Interaction of genes controlling ultraviolet sensitivity in Neurospora crassa

uvsI mutants defective in UV mutagenesis define a fourth epistatic group of uvs genes in Aspergillus

Three UV-sensitive mutations of A. nidulans, uvsI, uvsJ and uvsA, were tested for epistatic relationships with members of the previously established groups, here called the "UvsF", "UvsC", and "UvsB" groups. uvsI mutants are defective for spontaneous and induced reversion of certain point mutations and differ also for other properties from previously analyzed uvs types. They are very sensitive to the killing effects of UV-light and 4-NQO (4-nitro-quinoline-N-oxide) but not to MMS (methylmethane sulfonate). When double- and single-mutant uvs strains were compared for sensitivity to these three agents, synergistic or additive effects were found for uvsI with all members of the three groups. The uvsI gene may therefore represent a fourth epistatic group, possibly involved in mutagenic repair. On the other hand, uvsJ was clearly epistatic with members of the UvsF group and fitted well into this group also by phenotype. The uvsA gene was tentatively assigned to the UvsC group. uvsA showed epistatic interactions with uvsC in all tests, and like UvsC-group mutants is UV-sensitive mainly in dividing cells. However, the uvsA mutation does not cause the defects in recombination and UV mutagenesis typical for this group.

Inducible responses to DNA damaging or stress inducing agents in Neurospora crassa

Two-dimensional polyacrylamide gel electrophoresis has been used to analyze proteins from wild type and mutagen sensitive mutants of Neurospora crassa under constitutive conditions and after treatment with mutagens and other stress inducing agents. Several proteins have been detected that are either induced or show changes in electrophoretic mobility in response to UV irradiation, 4-NQO, X-ray, paraquat and heat shock. Ten proteins were found to respond to more than one of the stress inducing agents, demonstrating a complex pattern of polypeptide inductions. The significance of these findings and the possible effects of some of these proteins on the DNA repair process and stress management are discussed.

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.

Identification of DNA repair and damage induced proteins from Neurospora crassa

The response of Neurospora crassa to DNA damage induced by UV irradiation has been studied using two-dimensional polyacrylamide gel electrophoresis (2-D PAGE). Whole cell extracts of irradiated and untreated cultures were compared. Five polypeptides that show changes in response to DNA damage have been identified. Several mutagen sensitive strains of Neurospora were also tested for polypeptide changes on 2-D PAGE. Profiles of whole cell extracts of these mutant strains were compared to wild type. Two changes were observed in the meiotic mutant, mei-3 and one change was detected in the excision repair mutant, upr-1. Two changes were also detected in the allelic mutants, uvs-3 and nuh-4. Profiles of uvs-3 and nuh-4 revealed one polypeptide that was missing and another polypeptide which appeared to shift to a more basic position. This same shift was detected in wild type after induction by UV irradiation or heat shock.

Epistatic Grouping of Repair-Deficient Mutants in Neurospora: Comparative Analysis of Two uvs-3 Alleles, uvs-6 and Their mus Double Mutant Strains

The nuclease halo mutant, nuh-4, of Neurospora crassa was identified conclusively as an allele of uvs-3, a gene involved in error-prone DNA repair. Like uvs-3, nuh-4 showed spontaneous mutator effects, and any previous contradictory findings were found to be due to newly arisen mutants. In normal strains the two alleles are noncomplementing and indistinguishable for sensitivity to UV and methyl methanesulfonate (MMS). Like uvs-3, nuh-4 lacked secretion of the extracellular enzyme, DNase A, a Ca(++)-dependent strand-nonspecific endonuclease which was found to be phosphate repressible. However, nuh-4 differed from uvs-3 in showing much higher conidial viability and lower sensitivity to ionizing radiation and mitomycin C.--Epistatic relationships of the two uvs-3 alleles with seven other MMS-sensitive mutants were determined and compared with those of the highly X-ray-sensitive mutant, uvs-6. Three epistatic groups were found, based on survival of double mutant strains relative to that of their component single mutant strains after treatment with MMS. Both, uvs-3 and nuh-4, were epistatic to mus-9 which also is a mutator. None of the three produced viable double mutants in crosses to uvs-6. On the other hand, uvs-6, but not the uvs-3 alleles, was found to be epistatic to mus-7 and mus-10. The excision-defective uvs-2 and mus-8 both showed synergism with the uvs-3 alleles and with uvs-6, forming a third, separate epistatic group.

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.

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.

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.