Postreplication repair in Neurospora crassa

Does trans-lesion synthesis explain the UV-radiation resistance of DNA synthesis in C. elegans embryos?

Over 10-fold larger fluences were required to inhibit both DNA synthesis and cell division in wild-type C. elegans embryos as compared with other model systems or C. elegans rad mutants. In addition, unlike in other organisms, the molecular weight of daughter DNA strands was reduced only after large, superlethal fluences. The molecular weight of nascent DNA fragments exceeded the interdimer distance by up to 19-fold, indicating that C. elegans embryos can replicate through non-instructional lesions. This putative trans-lesion synthetic capability may explain the refractory nature of UV radiation on embryonic DNA synthesis and nuclear division in C. elegans.

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.

Excision of pyrimidine dimers from the DNA of Neurospora

Germinated conidia of Neurospora have been monitored for their ability to excise pyrimidine dimers. Dimer concentration was measured in DNA extracted immediately after UV treatment, and it was compared to that of DNA from cells which had a post-UV incubation before extraction. Two methods were used to assay dimer level in DNA: measurement of the number of single-strand breaks (as revealed in alkaline sucrose gradients) produced by a dimer-specific endonuclease; monitoring the ability to compete for binding to dimer-specific antibodies in a radioimmunoassay. Both methods showed efficient excision of dimers by wild-type and by uvs-2, even though an earlier study had reported that uvs-2 was unable to excise dimers. UV-induced mutation shows a dose-rate effect: acute UV yields several times as many mutations as does the same dose of chronic UV. There is a parallel effect on dimer accumulation. The concentration of dimers at the conclusion of the UV treatment shows a strong correlation with the resultant mutation frequency.

Assay of cytotoxicity and mutagenicity of alkylating agents by using Neurospora spheroplasts

A system relying on the use of Neurospora crassa spheroplasts has been developed for the assay of cytotoxicity and mutagenicity of chemical compounds. Mutagenicity was assayed by using reversion of alleles in the am gene selected to recognize certain specified transitions and also undefined point mutations. Cytotoxicity was quantified by measuring a 'cytotoxicity parameter', m, which appears in the exponential function that fits the survival/dose curve for each compound (under standard incubation conditions). Of the compounds tested, nitrogen mustard (Cl(CH2)2 NMe(CH2)2Cl) was cytotoxic and non-mutagenic, and ethyl nitrosourea was highly mutagenic but not cytotoxic. Of the remaining compounds tested, methyl nitrosourea, butadiene diepoxide, and cis platin (cis diammonia platinum II chloride) all showed comparable mutagenicity per survivor, although the values of m covered a wide range. Differences were found between the different compounds in the effects of the uvs-2 allele on survival and on the preponderance of G to A transitions.

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.