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

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.

Effect of pre-treatment with 5-bromouracil in an ultraviolet sensitive strain of Neurospora crassa

5-Bromouracil was shown to enhance the mutation frequency in an ultraviolet sensitive strain ofNeurospora crassa (uvs-2) that was previously irradiated with ultraviolet light. The design of the experiment eliminated light or dark repair as possible hypotheses to account for the observed enhancement. Since a similar effect was noted at therib-1 locus inN. crassa, which is not a UV-sensitive strain, it appears that the error prone repair mechanism inN. crassamay be related to post-replication repair. This enhancement may be attributed to the interference of 5-bromouracil with post-replication repair. It is also postulated that the maximum increase in the mutation frequency occurs when the greatest number of unrepaired pre-mutational UV lesions are repaired in the presence of 5-bromouracil.

The excision of pyrimidine dimers from the DNA of mutant and wild-type strains of Ustilago

UV-induced pyrimidine dimers were excised from the DNA of wild-type and four mutant strains of Ustilago maydis. Excision was partially dose dependent. The kinetics of excision differed in recombination deficient strains (rec 1 and rec 2) from those found in a recombination proficient radiation-sensitive strain (uvs 3). At fluences above 100 J-m-2 excision was saturated in uvs 3 but not in rec 1 or rec 2. Fluences above 300 J-m-2 started to saturate excision in wild-type. pol1-1, a temperature-sensitive DNA polymerase mutant, was excision proficient at both the permissive (22 degree) and restrictive (32 degree) temperatures. Wild-type cells were observed to excise CC before CT or TT in high dose experiments.

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.

Characterization of excision repair in Neurospora crassa

The excision of pyrimidine dimers from the deoxyribonucleic acid (DNA) of Neurospora crassa was examined. Postirradiation incubation in the presence of several chemicals known to inhibit various repair systems indicated that caffeine reduced the rate of excision twofold, but did not inhibit excision completely as did proflavine and quinacrine. Examination of the time course of excision showed that repair occurs at a relatively rapid rate: approximately 60 dimers excised per min after 500 ergs/mm(2). Further evidence for rapid excision was obtained by sedimentation analysis of DNA; the maximal number of breaks introduced during repair was three, suggesting that breaks are repaired almost as fast as they are made and that only a few dimers are repaired at a time. Repair synthesis was measured by prelabeling the DNA with (15)N and D(2)O, and then subjecting the DNA to equilibrium density gradient centrifugation after postirradiation incubation with (32)P. Accumulation of single-strand breaks with increasing dose of ultraviolet radiation suggested that the limiting step was subsequent to the incision and excision steps of repair. Equilibrium CsCl centrifugation demonstrated that the limiting step in excision was repair synthesis.