Isolation and characterization of MMS-sensitive mutants of Neurospora crassa

DNA interstrand crosslink repair by XPF-ERCC1 homologue confers ultraviolet resistance in Neurospora crassa

Ultraviolet (UV) light is a mutagen that causes DNA damage. Some UV-sensitive Neurospora crassa strains have been reported to exhibit a partial photoreactivation defect (PPD) phenotype, and the possible cause of this has been unknown for more than half a century. In this study, in the process of elucidating the possible causes of a PPD phenotype, we discovered that the XPF homologue MUS-38 is involved in repairing the UV-induced DNA interstrand crosslink (ICL) in N. crassa. Furthermore, the sensitivity of the Δmus-38 and Δmus-44 strains to ICL agents was significantly higher than that of other nucleotide excision repair (NER)-related gene knockout (KO) strains, indicating that the MUS-38/MUS-44 complex is involved in an NER-independent ICL repair mechanism. Based on reports concerning the mammalian homologues XPF and ERCC1 we obtained separation-of-function mutants defective only in NER in mus-38 and mus-44. Additionally, the photoreactivation ability of these mutants was significantly higher than that of the KO strains. These results indicate that the PPD phenotype is caused by a defect in the repair-ability of ICL induced by UV and that an NER-independent ICL repair by MUS-38 and MUS-44 confers resistance to UV in N. crassa.

mus-52 disruption and metabolic regulation in Neurospora crassa: Transcriptional responses to extracellular phosphate availability

Advances in the understanding of molecular systems depend on specific tools like the disruption of genes to produce strains with the desired characteristics. The disruption of any mutagen sensitive (mus) genes in the model fungus Neurospora crassa, i.e. mus-51, mus-52, or mus-53, orthologous to the human genes KU70, KU80, and LIG4, respectively, provides efficient tools for gene targeting. Accordingly, we used RNA-sequencing and reverse transcription-quantitative polymerase chain reaction amplification techniques to evaluate the effects of mus-52 deletion in N. crassa gene transcriptional modulation, and thus, infer its influence regarding metabolic response to extracellular availability of inorganic phosphate (Pi). Notably, the absence of MUS-52 affected the transcription of a vast number of genes, highlighting the expression of those coding for transcription factors, kinases, circadian clocks, oxi-reduction balance, and membrane- and nucleolus-related proteins. These findings may provide insights toward the KU molecular mechanisms, which have been related to telomere maintenance, apoptosis, DNA replication, and gene transcription regulation, as well as associated human conditions including immune system disorders, cancer, and aging.

Exploring the processes of DNA repair and homologous integration in Neurospora

This review offers a personal perspective on historical developments related to our current understanding of DNA repair, recombination, and homologous integration in Neurospora crassa. Previous reviews have summarized and analyzed the characteristics of Neurospora DNA repair mutants. The early history is reviewed again here as a prelude to a discussion of the molecular cloning, annotation, gene disruption and reverse genetics of Neurospora DNA repair genes. The classical studies and molecular analysis are then linked in a perspective on new directions in research on mutagen-sensitive mutants.

The Neurospora crassa UVS-3 epistasis group encodes homologues of the ATR/ATRIP checkpoint control system

The mutagen sensitive uvs-3 and mus-9 mutants of Neurospora show mutagen and hydroxyurea sensitivity, mutator effects and duplication instability typical of recombination repair and DNA damage checkpoint defective mutants. To determine the nature of these genes we used cosmids from a genomic library to clone the uvs-3 gene by complementation for MMS sensitivity. Mutation induction by transposon insertion and RIP defined the coding sequence. RFLP analysis confirmed that this sequence maps in the area of uvs-3 at the left telomere of LG IV. Analysis of the cDNA showed that the UVS-3 protein contains an ORF of 969 amino acids with one intron. It is homologous to UvsD of Aspergillus nidulans, a member of the ATRIP family of checkpoint proteins. It retains the N' terminal coiled-coil motif followed by four basic amino acids typical of these proteins and shows the highest homology in this region. The uvsD cDNA partially complements the defects of the uvs-3 mutation. The uvs-3 mutant shows a higher level of micronuclei in conidia and failure to halt germination and nuclear division in the presence of hydroxyurea than wild type, suggesting checkpoint defects. ATRIP proteins bind tightly to ATR PI-3 kinase (phosphatidylinositol 3-kinase) proteins. Therefore, we searched the Neurospora genome sequence for homologues of the Aspergillus nidulans ATR, UvsB. A uvsB homologous sequence was present in the right arm of chromosome I where the mus-9 gene maps. A cosmid containing this genomic DNA complemented the mus-9 mutation. The putative MUS-9 protein is 2484 amino acids long with eight introns. Homology is especially high in the C-terminal 350 amino acids that correspond to the PI-3 kinase domain. In wild type a low level of constitutive mRNA is present for both genes. It is transiently induced upon UV exposure.

Neurosprora crassa RAD5 homologue, mus-41, inactivation results in higher sensitivity to mutagens but has little effect on PCNA-ubiquitylation in response to UV-irradiation

The DNA replication machinery stalls at damaged sites on DNA. Postreplicaton repair (PRR) is a system to avoid cell death in such circumstances of deadlock. In Saccharomyces cerevisiae, the Rad6/Rad18 heterodimer plays pivotal roles in PRR. It promotes translesion synthesis via the monoubiquitylation of the DNA sliding clamp, PCNA. Ubc13/Mms2/Rad5 can extend the ubiquitin chain from this monoubiquitylated PCNA with a non-canonical lysine 63-linked ubiquitin-chain, resulting in an error-free mode of bypass. In this study, we identified and characterized the RAD5 homolog in Neurospora crassa, which we named mus-41. A mus-41 mutant was sensitive to several DNA-damaging agents including UV and MMS. Genetic analyses indicated that uvs-2 (RAD18 homolog) was epistatic to mus-41, suggesting a role for mus-41 in postreplication repair. Additionally, it was shown that mus-41 has a role independent from TLS gene upr-1 (REV3 homolog) and works in the error-free pathway, indicating that the function of mus-41 as a RAD5 homolog is also conserved in N. crassa. However, mus-41 is not essential for the ubiquitylation of PCNA that is detected in the wild-type background, suggesting that there is another ubiquitin ligase catalyzing ubiquitylation of PCNA in response to UV in N. crassa.

The novel gene mus7(+) is involved in the repair of replication-associated DNA damage in fission yeast

The progression of replication forks is often impeded by obstacles that cause them to stall or collapse, and appropriate responses to replication-associated DNA damage are important for genome integrity. Here we identified a new gene, mus7(+), that is involved in the repair of replication-associated DNA damage in the fission yeast Schizosaccharomyces pombe. The Deltamus7 mutant shows enhanced sensitivity to methyl methanesulfonate (MMS), camptothecin, and hydroxyurea, agents that cause replication fork stalling or collapse, but not to ultraviolet light or X-rays. Epistasis analysis of MMS sensitivity indicates that Mus7 functions in the same pathway as Mus81, a subunit of the Mus81-Eme1 structure-specific endonuclease, which has been implicated in the repair of the replication-associated DNA damage. In Deltamus7 and Deltamus81 cells, the repair of MMS-induced DNA double-strand breaks (DSBs) is severely impaired. Moreover, some cells with either mutation are hyper-elongated or enlarged, and most of these cells accumulate in late G2 phase. Spontaneous Rad22 (recombination mediator protein RAD52 homolog) foci increase in S phase to late G2 phase in Deltamus7 and Deltamus81 cells. These results suggest that replication-associated DSBs accumulate in these cells and that Rad22 foci form in the absence of Mus7 or Mus81. We also found that the rate of spontaneous conversion-type recombination is reduced in mitotic Deltamus7 cells, suggesting that Rhp51- (RAD51 homolog) dependent homologous recombination is disturbed in this mutant. From these data, we propose that Mus7 functions in the repair of replication-associated DSBs by promoting RAD51-dependent conversion-type recombination downstream of Rad22 and Mus81.

Nonhomologous chromosomal integration of foreign DNA is completely dependent on MUS-53 (human Lig4 homolog) in Neurospora

Homologous integration of a foreign DNA segment into a chromosomal target sequence enables precise disruption or replacement of genes of interest and provides an effective means to analyze gene function. However, integration after transformation is predominantly nonhomologous in most species other than yeast. Here, we show that homologous integration in the filamentous fungus Neurospora requires the homologous-recombination proteins MEI-3 (yeast Rad51 homolog) and MUS-25 (yeast Rad54 homolog), whereas nonhomologous integration requires nonhomologous end-joining protein MUS-52 (yeast Ku80 homolog). Two additional minor integration pathways are present, one MEI-3-independent and homologous, the other MUS-52-independent and nonhomologous. Homologous and nonhomologous mechanisms compete when external DNA is integrated. In Neurospora, both nonhomologous integration pathways, MUS-52-dependent and MUS-52-independent, require MUS-53 (a homolog of human Lig4), which functions in the final step of nonhomologous end-joining. Because nonhomologous integration is eliminated in a LIG4-disrupted strain, integration occurs only at the targeted site in mus-53 mutants, making them an extremely efficient and safe host for gene targeting.

Growth defect and mutator phenotypes of RecQ-deficient Neurospora crassa mutants separately result from homologous recombination and nonhomologous end joining during repair of DNA double-strand breaks

RecQ helicases function in the maintenance of genome stability in many organisms. The filamentous fungus Neurospora crassa has two RecQ homologs, QDE3 and RECQ2. We found that the qde-3 recQ2 double mutant showed a severe growth defect. The growth defect was alleviated by mutation in mei-3, the homolog of yeast RAD51, which is required for homologous recombination (HR), suggesting that HR is responsible for this phenotype. We also found that the qde-3 recQ2 double mutant showed a mutator phenotype, yielding mostly deletions. This phenotype was completely suppressed by mutation of mus-52, a homolog of the human KU80 gene that is required for nonhomologous end joining (NHEJ), but was unaffected by mutation of mei-3. The high spontaneous mutation frequency in the double mutant is thus likely to be due to NHEJ acting on an elevated frequency of double-strand breaks (DSBs) and we therefore suggest that QDE3 and RECQ2 maintain chromosome stability by suppressing the formation of spontaneous DSBs.

Srs2 and RecQ homologs cooperate in mei-3-mediated homologous recombination repair of Neurospora crassa

Homologous recombination and post-replication repair facilitate restart of stalled or collapsed replication forks. The SRS2 gene of Saccharomyces cerevisiae encodes a 3′–5′ DNA helicase that functions both in homologous recombination repair and in post-replication repair. This study identifies and characterizes the SRS2 homolog in Neurospora crassa, which we call mus-50. A knockout mutant of N.crassa, mus-50, is sensitive to several DNA-damaging agents and genetic analyses indicate that it is epistatic with mei-3 (RAD51 homolog), mus-11 (RAD52 homolog), mus-48 (RAD55 homolog) and mus-49 (RAD57 homolog), suggesting a role for mus-50 in homologous recombination repair. However, epistasis evidence has presented that MUS50 does not participate in post-replication repair in N.crassa. Also, the N.crassa mus-25 (RAD54 homolog) mus-50 double mutant is viable, which is in contrast to the lethal phenotype of the equivalent rad54 srs2 mutant in S.cerevisiae. Tetrad analysis revealed that mus-50 in combination with mutations in two RecQ homologs, qde-3 and recQ2, is lethal, and this lethality is suppressed by mutation in mei-3, mus-11 or mus-25. Evidence is also presented for the two independent pathways for recovery from camptothecin-induced replication fork arrest: one pathway is dependent on QDE3 and MUS50 and the other pathway is dependent on MUS25 and RECQ2.

Highly efficient gene replacements in Neurospora strains deficient for nonhomologous end-joining

Gene disruption and overexpression play central roles in the analysis of gene function. Homologous recombination is, in principle, the most efficient method of disrupting, modifying, or replacing a target gene. Although homologous integration of exogenous DNA into the genome occurs readily in Saccharomyces cerevisiae, it is rare in many other organisms. We identified and disrupted Neurospora crassa genes homologous to human KU70 and KU80, which encode proteins that function in nonhomologous end-joining of double-stranded DNA breaks. The resulting mutants, named mus-51 and mus-52, showed higher sensitivity to methyl methanesulfonate, ethyl methanesulfonate, and bleomycin than wild type, but not to UV, 4-nitroquinoline 1-oxide, camptothecin, or hydroxyurea. Vegetative growth, conidiation, and ascospore production in homozygous crosses were normal. The frequency of integration of exogenous DNA into homologous sequences of the genome in the KU disruption strains of N. crassa was compared with that in wild type, mei-3, and mus-11. In mei-3 and mus-11, which are defective in homologous recombination, none or few homologous integration events were observed under any conditions. When mtr target DNA with approximately 2-kb 5' and 3' flanking regions was used for transformation of the KU disruption strains, 100% of transformants exhibited integration at the homologous site, compared to 10 to 30% for a wild-type recipient. Similar results were obtained when the ad-3A gene was targeted for disruption. These results indicate that KU disruption strains are efficient recipients for gene targeting.

The Neurospora crassa mus-19 gene is identical to the qde-3 gene, which encodes a RecQ homologue and is involved in recombination repair and postreplication repair

An allele called mus-19 was identified by screening temperature-sensitive and mutagen-sensitive mutants of Neurospora crassa. The mus-19 gene was genetically mapped to a region near the end of the right arm of linkage group I, where a RecQ homologue called qde-3 had been physically mapped in the Neurospora database. Complementation tests between the mus-19 mutant and the qde-3(RIP) mutant showed that mus-19 and qde-3 were the same gene. The qde-3 genes of both mutants were cloned and sequenced; and the results showed that they have mutation(s) in their qde-3 genes. The original mus-19 and qde-3(RIP) mutants are defective in quelling, as reported for other qde-3 mutants. The mutants show high sensitivity to methyl methanesulfonate, ethyl methanesulfonate, N-methyl- N'-nitro- N-nitrosoguanidine, tert-butyl hydroperoxide, 4-nitroquinoline-1-oxide, hydroxyurea and histidine. Epistasis analysis indicated that the qde-3 gene belongs both to the uvs-6 recombination repair pathway and the uvs-2 postreplication repair pathway. The qde-3 mutation has no effect on the integration of a plasmid carrying the mtr gene by homologous recombination. In homozygous crosses, the qde-3 mutant is defective in ascospore production.

Histone H1 Is required for proper regulation of pyruvate decarboxylase gene expression in Neurospora crassa

We show that Neurospora crassa has a single histone H1 gene, hH1, which encodes a typical linker histone with highly basic N- and C-terminal tails and a central globular domain. A green fluorescent protein-tagged histone H1 chimeric protein was localized exclusively to nuclei. Mutation of hH1 by repeat-induced point mutation (RIP) did not result in detectable defects in morphology, DNA methylation, mutagen sensitivity, DNA repair, fertility, RIP, chromosome pairing, or chromosome segregation. Nevertheless, hH1 mutants had mycelial elongation rates that were lower than normal on all tested carbon sources. This slow linear growth phenotype, however, was less evident on medium containing ethanol. The pyruvate decarboxylase gene, cfp, was abnormally derepressed in hH1 mutants on ethanol-containing medium. This derepression was also found when an ectopically integrated fusion of the cfp gene promoter to the reporter gene hph was analyzed. Thus, Neurospora histone H1 is required for the proper regulation of cfp, a gene with a key role in the respiratory-fermentative pathway.

A new UV-sensitive mutant that suggests a second excision repair pathway in Neurospora crassa

In an attempt to understand the relationship between photorepair and dark repair in Neurospora crassa, a new mutant was isolated, which showed defects in both repair processes. The new mutant, mus-38, is moderately sensitive to UV and shows imperfect photoreactivation following UV irradiation. DNA was purified from this mutant and the other UV-sensitive mutants, and analyzed for the removal of cyclobutane pyrimidine dimers (CPDs). UV-specific endonuclease-sensitive sites (ESS) completely disappeared with 1 h of photoreactivation in mus-38 DNA, although the survival recovery with photoreactivation was greatly reduced in this mutant. This suggests that the insufficient survival recovery with photoreactivation in mus-38 does not result from a failure of photo-reversal of CPDs. Removal of ESS during liquid holding (dark repair) was slower in mus-38 compared to wild type. To test the possibility that this mutant was involved in excision repair, the double mutant was made between mus-38 and mus-18, which encodes a UV-damage-specific endonuclease. CPD excision in the mus-18 null mutant was severely affected but not completely inhibited. The double mutant showed a complete loss of the excision activity and was super sensitive to UV. These results indicate that mus-38 participates in an excision pathway that is different from the mus-18 pathway. The mus-38 mutant was sensitive not only to UV but also to some chemical mutagens which make adducts on DNA. Thus, mus-38 is possibly involved in an excision-repair pathway that is related to the Saccharomyces cerevisiae RAD3 pathway.

Mutagenesis and epistatic grouping of the Neurospora meiotic mutants, mei-2 and mei-3, which are sensitive to mutagens

To understand the possible roles of the Neurospora meiotic genes mei-2 and mei-3 in DNA repair, the frequencies of spontaneous and UV-induced mutation at the ad-3 loci were investigated and double mutants between mei mutants and other DNA-repair mutants were analyzed for mutagen sensitivity. Spontaneous mutation frequency in mei-2 was similar to that of the wild type, while the frequencies were high in both of two mei-3 strains which contained different mei-3 alleles. In addition, the frequency of spontaneous mutation in mei-3 varied greatly from experiment to experiment, which clearly showed a mutator phenotype for mei-3 mutation. UV irradiation increased mutation frequencies in both mei-2 and mei-3. The mutagen sensitivity of the double mutant, mei-2 mei-3, was no greater than that of the single mutants when treated with UV and methyl methanesulfonate (MMS). Thus, both mei genes appear to be involved in the same repair group. When analyzed in combination with other mutations, both mei mutations showed an epistatic relationship to uvs-6 and a synergistic relationship to mus-18 and uvs-2. However, uvs-3 was epistatic to mei-2, but the relationship of uvs-3 to mei-3 could not be tested directly, because the double mutant was barely viable. These results indicate that mei-2 and mei-3 are involved in the same repair group as uvs-6, and uvs-3 is possibly involved in the same group. Alternatively, the mei genes, or uvs-3 itself, may be related to more than one DNA repair group.

The Neurospora uvs-2 gene encodes a protein which has homology to yeast RAD18, with unique zinc finger motifs

A clone containing the DNA repair gene uvs-2 of Neurospora crassa was identified from a Neurospora genomic DNA library using the sib-selection method. Transformants were screened for resistance to methyl methane sulfonate (MMS). A DNA fragment that complements the uvs-2 mutation was subcloned by monitoring its ability to transform the uvs-2 mutant to MMS resistance. Deletion analysis of the cloned DNA indicated that the size of the uvs-2 gene is approximately 1.6 kb. The identity of the uvs-2 gene was verified by restriction fragment length polymorphism (RFLP) mapping. The sensitivity of the transformant to three different mutagens was similar to that of the wild-type strain. Nucleotide sequences of genomic DNA and cDNA of the uvs-2 gene indicated that it has an open reading frame (ORF) of 1572 bp with a 69 bp intron in the middle of the sequence. Two transcription initiation sites located around 73 bp and 290 bp upstream of the translation initiation codon were identified by primer extension experiments. Northern analysis revealed that the nature transcript of the uvs-2 gene was about 1.8 kb long. The uvs-2 gene ORF is deduced to encode a polypeptide of 501 amino acids with a molecular mass of 54 kDa. The proposed polypeptide has 25% identity to the RAD18 polypeptide of Saccharomyces cerevisiae and contains two unique zinc finger motifs for nucleic acid binding. Similarities between the phenotypes of the rad18 and uvs-2 mutants suggest that the uvs-2 gene encodes a protein which is involved in postreplication repair, rather than excision repair.

The induction and repair of (6-4) photoproducts in Neurospora crassa

The (6-4) photoproduct lesion found in DNA after UV irradiation is repaired by germinating Neurospora crassa conidia. Wild-type Neurospora removes 80% of the (6-4) photoproduct in approximately 20 min and maximal repair is accomplished by 30 min with approximately 89% of the original lesions removed. Mutagen-sensitive Neurospora mutants belonging to the established excision repair epistasis group, UVS-2, are not defective in the removal of cyclobutane pyrimidine dimers. Furthermore, we find these mutants capable of removing (6-4) photoproducts from their DNA at a rate similar to wild type. Comparable kinetics are also observed in key members of the other two epistasis groups.

A novel phenotype of an excision-repair mutant in Neurospora crassa: mutagen sensitivity of the mus-18 mutant is specific to UV

A UV-sensitive mutant has been isolated from UV-mutagenized conidia of Neurospora crassa. The mutation responsible for the lesion was mapped in linkage group VL, proximal to the nucleolus organizer region. We designated the mutant mus-18. The sensitivity of the mus-18 mutant to UV-irradiation was not particularly high, being less than twice that of the wild-type strain. However, the frequency of mutations at the ad-3 loci induced by UV was extremely high even at low doses, under conditions where survival rates of mus-18 cells were almost identical to those of wild-type cells. Photo-reactivation of UV damage was normal in the mus-18 mutant. Sensitivity to other mutagens, such as gamma rays, 4-nitroquinoline-1-oxide, N-methyl-N'-nitro-N-nitrosoguanidine, mitomycin C and methyl methanesulfonate, was similar to that of the wild type. Fertility of the mus-18 mutant was normal in homozygous crosses. These results suggest that mus-18 is an excision-repair mutant. Measurement of endonuclease-sensitive sites (ESS) after liquid-holding recovery from UV damage revealed that ESS remained unrepaired for longer than 18 h in the mus-18 mutant, while most were eliminated within 6 h in wild-type cells and in other UV-sensitive mutants. This result suggests that mus-18 is defective in the incision step of dimer excision. The mus-18 mutant provides the first example of an excision-defective mutation in eukaryotes, which is specific to UV damage.

Epistasis, photoreactivation and mutagen sensitivity of DNA repair mutants upr-1 and mus-26 in Neurospora crassa

Double mutants were constructed combining mus-26, formerly designated uvs-(SA3B), with other UV-sensitive mutants. Tests of sensitivity of these double mutants to UV and to chemical mutagens revealed that mus-26 and upr-1 belong to the same epistatic group. The UV dose-response curve of mus-26 showed a characteristic plateau in the range of 100-200 J/m2. The same characteristic was also shown in the dose-response curves of upr-1 and the double mutant, upr-1 mus-26. Photoreactivation of UV damage in mus-26, upr-1 and upr-1 mus-26 was defective but not null. Assays were made of the reversion rate of ad-8 in strains that also carried UV-sensitive mutations. The reversion frequencies of the strains with upr-1 and upr-1 mus-26 were very low for the UV dose range below 300 J/m2, similarly to mus-26. Previously reported homozygous sterility of mus-26 was not caused by the mus-26 locus itself, and fertile strains were obtained among progeny. The results of this study suggest that mus-26 and upr-1 have similar properties in DNA repair.

Sensitivity to bleomycin and hydrogen peroxide of DNA repair-defective mutants in Neurospora crassa

Mutations were induced in Neurospora which cause increased sensitivity to MMS (methyl methane-sulfonate) and other mutagens. Genetic analysis of such mus demonstrated that some of them defined new DNA repair genes (mus-21, and mus-27 to mus-30), while others represented new alleles in previously known genes. To characterize them further, and especially to identify rec- types which have not yet been found in this species, many MMS-sensitive strains were tested for cross-sensitivities to bleomycin (BLM) and to hydrogen peroxide (H2O2) to which some rec- of other species are hypersensitive. In Neurospora, many of the MMS-sensitive mutants were found to be cross-sensitive to BLM and frequently these were also hypersensitive to ionizing radiation. Bleomycin sensitivity was demonstrated for all alleles of 10 different genes, 4 of them new ones, with mus-27 being the most sensitive of the latter (resembling uvs-6; Koga and Schroeder, 1987, Mutation Res., 183, 139). In contrast, very few of the MMS-sensitive mutants were hypersensitive to H2O2 and, in general, results of H2O2 tests were variable and differences between strains small. However, consistent deviations from wild type were observed in a few cases (most clearly for mus-9 and mus-11) when results from treatments of germinating conidia were compared with those of non-growing ones.

A new mutagen-sensitive mutant in Neurospora, mus-16

A new gene, mus-16, is determined by the nitrogen mustard-sensitive Neurospora mutant of Baker, Parish and Curtis (1984) which is defective in the removal of DNA-DNA and DNA-protein crosslinks. This gene is on the left arm of linkage group V between caf-1 and lys-1. The mus-16(JMB) mutant is sensitive to the alkylating agents methyl methanesulfonate (MMS) [dose reduction factor (drf) 8-10 X], N-methyl-N'-nitro-N-nitrosoguanidine (drf 5-6 X), the amino acid histidine and the drug hydroxyurea. It is not sensitive to ultraviolet-light, gamma-irradiation, or mitomycin C (MMC). It shows normal spontaneous mutation rates but increased induction of mutation by MMS. Homozygous crosses are barren, showing no signs of sporulation. Mitotic spontaneous chromosome instability is increased. The mus-16 mutation is similar to several non-excision repair-defective mutants in Neurospora. Some of these may be defective in repair of alkylation damage. The MMC data supports earlier data that in fungi MMC is incapable of forming DNA-DNA crosslinks.

Relationship of histidine sensitivity to DNA damage and stress induced responses in mutagen sensitive mutants of Neurospora crassa

Previous work in other laboratories has shown that several mutagen sensitive mutants of Neurospora crassa are extremely sensitive to low levels of histidine in the culture medium. We have shown that wild type Neurospora accumulates nicks or breaks in the DNA in the presence of histidine. The number of nicks accumulating in histidine sensitive mutants is found to increase in relation to their sensitivity to histidine. Although these nicks can be repaired by both wild type and histidine sensitive mutants when histidine is removed from the medium, a steady state number of nicks exists as long as histidine is present. We suggest that the presence of these nicks or breaks induces an increase in recombination in these possibly recombination defective mutants and that this is the source of the high level of histidine sensitivity. We speculate on the mechanisms by which histidine induces this DNA damage. This report also shows that several polypeptides are induced by the wild type organism in the presence of histidine. Some of these polypeptides are also induced during other stress situations, such as heat shock and DNA damage due to ultraviolet irradiation. Two of the histidine induced proteins cannot be induced by any of the histidine sensitive mutants.

Damage-resistant DNA synthesis in eukaryotes

The molecular basis of sensitivity of ionizing radiation and other damaging agents is not clearly defined in eukaryotes. While a large number of mutants have been described only a few have been demonstrated to have a defect in the repair of damage to DNA. An interesting characteristic of a sub-group of these mutants, in different species extending throughout the phylogenetic scale, is the presence of damage-resistant DNA synthesis. This phenomenon is observed in cells from individuals with the genetic disorder ataxia telangiectasia, in HeLa cells treated with fluorodeoxyuridine prior to UV irradiation, in mutants of the fungus Neurospora crassa, the slime mould Dictyostelium discoideum, the fruit fly Drosophila melanogaster and possibly in the "wasted" mouse mutant. In the case of ataxia telangiectasia sensitivity is only observed to ionizing radiation or radiomimetic chemicals whereas sensitivity to a wider spectrum of mutagens is reported for the lower eukaryotic mutants. In all cases a reduced inhibition of DNA synthesis is obtained after exposure to an agent to which the cell type is hypersensitive. It is unclear how damage-resistant DNA synthesis contributes to increased sensitivity in these cells, but is unlikely to be the major mechanism predisposing to radiation-induced cell death. The description of a derivative of an ataxia telangiectasia cell line with normal sensitivity to radiation but still maintaining resistant DNA synthesis partially uncouples radioresistant DNA synthesis and radiosensitivity. This paper is designed to review the phenomenon of damage-resistant DNA synthesis in a number of mutants.

A new ultraviolet-light sensitive mutant of Neurospora crassa with unusual photoreactivation property

A mutant, uvs-(SA3B), which shows high sensitivity to UV light segregated among the progeny in a back-cross of a presumptive MMS-sensitive mutant to a wild-type strain. At 37% survival, this mutant was approximately 5 times more sensitive to UV and also 6 times more sensitive to 4-NQO than the wild type. But it was only slightly sensitive to gamma-ray, MMS, MNNG, MTC and histidine. It showed an unusual photoreactivation response. Its time course of photorecovery was similar to the photoreactivation-defective strain upr-1 of Neurospora crassa. Mutation induction by UV at the ad-3 loci in this mutant strain was lower than that at the same loci in the wild-type strain. The uvs-(SA3B) mutant maps between met-1 and col-4 in linkage group IV, and it was not allelic with the mutagen-sensitive mutant mus-8 which is located in this area. We have concluded, therefore, that uvs-(SA3B) has resulted from mutation in a new DNA-repair gene. This new mutant was barren in homozygous crosses.

Effect of the homokaryotic or heterokaryotic state of the uvs-2 allele in Neurospora crassa on mitomycin C-induced killing and ad-3 mutation

Mitomycin C (MC) was tested for its killing and mutagenic activities in the ad-3 forward-mutation test in Neurospora crassa. The test was conducted in 4 dikaryons of N. crassa in order to determine the effect of the uvs-2 allele, which causes a defect in nucleotide excision repair, on MC-induced killing and ad-3 mutation. These dikaryons were homokaryotic for uvs-2+ (H-12), homokaryotic for uvs-2 (H-59), and heterokaryotic for uvs-2/uvs-2+ (H-70 and H-71). MC induced killing and ad-3 mutation in H-12, but the presence of uvs-2 in the homokaryotic state (H-59) resulted in a great increase in the killing and mutagenic activities of MC. This increased sensitivity to MC-induced killing and mutation conferred by uvs-2 in the homokaryotic state (H-59 vs. H-12) is a different effect than that noted by others for a defect in nucleotide excision-repair in Escherichia coli and Salmonella typhimurium or in human cells. The dikaryons heterokaryotic for uvs-2/uvs-2+ had the same sensitivity to MC as H-12, indicating that for MC-induced killing and ad-3 mutation uvs-2 is recessive to uvs-2+.