Ultraviolet-sensitive mutants of Neurospora. II. Radiation studies

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.

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.

Characterization of a Neurospora crassa photolyase-deficient mutant generated by repeat induced point mutation of the phr gene

We produced a photolyase-deficient mutant by repeat induced point mutation using the Neurospora crassa photolyase gene cloned previously. This mutation identified a new gene, phr, which was mapped on the right arm of linkage group I by both RFLP mapping and conventional mapping. To investigate the relationship between photoreactivation and dark repair processes, especially excision repair, double mutants of phr with representative repair-defective mutants of different types were constructed and tested for UV sensitivity and photoreactivation. The results show that the phr mutation has no influence on dark repair. Tests with CPD and TC(6-4) photoproduct-specific antibodies demonstrated that the phr mutant is defective in CPD photolyase and confirmed that there is no TC(6-4) photolyase activity in N. crassa. Furthermore, N. crassa photolyase is not a blue light receptor in the signal transduction that induces carotenoid biosynthesis.

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.

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.

Interpretation of UV-survival curves of Aspergillus conidiospores

Semi-logarithmic dose-response curves for survival of UV-irradiated conidiospores of A. nidulans have an initial shoulder (at low doses) followed by a decline which becomes linear. To explain the initial shoulder and the resulting extrapolation number (log S intercept of the linear extrapolation line) a general model is presented, which includes multi-target (n) and multi-hit (h) effects and allows for the effect of initial repair and of a compound parameter k, which stands for inherent sensitivity of the spores and for dose received inside the spores. From experiments on (a) the modification of k (spore wall colour and shelter effects), (b) a repair-deficient strain (shoulderless) and (c) preincubation during which DNA-replication takes place, it is concluded that the shoulder is generated by initial repair rather than by a multi-hit nature of the cell-killing process. In experiments where k takes different values (sub a and c), notably the position of the point of intersection of the linear lines gives conclusive information. In general, the log S intercept of the linear extrapolation line cannot be used to estimate the target number.

Gamma-ray-sensitive mutants of Neurospora crassa with characteristics analogous to ataxia telangiectasia cell lines

Well characterized gamma-ray sensitive mutants of the fungus Neurospora crassa have been screened for characteristics analogous to those of cell lines derived from humans with the genetic disease, ataxia telangiectasia (AT). Two Neurospora mutants, uvs-6 and mus-9, show the AT cell line characteristics of gamma-ray and bleomycin sensitivity, and little or no repression of DNA synthesis following treatment with these agents. Normal human or Neurospora cells show an extensive biphasic DNA synthesis repression (to 50% of control) and when DNA synthesis is analyzed by alkaline sucrose gradient centrifugation, repression of DNA synthesis by low doses of gamma-radiation occurs primarily in low molecular weight (MW) DNA pieces in both organisms. In AT cells and the uvs-6 mutant, no repression in synthesis of low or higher MW DNA is seen at low doses, while the mus-9 mutant shows little repression of high MW DNA, but an intermediate level of low MW DNA synthesis. Both mutants have been shown previously to have an increased level of spontaneous chromosome instability as do AT lines. The uvs-6 and mus-9 mutations are known to be due to two different genes in two different epistatic groups. These results demonstrate that AT-like cellular characteristics can arise from defects in at least two and probably any of several genes, and that lower eukaryotes such as Neurospora can provide an inexpensive and useful model for AT while avoiding the problems inherent in using transformed cell lines.

UV-induced recessive lethals in uvs strains of Neurospora which are deficient in UV mutagenesis

The frequencies of spontaneous and UV-induced recessive lethal mutations were compared for UV-sensitive and wild-type heterokaryons of Neurospora crassa. These heterokaryons were homokaryotic either for one of two alleles of uvs-3, or for uvs-6 or uvs+. For uvs-3, which is known to have mutator effects, spontaneous recessive lethals were found to be 4-6 times more frequent than observed in uvs+. After correction for clonal distribution of spontaneous mutants, an observed 2-fold increase for uvs-6 was not statistically significant and may have been due to chance occurrence of a few large clones of mutants. Treatment with low doses of UV (50-200 J/m2) produced very similar overall rates of increase for recessive lethals in uvs and uvs+ heterokaryons. This means, that in contrast to results obtained when mutation to ad-3 was measured, both uvs-3 alleles showed highly significant increases for recessive lethals when treated with UV. It is proposed that certain types of UV damage may be processed into recessive lethal mutations by an alternate mechanism from that responsible for viable mutations.

Isolation and characterization of MMS-sensitive mutants of Neurospora crassa

Seven different mutants that show high sensitivity to MMS killing were isolated and mapped at different loci. One group, mms-(SA1), mms-(SA2) and mms-(SA6), showed high sensitivity to MMS but not to UV or gamma-rays. Another group, mms-(SA4) and mms-(SA5), showed extremely high sensitivity to UV and MMS. And mms-(SA3) and mms-(SA7) were moderately sensitive to both UV and MMS. Mms-(SA4) and mms-(SA1) were identified as alleles of uvs-2 and mus-7, respectively, which had been previously isolated. The mms-(SA1), mms-(SA6) and mms-(SA7) strains were barren in homozygous crosses, and the mms-(SA5) strain was barren in heterozygous crosses. The mms-(SA1), mms-(SA3) and mms-(SA5) strains showed high sensitivity to histidine. In summary, at least two new loci involved in the repair of MMS damage have been identified. The possibility that some of these new mutants are in new repair pathways is suggested.

Genetic effects of N-methyl-N'-nitro-N-nitrosoguanidine and its homologs

Since the discovery of the mutagenic activity of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in 1960, this compound has become one of the most widely used chemical mutagens. The present paper gives a survey on the chemistry, metabolism, and mode of interaction of MNNG with DNA and proteins, and of the genotoxic effects of this agent on microorganisms, plants, and animals, including human cells cultured in vitro. Data on the carcinogenicity and teratogenicity of MNNG as well as on the genotoxic effects of homologs of MNNG are also presented.

The role of pyrimidine dimers in postreplication repair in Neurospora

Using the Micrococcus luteus dimer specific endonuclease assay of Wilkins (1973), and photoreactivation we have examined the induction and fate of ultraviolet induced pyrimidine dimers in the excision defective strain, uvs-2, of Neurospora crassa. Dimer induction was fluence dependent from 0 to 800 ergs/mm2 UV. An interdimer distance of 19.6 x 10(6) DNA molecular weight was found after a fluence of 220 ergs/mm2. We confirm the earlier report that this mutant is completely excision defective (Worthy and Epler 1972). Photoreactivation (PR), which greatly enhanced survival (by 10 fold after 440 ergs/mm2 UV), reduced significantly (40-44%) the number of UV-endonuclease sensitive sites found in irradiated DNA. This treatment also alleviated immediately some of the temporary blocks to high molecular weight DNA synthesis (elongation or ligation) seen in irradiated cells. We have also attempted to elucidate the mechanism of cellular postreplication repair used to overcome the UV inhibition to DNA synthesis. It was determined that during postreplication repair, Neurospora does not use recombination to bypass dimers and that single stranded DNA gaps opposite dimers do not appear to be present during the time when DNA being synthesized is made only in short pieces.

Postreplication repair in Neurospora crassa

Changes in the molecular weight of nascent DNA made after ultraviolet (UV) irradiation have been studied in the excision-defective Neurospora mutant uvs-2 using isotopic pulse labeling, alkaline gradient centrifugation and alkaline filter elution. Both the size of nascent DNA and the rate of incorporation of label into DNA was reduced by UV light in a dose dependent manner. However, this DNA repair mutant did recover the ability to synthesize control-like high molecular weight DNA 3 hours after UV treatment, although the rate of DNA synthesis remained depressed after the temporary block to elongation (or ligation) had been overcome. Photoreactivation partially eliminated the depression of DNA synthesis rate and UV light killing of cells, providing strong evidence that the effects on DNA synthesis and killing were caused by pyrimidine cyclobutane dimers. The caffeine inhibition repair studies performed were difficult to quantitate but did suggest either partial inhibition of a single repair pathway or alternate postreplication DNA repair pathways in Neurospora. No enhancement in killing was detected after UV irradiation when cells were grown on caffeine containing plates.

Mutagenesis at the ad-3A and ad-3B loci in haploid UV-sensitive strains of Neurospora crassa. V. Comparison of dose--response curves of single- and double-mutant strains with wild-type

The interactions of mutant alleles that individually confer radiation sensitivity in Neurospora crassa are being studied with regard to their effects on radiation-induced inactivation and forward-mutation induction at the ad-3 loci. This paper reports attempts to construct 3 double-mutant strains containing the following pair-wise combinations of repair-deficient mutants: upr-1,uvs-2; uvs-2,uvs-6; and uvs-3,uvs-6. The double-mutant strain with the 2 excision-repair-deficient mutants upr-1 and uvs-2 shows increased sensitivity to X-ray-induced mutagenesis and inactivation, relative to that shown by either of the parental single-mutant strains. This double mutant is no more sensitive than the parental single-mutant strains to either UV mutagenesis or inactivation. The combination of the uvs-2 and uvs-6 double-mutant strain is considerably more sensitive to both UV and X-ray inactivation than either the uvs-2 or uvs-6 strain, but it shows no greater sensitivity than the parental strains to ad-3 mutation induction by either agent. The combination of the uvs-3 and uvs-6 alleles is inviable. Tetrad analysis and microscopical examination of ascospores shows that ascospores of presumptive genotype uvs-3, uvs-6 do not grow beyond the formation of a few hyphal threads. The lethal and mutagenic effects of UV and X-irradiation in these double-mutant strains are interpreted in terms of the repair systems in Neurospora and other microorganisms.

Mutagen sensitivities and mutator effects of MMS-sensitive mutants in Neurospora

7 mus (mutagen-sensitive) mutants of Neurospora crassa, which are more sensitive to the toxic effects of MMS (methyl methanesulfonate) than wild-type, were investigated for cross-sensitivities to other mutagens and inhibitors. These mutants have recently been mapped in 5 new genes, mus-7 to mus-11, and mutant alleles from each gene were checked for their effects on mutation frequencies. It was found that mutants in 3 of these 5 genes showed radiation-induced mutation frequencies similar to wild-type. These included 2 alleles of the gene mus-10, which were cross-sensitive only to UV and were the only mutants that produced some viable ascospores in homozygous crosses. The mutant of the second gene, mus-8, was especially sensitive to UV and mitomycin C and produced slightly reduced frequencies of spontaneous mutation. In contrast, the mutant of the third gene, mus-7, was not UV-sensitive but showed some cross-sensitivity to X-rays; mus-7 was highly sensitive to MMS and also to histidine, which inhibits various repair-defective mutants at concentrations well below those that reduce wild-type growth. None of these mus resemble mutants previously found in Neurospora, nor do they conform clearly to mutant types identified in E. coli or yeast. On the other hand mutants in 2 further genes, mus-11, and especially 2 alleles of mus-9, are very similar to uvs-3 of Neurospora and generally resemble mutants that are considered to be defective in "error-prone" repair. They were UV- as well as X-ray-sensitive, and showed strong spontaneous mutator effects but almost no increase in recessive lethal frequencies in heterokaryons after UV-treatments.

Isolation and genetic analysis of MMS-sensitive mus mutants of neurospora

With the aim of obtaining mutants that affect DNA repair or recombination, mutants sensitive to methylmethane sulfonate (MMS) have been isolated in the ascomycete Neurospora crassa. Seven of these mutants were backcrossed repeatedly to produce isogenic strains for measurements of relative mutagen sensitivities and for analysis of recombination frequencies. The new mus (mutagen sensitives) were compared to four previously known radiation-sensitive mutants which were shown to be cross-sensitive to MMS. Tests for allelism assigned the mus mutants to five new genes, mus-7 to mus-11, each mapping in a different linkage group. In homozygous crosses all mutants were sterile, except the two alleles of gene mus-10 which occasionally produced some viable ascospores. Complementation tests on MMS-media identified double mutant strains from many intercrosses. Such strains can be used for analysis of interactions between mutant alleles from different genes and of possible epistatic groupings for repair-deficient mutants in Neurospora. Four of these double mutant strains, all containing mus-8 and previously known mutants, were checked for survival on MMS media and their sensitivities were compared to those of their parental single mutant strains. Results indicate that mus-8 may be epistatic to uvs-2 which is deficient in excision repair, but not to mutants like uvs-3 that appear to be deficient in error-prone repair.

Nucleases and their control in wild-type and nuh mutants of Neurospora

A review of all of the work on Neurospora nucleases strongly suggests that five nucleases, originally isolated on the basis of markedly different properties, may actually be derived from a single inactive precursor polypeptide via different routes of proteolysis. One of these nucleases may be involved in DNA repair and/or recombination. Two repair-deficient mutants of Neurospora, uvs-3 and nuh-4, may have a lesion in protease(s) which control the level of this nuclease or in some function which regulates the protease(s). Both of these mutants map in the same gene region and they may be defective in recombination, since they are sensitive to various mutagens and to mitomycin C and they show high frequency of spontaneous, but not radiation-induced, recessive lethal mutations and/or deletions.

Mutagen-sensitive mutants in Neurospora

Initial work on the fungus Neurospora crassa has shown that a least two DNA-repair systems exist in this eukaryote: excision repair and a mutation-prone repair. The evidence suggests that there is also a third repair system. Recently, new mutagen-sensitive strains have been isolated in several laboratories, but they are not yet fully characterized. A hunt for cytoplasmically inherited UV sensitivity has failed to turn up any such mutants among 25 new UV-sensitive isolates.

Isolation and genetic analysis of nuclease halo (nuh) mutants of Neurospora crassa

Nuclease halo (nuh) mutants of the ascomycete Neurospora crassa have been isolated which are characterized reduced release of deoxyribonuclease (DNase) activities from colonies grown on sorbose-containing agar media. To identify nuh mutants, mutagenized isolates were transferred to commercial DNase test agar, or grown on minimal medium and then overlayed with agar that contained heat-denatured DNA. DNase activity was visualized by acid precipitation which produced clear rings of digestion (haloes) around the colonies. To identify the number of genes in which mutations lead to reduced release of nuclease activity, eleven nuh mutants were checked for close linkage and linked pairs were tested for complementation. These mutants were assigned to eight genes, and all except one were mapped in six small regions of the Neurospora linkage maps. In addition, among a large number of existing mutants which were tested for nuclease haloes, two mutants were found that showed the Nuh phenotype, namely uvs-3 and uvs-6. One of the isolated nuh mutants was also found to be sensitive to UV and was mapped close to uvs-3; it may represent a new allele of this gene. As a first step towards identification of genuine nuclease mutants, extensively backcrossed strains of mutants from different genes have been assayed for nuclease activity with denatured DNA in extracts. A pronounced reduction, compared to wild type at the same stage of growth, was found in uvs-3 and also in nuh-3, a mutant that is not UV-sensitive.

Evidence for joint genic control of spontaneous mutation and genetic recombination during mitosis in Saccharomyces

A procedure for detection of mutants exhibiting either enhanced or reduced spontaneous mutation during mitosis and/or meiosis has been developed to probe the joint genic control of spontaneous mutation and recombination in yeast. A semidominant mutator, rem1-1, recovered by this technique, exhibits enhanced spontaneous mutation,intragenic recombination, and intergenic recombination during mitosis. Diploids homozygous for rem1-1 exhibit normal levels of meiotic intragenic and intergenic recombination and diminished ascospore viability.

Gamma-ray inactivation of conidia from heterokaryons of Neurospora crassa containing UV-sensitive mutations

Two-component heterokaryons were formed with the fungus Neurospora crassa. The UV-sensitive mutations upr-I, uvs-4, and uvs-6 were utilized. Conidia produced by these heterokaryons were exposed to gamma-rays and survival curves were established for the three conidial fractions produced by each heterokaryon. Results showed that upr-I, when included in only one nuclear component, did not affect the sensitivity of any conidial fraction; however, when included in both components, all three conidial fractions exhibited two- to four-fold decreases in survival at the 30 krad exposure. The uvs-4 mutation, when included in one or both components, did not increase the sensitivity of any conidial fraction and appeared, in contrast, to impart a small increase in resistance to inactivation by gamma-rays. When included in only one component, uvs-6 increased the sensitivity of homokaryotic uvs-6 conidia but had no affect on the other two conidial fractions. When included in both components, uvs-6 resulted in exponential inactivation curves and at the 30 krad exposure, 100-fold decreases in survival for all three conidial fractions.

Ultraviolet-inactivation of conidia from heterokaryons of Neurospora crassa containing uv-sensitive mutations

The effect of three UV-sensitive mutations of Neurospora crassa, upr-I, uvs-4 and uvs-6, on the ultraviolet-inactivation of conidia from two-component heterokaryons was investigated. In two-component heterokaryons with wild-type sensitivity to radiation inactivation, all three conidial fractions exhibited similar ultraviolet-inactivation curves. Each UV-sensitive mutation studied uniquely modified the ultraviolet-inactivation curves of conidia from two-component heterokaryons. In heterokaryons heterokaryotic for upr-I, the upr-I mutation was recessive and the repair function determined by the wild type allele was functional to some degree in homokaryotic upr-I conidia. All three conidial fractions of heterokaryons containing upr-I in both components showed increased sensitivity to ultraviolet light. The uvs-4 mutation was recessive and resulted in conidia with increased UV-sensitivity only when included in both components of a heterokaryon. Homokaryotic uvs-4 conidia, which arose from heterokaryons containing both uvs-4 and wild-type components, exhibited wild-type survival. Therefore, as with upr-I, there was a carryover the repair capability to conidia which were genetically UV-sensitive. The uvs-6 mutation, when included in one component of a two-component heterokaryon, resulted in increased UV-sensitivity of both heterokaryotic and homokaryotic uvs-6 conidia. When both components contained uvs-6, the UV-sensitivity of all three conidial fractions was increased and all showed similar inactivation curves. Thus, as with upr-I and uvs-4, there was a carryover of the wild-type repair capability to genetically uvs-6 conidia. Heterokaryon tests for complementation between two non-allelic UV-sensitive mutations showed that in heterokaryotic conidia, complete complementation occurred between upr-I and uvs-4.

Genetic control of radiation sensitivity and DNA repair in Neurospora

Radiation sensitivity in the fungus Neurospora crassa is under the control of at least eight distinct loci and is also affected by cytoplasmic factors. Although radiation-sensitive mutants which affect inter- or intragenic meiotic recombination have not been isolated, mutants which are defective in the repair of pyrimidine dimers have been found. Evidence from both mutational and biochemical studies shows that Neurospora has an excision-repair system for pyrimidine dimers which is very similar to the one found in Escherichia coli. Wild-type strains excise dimers, but two mutants, uvs2 and upr1, are UV sensitive and excision defective. Like the E. coli excision-defective mutants, the Neurospora mutants show a greatly increased frequency of UV-induced mutation at low UV doses, and they do not affect recombination. However, they differ from the E. coli mutants in being significantly more sensitive to ionizing radiation than wild-type strains. A third mutant, uvs6, resembles the DNA polymerase-I-negative mutants of E. coli. It is sensitive to both UV and X-irradiation, has a wild-type pattern of UV-induced mutation, and increases spontaneous deletion frequencies. Its polymerases have not been examined. The high frequency of UV-induced mutation in excision-defective strains suggests that a "mutation prone" system of DNA repair exists in Neurospora. This is supported by the ppoperties of the uvs3 strain, which shows no UV-induced mutation. Like postreplication-repair-defective E. coli mutants, it is UV and ionizing radiation sensitive and sensitive to both monofunctional and bifunctional alkylating agents. This mutant is sterile. As expected, the double mutant uvs3 upr1 strain is much more sensitive to UV than either single-mutant strain. Two other loci, muc2 and gs6, may affect DNA repair. Mutations at the five remaining loci, uvs1, uvs4, uvs5, gs3, and gs20, lead to a constellation of properties unlike those of any DNA-repair-deficient E. coli mutant. The occurrence of these mutations could mean that other DNA repair systems exist in Neurospora, or, like the lon mutants of E. coli, they might indicate that cell sensitivity to radiation damage can be increases in other ways.

Interaction of genes controlling ultraviolet sensitivity in Neurospora crassa

Two independently segregating ultraviolet (UV) sensitivity genes in Neurospora crassa interact synergistically resulting in UV sensitivity approximately twice that expected based on an evaluation of the sensitivities of the individual mutants. The mutant genes singly and together reduce photoreactivation (PR) in vivo although a PR enzyme is produced which exhibits normal activity in vitro.

Repair of ultraviolet light-induced damage to the deoxyribonucleic acid of Neurospora crassa

A method is described for labeling a specific pyrimidine in the deoxyribonucleic acid (DNA) of Neurospora crassa. In cells grown in the presence of [5-(3)H]-uridine, more than 97% of the radioactivity associated with the DNA had been incorporated into cytosine. The specific activity of the labeled DNA was approximately 3 x 10(3) counts per min per mug. The DNA was isolated by elution from hydroxyapatite columns with sodium phosphate buffer (0.40 m, pH 6.8). This procedure was used to demonstrate that in vegetative cells of N. crassa both photoreactivation and excision repair are operative, as measured by the removal of ultraviolet light-induced cytosine-containing dimers.