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

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.

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.

The phenoloxidases of the ascomycete Podospora anserina. XIII. Action and interaction of genes controlling the formation of laccase

1. Eight mutants were isolated following mutagen treatment which are deficient in laccase formation. Seven of these had a pleiotropic effect and exhibited defects in growth rate and in mycelial and sexual morphology. 2. By means of tetrad analysis the mutations were assigned to 6 loci. Three mutations were in the incolora locus, the others were non-allelic. Only two of these loci were closely linked. 3. All genes exhibit numerous interactions. These concern the morphological expression of the laccase genes and also the laccase spectra. 4. The mutants could be separated into four classes on the basis of the amount and type of laccase produced. 5. Five of the loci studied appear to be structural genes because mutations alters the physical properties of the laccase protein. The sixth gene has a regulatory role.

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.