Genetic and enzymatic analysis of a gene controlling UV sensitivity in Neurospora crassa

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.

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.

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.

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.

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 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.

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.

Ultraviolet light-induction and photoreactivation of thymine dimers in a cyanobacterium, Anacystis nidulans

Partially photoreactivable mutant of Anacystis nidulans demonstrates partial photorepair of thymine dimers. The wild type which is completely photoreactivable at the conditions studied shows higher level of thymine dimer photolysis.

Inactivation of carotenoid-producing and albino strains of Neurospora crassa by visible light, blacklight, and ultraviolet radiation

Suspensions of Neurospora crassa conidia were inactivated by blacklight (BL) radiation (300 to 425 nm) in the absence of exogenous photosensitizing compounds. Carotenoid-containing wild-type conidia were less sensitive to BL radiation than albino conidia, showing a dose enhancement factor (DEF) of 1.2 for dose levels resulting in less than 10% survival. The same strains were about equally sensitive to shortwave ultraviolet (UV) inactivation. The kinetics of BL inactivation are similar to those of photodynamic inactivation by visible light in the presence of a photosensitizing dye (methylene blue). Only limited inactivation by visible light in the absence of exogenous photosensitizers was observed. BL and UV inactivations are probably caused by different mechanisms since wild-type conidia are only slightly more resistant to BL radiation (DEF = 1.2 at 1.0% survival) than are conidia from a UV-sensitive strain (upr-1, uvs-3). The BL-induced lethal lesions are probably no cyclobutyl pyrimidine dimers since BL-inactivated Haemophilus influenzae transforming deoxyribonucleic acid is not photoreactivated by N. crassa wild-type enzyme extracts, whereas UV-inactivated transforming deoxyribonucleic acid is photoreactivable with this treatment.

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.