Reversion of acridine mustard-induced ad-3 mutants of Neurospora crassa

Genetic and biochemical properties of revertants at the rudimentary locus in Drosophila melanogaster

The rudimentary (r) locus of Drosophila melanogaster encodes the first three enzymes for the de novo synthesis of pyrimidines: carbamyl phosphate synthase (CPSase), aspartyl transcarbamylase (ATCase), and dihydroorotase (DHOase). Revertants of leaky r mutants deficient in CPSase have been recovered. Of those examined, most (28/30) appear to result from reversion of the original mutation or a second site mutation in the mutant gene: They map to the r locus, have levels of CPSase and Km's for glutamine similar to wild type, and have comparatively little change in ATCase and DHOase activity. Two of the revertants appear to involve regulatory mutations that lead to an increase in defective CPSase molecules: They map to the r locus, have Km's for glutamine like that of the CPSase in the progenitor stock, and lead to relatively small increases in CPSase activity that are roughly paralleled by increases in ATCase and DHOase. The recovery of these putative regulatory mutations increasing the activity of the three enzymes supports the conclusion that the three activities are part of a trifunctional polypeptide or that their genes are transcribed together as parts of a multicistronic transcript.

Effects of pH on the mutagenicity of sodium azide in Neurospora crassa and Salmonella typhimurium

Sodium azide at various pH values did not cause a significant increase in the frequency of forward mutation above the control frequency at the adenine-3 (ad-3) region in resting conidia and in conidia from growing cultures of heterokaryons 12 and 59 of Neurospora crassa. Conidia from ad-3 mutants were plated with sodium azide at various pH values, and no obvious increase in reverse mutation above the controls was observed. Data are presented showing that sodium azide at pH 3 is inactivating conidia by interacting with the cytoplasma rather than the nucleus, and this may be the primary reasons that no mutation at the ad-3 region was detected. The dependence of sodium azide mutagenicity on pH was investigated in histidine-requiring mutants of Salmonella typhimurium using a suspension test. There were no significant differences in the reversion frequencies among the pH values (3-8) tested. Thus, no pH dependence is associated with sodium azide mutagenicity, nor are growth and/or DNA replication required for mutagenicity by sodium azide, in S. typhimurium.

Mutagenicity and mutagenic specificity of metronidazole and niridazole in Neurospora crassa

Mutagenicity and mutagenic specificity of niridazole and metronidazole, two chemotherapeutic agents used in the treatment of human parasitic diseases, were studied with the ad-3 test system of Neurospora crassa. The results show that neither compound is mutagenic in resting conidia. In growing vegetative cells, however, both compounds are mutagenic in N. crassa. Genetic analysis of the mutants indicated that niridazole induces predominantly base-pair substitution mutations. None of the niridazole-induced mutants resulted from multilocus deletions. The spectra of genetic alterations induced by metronidazole are similar to those induced by the mono-functional alkylating agents ethyleneimine (EI), ethylmethanesulfonate (EMS), and ICR-177. It is therefore suggested that the mechanism of mutation induction by metronidazole in Neurospora is similar to that of monofunctional alkylating agents.