.9%; and mutants not reverting at all, 6.8%. The frequency of the 4 classes of genetic alteration differs markedly among mutants with different complementation responses: 1. (1) Mutants with nonpolarized complementation patterns—92.0% base-pair substitutions and 8.0% reverting only spontaneously. 2. (2) Mutants with polarized complementation patterns—56% base-pair substitutions, 12% base-pair insertions or deletions, 12% reverting only spontaneously and 20% not reverting at all. 3. (3) Noncomplementing mutants—46.5% base-pair substitutions, 25.6% base-pair insertions or deletions, 11.6% reverting only spontaneously and 16.3% not reverting at all. The above association has been interpreted as an indication that the non-polarized mutants result from missense mutations. Both the polarized and the non-complementing mutants derive from a variety of genetic alterations. The data agree with the hypothesis that some of the mutants in the latter 2 groups produce part of a polypeptide fragment or no peptide at all. In several cases it was found that mutants with the same complementation pattern also have the same revertibility with NA, EMS, and ICR-170. We have concluded that since such mutants arose independently, they must have resulted from alterations of identical sites, or “hot spots”, in the ad-3B locus.