Correlation between base-pair transition and complementation pattern in nitrous acid-induced ad-3B mutants of Neurospora crassa

History of the science of mutagenesis from a personal perspective

A career in the study of mutagenesis spanning 50 years is a gift few scientists have been bestowed. My tenure in the field started in 1953, the year the structure of DNA became known (Watson and Crick [1953]: Nature 171:737). Before that time, it was suspected that DNA was the genetic material based on the research of Oswald T. Avery (Avery et al. [1944]: J Exp Med 79:137), but many scientists still believed that proteins or polysaccharides could be the genetic material. The present article describes a lifetime of personal experience in the field of chemical mutagenesis. The methods used to treat viruses with chemical mutagens were well developed in the 1950s. Here I review the early use of nitrous acid and hydroxylamine as mutagens in eukaryotes, the development of methods for the metabolic activation of mutagens by microsomal preparations, and the selection of a mutant tester set for the qualitative characterization of the mutagenic activity of chemicals. These studies provided critical background information that was used by Bruce Ames in the development of his Salmonella/microsome assay, widely known as the Ames test (Ames et al. [1973]: Proc Nat Acad Sci USA 70:2281-2285). This article also describes how a set of diagnostic chemical mutagens was selected and used to identify the molecular nature of gene mutations. Today, DNA sequencing has replaced the use of diagnostic mutagens, but studies of this kind formed the foundation of modern mutation research. They also helped set the stage for the organization of the Environmental Mutagen Society and the Environmental Mutagen Information Center, which are described. The article ends with the development of mammalian single-cell mutation assays, the first system for studying in vivo mutagenesis using recoverable vectors in transgenic animals, other mutation assays in intact mammals, and my thoughts on the critically important area of germ cell mutagenesis. This narrative is not a complete autobiographical account, in that I have selected only those experiences that I feel are important for the history of the field and the edification of today's students. I hope I have shown that science not only is a valuable pursuit but can also be fun, stimulating, and satisfying. A good sense of humor and the knowledge that many discoveries come by serendipity are essential.

Mutagenic activity and specificity of hydrogen peroxide in the ad-3 forward-mutation test in two-component heterokaryons of Neurospora crassa

In the ad-3 forward-mutation test, hydrogen peroxide was at best a weak mutagen in nongrowing conidia from a DNA repair-proficient heterokaryon (H-12, uvs-2+/uvs-2+) but was a moderate mutagen in nongrowing conidia from a DNA-repair-deficient heterokaryon (H-59, uvs-2/uvs-2) over a narrow range of high concentrations. H-59 also was more sensitive than H-12 to the killing activity of hydrogen peroxide at high concentrations. Thus, a DNA-repair pathway, of which the gene product of the uvs-2+ allele is a part, appears to be involved in the repair of hydrogen peroxide-induced DNA lesions at low survival in these strains. There was slightly, but significantly, more killing by hydrogen peroxide of nongrowing conidia from H-12 and H-59 in the presence of O2 than in the absence of O2 (presence of N2). Thus, the killing activity of hydrogen peroxide was enhanced by O2. The Mutational Spectra of hydrogen peroxide-induced ad-3 mutants shows that hydrogen peroxide induced mainly gene/point mutations but also some multilocus deletion mutations in H-12 and H-59. Multiple-locus mutations occurred only in H-59, but the frequency was very low. The frequencies of the 3 kinds of intracistronic complementation pattern among ad-3BR mutants (gene/point mutations) suggest that hydrogen peroxide induced both base-pair substitutions and frameshift mutations in both strains.

Quantitative and qualitative aspects of spontaneous specific-locus mutation in the ad-3 region of heterokaryon 12 of Neurospora crassa

The data from forward-mutation experiments to obtain specific-locus mutations at 2 closely linked loci in the adenine-3 (ad-3) region of heterokaryon 12 (H-12) of Neurospora crassa have been tabulated to determine the frequency of spontaneous ad-3 mutations and to determine the percentages resulting from each of the 2 major genotypic classes: gene/point mutations and multilocus deletion mutations. Gene/point mutations at the ad-3B locus (ad-3BR) have been characterized to determine the percentage showing allelic complementation to obtain a presumptive identification of the genetic alteration in each mutation at the molecular level. Data from experiments performed at 2 different laboratories have been compared to assess the interlaboratory reproducibility of quantitative data on H-12. No difference was found between the frequencies of spontaneous specific-locus mutations in the ad-3 region. Genetic analysis of 172 ad-3 mutants demonstrated that specific-locus mutations in the ad-3 region result from both gene/point mutations (82.0% [141/172]), and multilocus deletion mutations (14.5% [25/172]). Heterokaryon tests for allelic complementation demonstrated that 52.5% (53/101) spontaneous ad-3BR mutants show allelic complementation, and result from single base-pair alterations. In addition, 100% (25/25) of the spontaneous multilocus deletion mutations result from the 3 smallest sized genotypic subclasses. The implications of the present experimental data for the validation of the ad-3 specific-locus assay system in Neurospora are discussed.

Comparison of the spectra of genetic damage in N4-hydroxycytidine-induced ad-3 mutations between nucleotide excision repair-proficient and -deficient heterokaryons of Neurospora crassa

A comparison has been made of the mutagenic effects of N4-hydroxycytidine (HC) in the adenine-3 (ad-3) region of two-component heterokaryons of Neurospora crassa: nucleotide excision repair-proficient (uvs-2+/uvs-2+) heterokaryon 12 (H-12) and nucleotide excision repair-deficient (uvs-2/uvs-2) heterokaryon 59 (H-59). HC was found to produce mutations predominantly, if not exclusively, by AT to GC base-pair transitions in Escherichia coli strain K12 by Janion and Glickman (1980, Mutation Res., 72, 43-47) and Sledziewska-Gojska et al. (1992, Mutagenesis, 7, 41-46). The ad-3 forward-mutation, specific-locus assay system permits the recovery of ad-3A and/or ad-3B mutants resulting from gene/point mutation, multiple-locus mutation, and multilocus deletion mutation. Uvs-2, which is homokaryotic in H-59, results in a recovery of HC-induced ad-3 forward mutations at a frequency in H-59 that is comparable to that found in H-12. Genetic analysis of ad-3 mutants recovered from experiments with HC treatment demonstrates that predominantly gene/point mutations were found in both strains: 99.3% (540/544) in H-12, and 97.4% (531/545) in H-59. Genetic analysis of allelic complementation among the ad-3BR mutations demonstrated that HC induced the highest percentage of complementing mutants ever found with base analogs both in H-12 (99.7% [328/329]) and H-59 (91.2% [290/318]). As a result of these findings, the majority of HC-induced ad-3 mutations are postulated to have resulted from missense mutations. Thus, we conclude that the results in Neurospora are consistent with the observations in E. coli strain K-12, where HC induces predominantly AT to GC base-pair transitions.

Utilization of the specific-locus assay in the ad-3 region of two-component heterokaryons of Neurospora for risk assessment of environmental chemicals

The utilization of the specific-locus assay in the ad-3 region of two-component heterokaryons of Neurospora crassa is compared with that of other eukaryotic assay systems for the evaluation of the mutagenic effects of environmental chemicals. In contrast to other in vitro specific-locus assays, the Neurospora assay can detect mutations not only at the ad-3A and ad-3B loci but also recessive lethal mutations elsewhere in the genome. Mutational damage in this system can be characterized readily by means of classical genetic techniques involving heterokaryon tests to determine genotype, and allelic complementation among ad-3BR mutations. The percentages of ad-3BR mutations showing allelic complementation with polarized or nonpolarized complementation patterns provide a presumptive identification of the genetic alterations at the molecular level in individual mutants. Dikaryon and trikaryon tests (using 3 strains carrying multilocus deletion mutations as tester strains) distinguish ad-3 mutations resulting from gene/point mutation, multilocus deletion mutation, and various types of multiple-locus mutation. The array of ad-3 mutations recovered from forward-mutation experiments can be expressed in terms of Mutational Spectra, which make it possible to make comparisons of mutational types between different doses of the same mutagen, different mutagens, or the effects of the same mutagen on different strains. Another important feature of this specific-locus assay system is that the effects of mutagens can be studied in both DNA excision repair-proficient (H-12) and -deficient (H-59) two-component heterokaryons to evaluate both quantitative and qualitative differences between the spectra of induced ad-3 mutations. The utilization of this assay on large numbers of environmental chemicals has shown that some chemicals produce predominantly, or exclusively, gene/point mutations, whereas other agents produce both gene/point mutations and multilocus deletion mutations in H-12. When the mutagenic effects of the same chemicals were compared in H-12 and H-59, marked differences between forward-mutation frequencies and Mutational Spectra of ad-3 mutations were detected.

2-Amino-N6-hydroxyadenine induces gene/point mutations and multiple-locus mutations, but not multilocus deletion mutations, in the ad-3 region of a two-component heterokaryon of Neurospora crassa

The mutagenicity of 2-amino-N6-hydroxyadenine (AHA) has been studied in Neurospora crassa by treating a two-component heterokaryon (H-12) and recovering specific-locus mutations induced in the ad-3 region. This assay system permits the identification of ad-3A and/or ad-3B mutants resulting from gene/point mutations, multilocus deletion mutations, and multiple-locus mutations of various genotypes, involving one or both loci. Genetic characterization of the ad-3 mutants recovered from experiments with AHA in H-12 shows that 98.9% (270/273) of the ad-3 mutants are gene/point mutations (ad-3R), 1.1% (3/270) are unknowns, and none is a multilocus deletion mutation (ad-3IR). Among the gene/point mutations, 3.3% (9/273) are multiple-locus mutations (gene/point mutations with a closely-linked recessive lethal mutation [ad-3R + RLCL]). Another 25.3% (69/273) are multiple-locus mutations with a recessive lethal mutation located elsewhere in the genome [ad-3R + RL]. Heterokaryon tests for allelic complementation among the ad-3BR mutants showed that 90.8% (139/153) of the mutants were complementing, and 20.3% (31/153) were leaky. In addition, 32.5% (38/117) of the ad-3AR mutants were leaky. These data are consistent with the hypothesis that AHA produces specific-locus mutations in the ad-3 region of N. crassa by base-pair substitution. The data from the present experiments are compared with the data for 2-aminopurine (2AP)-induced ad-3 mutants in H-12 (de Serres and Brockman, 1991). Whereas, 2AP is a weak mutagen in H-12, AHA is extremely potent (Brockman et al., 1987). In contrast with 2AP, AHA induces ad-3 mutants exclusively by gene/point mutation in H-12. We conclude that whereas AHA induces ad-3 mutants predominantly by AT to GC base-pair transitions, 2AP induces ad-3 mutants by a wide variety of mechanisms including: (1) AT to GC and GC to AT base-pair transitions, (2) frameshift mutations, (3) other, as yet unidentified, intragenic alterations, (4) small multilocus deletion mutations, and (5) multiple-locus ad-3R mutations with closely linked recessive lethal mutations.

X-ray-induced specific-locus mutations in the ad-3 region of two-component heterokaryons of Neurospora crassa. VII. Genetic lesions resulting in gene/point mutations at the ad-3B locus have different dose-response relationships

Genetic characterization of X-ray-induced ad-3 mutants, induced in a two-component heterokaryon (H-12) of Neurospora crassa, has been performed to determine genotype, patterns of allelic complementation, and leakiness, and to distinguish gene/point mutations from multilocus deletions and multiple locus mutations (de Serres, 1989c, 1990a). The array of genotypes in the classes and subclasses in the spectrum of ad-3 mutants induced by a mutagenic agent constitute its mutagenicity profile; for X-rays this profile consists of 29 different genotypes. In the present paper, the data on gene/point mutations induced at the ad-3B locus (designated ad-3BR mutations) have been tabulated as a function of X-ray dose to determine the dose-dependent relationships of complementing and noncomplementing mutants. This analysis has shown that the overall percentages of mutants showing allelic complementation and the percentages of complementing mutants with nonpolarized patterns (both leaky and nonleaky) and noncomplementing mutants were dose-dependent; the former class decreased with increasing X-ray dose, and the latter class increased with increasing X-ray dose. The percentages of complementing mutants with polarized patterns were X-ray dose-independent. In addition, an unexpectedly high frequency of mutants with nonpolarized complementation patterns are discontinuous and probably result from multiple-site mutations.

Mutagenicity of 2-aminopurine, 6-N-hydroxylaminopurine, and 2-amino-N6-hydroxyadenine in Neurospora crassa

These data from our experiments with 3 purine analogs reveal striking differences in mutagenic potency. It seems highly likely that these analogs substitute readily for adenine and that they cause mutations in the main part, and in the case of AHA perhaps predominantly, by mispairing with cytosine. The most potent mutagens are those with the hydroxylamino group at the C6 position (AHA and HAP). Of these, the most potent is the analog with an amino group in the C2 position (AHA). The most interesting aspect of the present studies is their implications for other eukaryotic organisms. We have determined that AHA, which was shown to be a potent mutagen in bacteria [11], is an extremely potent mutagen in a eukaryotic organism. AHA is active at relatively low concentrations, and it gives rise to point mutations that appear to arise predominantly by AT----GC base-pair transitions. AHA should be an extremely useful genetic probe for studies on higher eukaryotic organisms. Its potency and specificity make it an unusual mutagen that can be expected to produce specific-locus mutants at high frequency with the genetic damage confined to the boundaries of the gene. These characteristics should make it useful not only for studies of specific-locus mutations and sex-linked recessive lethal mutations in Drosophila but also for specific-locus studies in mammalian cells in culture and in the whole animal. In these latter systems, it is extremely time consuming and sometimes impossible to distinguish between point mutations and multilocus deletions. The use of AHA as a mutagen in these systems should provide a useful new approach to genetic fine structure analysis.

Mutation tests in Neurospora crassa. A report of the U.S. Environmental Protection Agency Gene-Tox Program

Many mutation tests have been developed in Neurospora crassa during the almost 40 years of its use in mutation research. These tests detect two major classes of mutation: gene mutation and meiotic nondisjunction. Within the first class, forward- and reverse-mutation tests have been used. The forward-mutation tests include those that detect mutations at many loci and at specific loci. Both kinds of forward-mutation tests have been done in homokaryons (n) and heterokaryons (n + n'). From the publications that were not rejected by our pre-established criteria, data were extracted for 166 chemicals that had been tested for mutagenicity. Only 6 of the 166 chemicals have been tested in one or more gene mutation test and the meiotic nondisjunction test; these 6 chemicals were positive in the first and negative in the second. Of the 102 chemicals tested in one or more gene mutation tests, 94 were positive and 8 were negative. Of the 70 chemicals tested in the meiotic nondisjunction test, 7 were positive and 63 were negative. Two tests, the ad-3 forward-mutation test and the meiotic nondisjunction test, have been used most frequently. These two tests are especially important for hazard evaluation, because each detects a class of mutations that is likely to be deleterious or lethal in the F1 - disomics by the meiotic nondisjunction test and multilocus deletions by the ad-3 forward-mutation test in heterokaryons. Generally, direct-acting chemicals are mutagenic in the gene mutation tests, but few chemicals that required metabolic activation have been tested. Only 31 of the 166 chemicals tested in N. crassa have been tested for carcinogenicity. Among these chemicals, there is a good association between mutagenicity in gene mutation tests and carcinogenicity but a poorer association between meiotic nondisjunction and carcinogenicity; however, only a small number of chemicals has been tested in the meiotic nondisjunction test. Further use and development of certain mutation tests in N. crassa are desirable.

Mutagenesis at the ad-3A and ad-3B loci in haploid UV-sensitive strains of Neurospora crassa. VI. Genetic characterization of ad-3 mutants provides evidence for qualitative differences in the spectrum of genetic alterations between wild-type and nucleotide excision-repair-deficient strains

Genetic characterization of ad-3B mutants induced in wild-type and UV-sensitive strains has revealed qualitative differences between the spectra of genetic alterations at the molecular level. Ad-3B mutants induced in the two nucleotide excision-repair-deficient strains upr-1 and uvs-2 (Worthy and Epler, 1973) had significantly lower frequencies of nonpolarized complementation patterns and higher frequencies of noncomplementing mutants than ad-3B mutants induced in the wild-type strain in samples induced by either UV, gamma-rays, 4NQO or MNNG. In these same samples ad-3B mutants induced in uvs-4, uvs-5 or uvs-6 did not differ significantly from those induced in the wild-type strain. After ICR-170 treatment, ad-3B mutants induced in the UV-sensitive strains did not differ significantly from those induced in wild-type. The comparisons in the present and previous studies demonstrate that the process of mutation-induction in the ad-3 region is under the control of other loci that not only alter mutant recovery quantitatively (de Serres, 1980; Schüpbach and de Serres, 1981; Inoue et al., 1981a, b) but also qualitatively. These data have important implications for comparative chemical mutagenesis, since the spectrum of genetic alterations produced by a given agent can be modified markedly as a result of defects in DNA repair.

Genetic analysis of ad-3 mutants induced by AF-2 and two other nitrofurans in Neurospora crassa

Our previous studies have shown that the nitrofurans AF-2, SQ18506, and FANFT are potent mutagens in Neurospora crassa. The genetic damage produced by these chemicals at the ad-3 region in N crassa has been characterized by a series of genetic tests. The results of these tests indicate that all three agents induce a high frequency of point mutations and probably a low frequency of multilocus deletions. A comparison of the complementation patterns among the AF-2--induced ad-3B mutants and those induced by other chemical agents indicates that the spectra of intragenic alterations induced by AF-2 in N crassa are similar to those induced by monofunctional alkylating agents.

Mutation-induction in repair-deficient strains of Neurospora

Various mutants sensitive to UV-induced inactivation have been used to study the process of spontaneous and induced mutation in the ad-3 region of Neurospora crassa. Studies on haploid strains have shown that the process of mutation-induction in the ad-3 region is under genetic control. Studies on two-component heterokaryons have shown that this control effects both point mutations and multilocus deletions. Comparisons made between an excision-repair deficient two-component heterokaryon (59) have shown that the level of effect is markedly mutagen-specific. All possible effects on the process of mutation-induction in the ad-3 region have been found in the strains tested.

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.

Depression of uracil uptake by ammonium in Neurospora crassa

The mechanism of uracil uptake and one aspect of its regulation were studied in germinated conidia of Neurospora crassa. Uracil was found to be taken up by a transport mechanism that did not exhibit Michaelis-Menten kinetics. Rather, the kinetic patterns indicated two separate systems or a single transport mechanism with negative cooperativity. Cytosine and thymine inhibited uracil uptake, but uridine did not. The mutant strain uc-5-pyr-1, which failed to transport uracil, was used in reversion studies and to map the uc-5 locus. Spontaneous reversion rates at the uc-5 locus were found to be approximately 2 x 10(-8), indicating that the uc-5 lesion results from a single mutation. Loss of the uracil transport function through a single mutation favors the model of a single transport mechanism with negative cooperativity. Uracil uptake was significantly decreased in the presence of NH 4+, and evidence is presented for repression by NH4+ of a uracil transport system. Growth rates of pyrimidine-requiring and wild-type strains measured in the presence and absence of NH4+, with uracil as the pyrimidine supplement, showed that NH4+ decreased the growth rates of the pyrimidine-requiring strains significantly, while having no effect on wild-type growth rates.

Evidence for nuclear restriction of supersuppressor gene products in Neurospora heterokaryons

It was demonstrated that the gene products of the three ssu loci cannot enter the transcription/translation system coded by the other component nucleus in a heterokaryon. In such a situation, dominance or recessiveness cannot be established, and simple dominance tests are meaningless.

Growth patterns of adenine-3B; supersuppressor strains of Neurospora crassa

The interaction between several suppressible ad-3B alleles and several supersuppressor genes had been examined both quantitatively and qualitatively in a tetrad analysis. Quantitatively, there is a good deal of variation in mean growth rates of cultures showing supersuppression: part of this variation can be attributed to variation in the suppressibility of ad-3B alleles, part to variation of suppression efficiency of ssu alleles, and part to the action of modifying genes. Qualitatively, ad-3B; ssu cultures nearly always show cyclic stop-start growth patterns. Escape from stop-start growth to wild-type growth is common.

Topography of the ad-3 region of Neurospora crassa

An analysis of prototroph frequencies in crosses between 7 ad-3A and 10 ad-3B mutants has demonstrated that where a linear order of ad-3B alleles can be specified, the order is consistent with that obtained from complementation maps. The data also provide a working estimate of the sizes of the components of the ad-3 region.