AN UNSTABLE ALLELE OF THE am LOCUS OF NEUROSPORA CRASSA

John A. Kinsey ¹ and J. R. S. Fincham ²

¹ Department of Microbiology, University of Kansas Medical Center, Kansas City, Kansas 66103
² Department of Genetics, University of Edinburgh, Edinburgh EH9 3JN, United Kingdom

The mutant strain am₁₂₆ was isolated, using the direct selection procedure, after nitrous acid mutagenesis. It produced neither measurable NADP-dependent glutamate dehydrogenase (GDH) nor immunologically cross-reacting material. That the am₁₂₆ strain produced some form of GDH product was shown by the fact that it complemented several other am mutant strains. The GDH formed by complementation between am₁₂₆ and each of two other am mutants was relatively thermolabile, but could not be distinguished from wild-type GDH by electrophoresis in polyacrylamide gels. This, together with the relatively high yield of the complementation enzymes, suggests that the am₁₂₆ product is a polypeptide chain not grossly abnormal in structure. The spontaneous revertant frequency was between 0.3 and 3 prototrophic revertants per 10⁵ live cells. This frequency was at least 40 times greater than that for am₁₉, which had the second highest spontaneous revertant frequency among the mutants tested. Neither meiosis nor mutagenesis increased the revertant frequency, nor did incubation at elevated temperatures lower it. Sixty-eight revertant strains were examined for thermostability of their GHD. All appeared to be identical to wild type. Seven of the revertant strains were also tested for instability with regard to forward mutation to am auxtrophy. None was found to be unstable. Models for the genetic instability of the am₁₂₆ mutation are discussed.