Classical and molecular genetic analyses of his-3 mutants of Neurospora crassa. II. Southern blot analyses and molecular mechanisms of mutagenicity.

Previous studies (Overton et al., Mutation Res., 1989) on specific revertibility of 81 his-3 mutants have shown a correlation between complementation pattern and presumed genetic alteration similar to that shown by ad-3B mutants. In the present study, restriction enzyme analyses were used to further characterize the genetic alterations in individual his-3 mutants. The restriction fragment banding patterns of the majority of mutants were identical with that shown by wild-type 74-OR23-1A and were consistent with expectations based on previous data suggesting that they resulted from single base-pair alterations (Overton et al., Mutation Res., 1989). His-3 mutants with altered banding patterns were only found among those with polarized complementation patterns or noncomplementing mutants. One of the mutants with a polarized complementation pattern, 1-189-83, and another noncomplementing mutant, 1-189-85, are associated with genetic alterations proximal to the his-3 locus. In one other mutant, 1-226-565 (with a polarized complementation pattern), an insertion of approx. 2 kb has occurred in the proximal region of the his-3 locus. Two other mutants, 1-155-270 and 1-155-276 (both noncomplementing), contained a large insertion of approx. 12.8 kb in the proximal region of the his-3 locus.