T.M. Ong

Mutagenesis at the ad-3A and ad-3B loci in haploid UV-sensitive strains of Neurospora crassa IV. Comparison of dose—response curves for MNNG, 4NQO and ICR-170 induced inactivation and the mutation-induction

The genetic effects of MNNG, 4NQO and ICR-170 have been compared on 5 different UV-sensitive strains and a standard wild-type strain of Neurospora crassa with regard to inactivation and the induction of forward-mutations at the ad-3A and ad-3B loci. Whereas all UV-sensitive strains (upr-1, uvs-2, uvs-3, uvs-5 and uvs-6) are more sensitive to inactivation by MNNG and ICR-170 than wild-type, only uvs-5 shows survival comparable to wild-type after 4NQO treatment, all other strains are more sensitive to 4NQO. In contrast to the effects on inactivation, a wide variety of effects were found for the induction of ad-3A and ad-3B mutations: higher forward-mutation frequencies than were found in wild-type were obtained after treatment with MNNG or 4NQO for upr-1 and uvs-2, no significant increase over the spontaneous mutation frequency was found with uvs-3 after MNNG, 4NQO or ICR-170 treatment; mutation frequencies comparable to that found in wild-type were obtained with uvs-6 after MNNG, 4NQO or ICR-170 treatment and with upr-1 after ICR-170 treatment. Lower forward-mutation frequencies than were found in wild-type were obtained with uvs-2 after ICR-170 treatment and with uvs-5 after MNNG, 4NQO or ICR-170 treatment. These data clearly show that the process of forward-mutation at the ad-3A and ad-3B loci is under genetic control by mutations at other loci (e.g. upr-1, uvs-2, uvs-3, uvs-5 and uvs-6) and that the effect is markedly mutagen-dependent.

Genetic characterization of ad-3 mutants induced by chemical carcinogens, 1-phenyl-3-monomethyltriazene and 1-phenyl-3,3-dimethyltriazene, in Neurospora crassa

Abstract 180 ad-3 mutants of Neurospora crassa induced by 1-phenyl-3-monomethyl-triazene (PMMT) and 56 ad-3 mutants induced by 1-phenyl-3,3-dimethyltriazene (PDMT) were characterized by dikaryon, trikaryon and complementation tests. Results show that the spectrum of genetic alterations induced by PMMT is different from that of PDMT. This suggests that enzymatic dealkylation of PDMT to PMMT does not occur within Neuropsora crassa conidia, and that the mechanism of mutation induction of PDMT in N. crassa is different from that of PMMT. Hydrolytic breakdown products or its intact molecule or some other converted forms might be responsible for the mutagenic activity of PDMT. Mutation induction of PMMT in N. crassa appears to be via alkylation of DNA by carbonium ions produced by this compound, the same mechanism proposed for its carcinogenic activity. The frequencies of leakiness, allelic complementation and nonpolarized complementation patterns among PMMT-induced ad-3 mutants are similar to those of ad-3 mutants induced by other potent chemical carcinogens, such as MNNG and the aflatoxins.

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.

Mutagenic evaluation of antischistosomal drugs and their derivatives in Neurospora crassa.

The mutagenic activities of lucanthone, hycanthone, niridazole, and the indazole analogs of lucanthone (IA-3 and IA-5) or hycanthone (IA-4 and IA-6) were studied by assaying for the induction of specific locus mutations in the ad-3 region of N. crassa. The results show that lucanthone, hycanthone, and their indazole analogs (IA-3 through IA-6) are all mutagenic in N. crassa when conidia are treated with any of these compounds. On a per mole basis, hycanthone is the least toxic and mutagenic, whereas IA-3 is the most toxic and mutagenic compound among the six closely related agents. In general, compounds with a methyl group at the C-4 position are more mutagenic than compounds with a methanol group; 6-chloroindazole analogs are more mutagenic and more toxic than nonchlorinated analogs. Niridazole is not mutagenic when conidial suspensions are treated. However, the mutation frequency increased more than 50-fold when niridazole was added to the medium used to grow vegetative cultures. Thus, it appears that the mutagenic activity of this latter compound requires metabolic activation.

Mutation induction by rodent liver microsomal metabolites of aflatoxins B1 and G1 in Neurospora crassa

The mutagenic activities of aflatoxins B1 and G1 were studied in the ad-3 test system of Neurospora crassa by treatment of conidia with aflatoxin and liver homogenate for 2 h. No significant increase in the ad-3 mutation frequency over the spontaneous frequency was observed when either aflatoxin or mammalian liver homogenate was omitted from the test system. The ad-3 mutation frequencies increased to between 29 and 87/10(6) survivors, which is a 73- to 217-fold increase over the average spontaneous ad-3 mutation frequency (0.4/10(6) survivors), after conidia of N. crassa were treated with 0.67 mM aflatoxin B1, hamster liver homogenate, and a NADPH generating system. A 9- to 15-fold increase in the mutation frequency over the spontaneous mutation frequency was found when 0.67 mM of aflatoxin G1 instead of aflatoxin B1 was used in the test system. Treatment of conidia with 0.44 mM aflatoxin B1 mice liver homogenate and a NADPH generating system caused a small, but significant increase in the ad-3 mutation frequencies. No significant increase in the mutation frequency was found when a single sample of human liver homogenate was used in the test system. These studies show that metabolic activation is necessary for the expression of the mutagenic activity of aflatoxins B1 and G1 in N. crassa.

Mutagenic activities of nitrofurans in Neurospora crassa

Abstract The studies on the mutagenicity of several nitrofurans reported here indicate that the carcinogenic nitrofurans are mutagenic in Neurospora crassa and that the NO 2 group at the C5 position of the furan ring is necessary for the mutagenic activity of nitrofurans. The side chain at the C2 position of the furan ring for the mutagenic activity appears to be more specific than those for the antischistosomal and antibacterial activities.