WC-1 and the Proximal GATA Sequence Mediate a Cis-/Trans-Acting Repressive Regulation of Light-Dependent Gene Transcription in the Dark

Simple Summary

We observed that both the proximal GATA sequence in light-responsive elements (LREs) of the albino-3 promoter and the Zinc Finger Domain of WC-1 are involved in the dark-related repressive control mechanism of light-regulated genes.

Abstract

Light influences a wide range of physiological processes from prokaryotes to mammals. Neurospora crassa represents an important model system used for studying this signal pathway. At molecular levels, the WHITE COLLAR Complex (WCC), a heterodimer formed by WC-1 (the blue light photo-sensor) and WC-2 (the transcriptional activator), is the critical positive regulator of light-dependent gene expression. GATN (N indicates any other nucleotide) repeats are consensus sequences within the promoters of light-dependent genes recognized by the WCC. The distal GATN is also known as C-box since it is involved in the circadian clock. However, we know very little about the role of the proximal GATN, and the molecular mechanism that controls the transcription of light-induced genes during the dark/light transition it is still unclear. Here we showed a first indication that mutagenesis of the proximal GATA sequence within the target promoter of the albino-3 gene or deletion of the WC-1 zinc finger domain led to a rise in expression of light-dependent genes already in the dark, effectively decoupling light stimuli and transcriptional activation. This is the first observation of cis-/trans-acting repressive machinery, which is not consistent with the light-dependent regulatory mechanism observed in the eukaryotic world so far.

A relationship between carotenoid accumulation and the distribution of species of the fungus Neurospora in Spain

The ascomycete fungus Neurospora is present in many parts of the world, in particular in tropical and subtropical areas, where it is found growing on recently burned vegetation. We have sampled the Neurospora population across Spain. The sampling sites were located in the region of Galicia (northwestern corner of the Iberian peninsula), the province of Cáceres, the city of Seville, and the two major islands of the Canary Islands archipelago (Tenerife and Gran Canaria, west coast of Africa). The sites covered a latitude interval between 27.88° and 42.74°. We have identified wild-type strains of N. discreta, N. tetrasperma, N. crassa, and N. sitophila and the frequency of each species varied from site to site. It has been shown that after exposure to light Neurospora accumulates the orange carotenoid neurosporaxanthin, presumably for protection from UV radiation. We have found that each Neurospora species accumulates a different amount of carotenoids after exposure to light, but these differences did not correlate with the expression of the carotenogenic genes al-1 or al-2. The accumulation of carotenoids in Neurospora shows a correlation with latitude, as Neurospora strains isolated from lower latitudes accumulate more carotenoids than strains isolated from higher latitudes. Since regions of low latitude receive high UV irradiation we propose that the increased carotenoid accumulation may protect Neurospora from high UV exposure. In support of this hypothesis, we have found that N. crassa, the species that accumulates more carotenoids, is more resistant to UV radiation than N. discreta or N. tetrasperma. The photoprotection provided by carotenoids and the capability to accumulate different amounts of carotenoids may be responsible, at least in part, for the distribution of Neurospora species that we have observed across a range of latitudes.

Fungal photoreceptors: sensory molecules for fungal development and behaviour

Light regulates fungal development and behaviour and activates metabolic pathways. In addition, light is one of the many signals that fungi use to perceive and interact with the environment. In the ascomycete Neurospora crassa blue light is perceived by the white collar (WC) complex, a protein complex formed by WC-1 and WC-2. WC-1 is a protein with a flavin-binding domain and a zinc-finger domain, and interacts with WC-2, another zinc-finger domain protein. The WC complex operates as a photoreceptor and a transcription factor for blue-light responses in Neurospora. Proteins similar to WC-1 and WC-2 have been described in other fungi, suggesting a general role for the WC complex as a fungal receptor for blue light. The ascomycete Aspergillus nidulans uses red light perceived by a fungal phytochrome as a signal to regulate sexual and asexual development. In addition, other photoreceptors, rhodopsins and cryptochromes, have been identified in fungi, but their functional relevance has not been elucidated. The investigation of fungal light responses provides an opportunity to understand how fungi perceive the environment and to identify the mechanisms involved in the regulation by light of cellular development and metabolism.

Posttranslational photomodulation of circadian amplitude

The transcription-translation feedback loops that form our current view of how the core mechanism of the clock operates is being challenged, as more and more posttranslational events are seen as essential to a full understanding of oscillator function. But in addition to phosphorylation, other processes may be involved. Here, a novel mechanism of posttranslational photomodulation of circadian amplitude is described that uniquely ties together light perception, protein stabilization, and proteolysis. In the process, the waveform of a core clock component is sharpened or "sculpted," resulting in appropriately high amplitude and proper phasing to obtain normal clock function.

Radiobiological studies of plants orbited in Biosatellite II

The Biosatellite II Tradescantia experiment probed the effects of the space environment on spontaneous and radiation-induced mutation rates and on cytological changes in Tradescantia clone 02. Thirty two young flowering plants arranged in a plastic housing with the roots immersed in nutrient solution were exposed to gamma radiation from an on-board 85 Strontium source during the two-day orbital flight. Unirradiated plants were flown in a package in the spacecraft behind a tungsten radiation shield and identical non-flight control packages (with and without irradiation) were maintained at the launch site. After retrieval of the spacecraft near Hawaii, samples of root tip, ovary and stamen tissues were collected. These and the intact plants were flown to the Brookhaven National Laboratory for observations on the following end points: somatic mutation, cell size, loss of reproductive integrity resulting in stunted stamen hairs, pollen grain mortality, frequency of micronuclei in pollen, disturbed mitotic spindle function and chromosome aberrations. Analysis of data on somatic mutation, cell size and chromosome aberration end points showed no significant differences between flight and non-flight samples. However, pollen abortion, frequency of micronuclei in pollen and loss of reproductive integrity (stamen hair stunting) showed increases associated with weightlessness in irradiated material. Root tip and microspore cells showed effects of disturbed mitotic spindle function in orbited plants both with and without irradiation. Clearly differences exist between flight and non-flight material and the significance and possible mechanisms for these effects are being studied in continuing non-flight tests.

PAS domain-mediated WC-1/WC-2 interaction is essential for maintaining the steady-state level of WC-1 and the function of both proteins in circadian clock and light responses of Neurospora

In the frq-wc-based circadian feedback loops of Neurospora, two PAS domain-containing transcription factors, WHITE COLLAR-1 (WC-1) and WC-2, form heterodimeric complexes that activate the transcription of frequency (frq). FRQ serves two roles in these feedback loops: repressing its own transcription by interacting with the WC complex and positively upregulating the levels of WC-1 and WC-2 proteins. We report here that the steady-state level of WC-1 protein is independently regulated by both FRQ and WC-2 through different posttranscriptional mechanisms. The WC-1 level is extremely low in wc-2 knockout strains, and this low level of expression is independent of wc-1 transcription and FRQ protein expression. In addition, our data show that the PAS domain of WC-2 mediates the interactions of this protein with both WC-1 and FRQ in vivo. Such interactions are essential for maintaining the steady-state level of WC-1 and the proper function of WC-1 and WC-2 in circadian clock and light responses.

Assignment of circadian function for the Neurospora clock gene frequency

Circadian clocks consist of three elements: entrainment pathways (inputs), the mechanism generating the rhythmicity (oscillator), and the output pathways that control the circadian rhythms. It is difficult to assign molecular clock components to any one of these elements. Experiments show that inputs can be circadianly regulated and outputs can feed back on the oscillator. Mathematical simulations indicate that under- or overexpression of a gene product can result in arrhythmicity, whether the protein is part of the oscillator or substantially part of a rhythmically expressed input pathway. To distinguish between these two possibilities, we used traditional circadian entrainment protocols on a genetic model system, Neurospora crassa.

Blue light photoreception in Neurospora circadian rhythm: evidence for involvement of the flavin triplet state

The mechanism of the photoreceptor acting on the circadian conidiation rhythm of Neurospora crassa was studied, with the following results: (1) the efficiency of 8-haloflavins as sensitizers increased with their triplet yields. (2) Phase shifts were not abolished by removal of oxygen prior to illumination. (3) Oxygen inhibited phase shifts when introduced into the cultures after light treatment. It is proposed that the blue light photoreceptor for the circadian clock of Neurospora crassa acts (1) from its triplet state, but (2) not via singlet oxygen; (3) signal transduction involves (an) oxygen-sensitive intermediate(s).

Light-induced dephosphorylation of a 33 kDa protein in the wild-type strain of Neurospora crassa: the regulatory mutants wc-1 and wc-2 are abnormal

Light induces the dephosphorylation of a 33 kdalton protein within 8 min in the wild-type strain of Neurospora crassa. The regulatory mutants, wc-1 and wc-2, have an altered pattern of phosphoproteins in darkness and also after irradiation. Because the wc genes have previously been implicated in photodifferentiation (F. Degli Innocenti and V. E. A. Russo, Genetic analysis of blue light-induced responses in Neurospora crassa, in H. Senger (ed.), Blue Light Effects in Biological Systems, Springer-Verlag, Berlin, Heidelberg, 1984, pp. 213-219), we suggest that protein dephosphorylation may constitute a necessary step in the light-transduction chain of Neurospora crassa.

X-ray-induced specific-locus mutations in the ad-3 region of two-component heterokaryons of Neurospora crassa. V. Irreparable mutants of genotype ad-3A ad-3B, ad-3A ad-3B nic-2, and ad-3B nic-2 result from multilocus deletion and an unexpectedly high frequency of multiple-locus mutations

More extensive complementation tests than those performed initially (Webber and de Serres, 1965) on a series of 832 X-ray-induced specific-locus mutations in the adenine-3 (ad-3) region of a two-component heterokaryon (H-12) of Neurospora crassa (de Serres, 1989a) showed that unexpectedly high frequencies of specific-locus mutations in the ad-3 region have additional, but separate, sites of recessive lethal (RLCL) damage in the immediately adjacent genetic regions. The frequencies of these X-ray-induced multiple-locus mutants in the ad-3 region are orders of magnitude higher than that expected on the basis of target theory and classical models of chromosome structure during interphase (de Serres, 1989a). Genetic fine structure analyses, by means of homology tests with tester strains carrying genetic markers in the ad-3 and immediately adjacent regions, have been performed to map the presumed multiple-locus mutations. In a previous paper (de Serres, 1989c), X-ray-induced irreparable ad-3 mutants of the following genotypes and numbers (ad-3A or ad-3B were analyzed, and the high frequency of multiple-locus mutations was confirmed. In the present paper, X-ray-induced irreparable ad-3 mutants of the following genotypes and numbers (ad-3A ad-3B, ad-3A ad-3B nic-2, and ad-3B nic-2 have also been subjected to the same genetic fine structure analysis. These experiments, in the previous (de Serres, 1989c) and present papers, were designed to determine the extent of the functional inactivation in the ad-3 and immediately adjacent genetic regions in individual mutants classified as presumptive multilocus deletions or multiple-locus mutations.

Fast induction of translatable mRNA by blue light in Neurospora crassa wt: the wc-1 and wc-2 mutants are blind

After blue-light irradiation of Neurospora crassa (wt) mycelia we observed an increase of about 13 translatable mRNA species within a period of 30 min. The induction of translatable mRNA species followed a specific temporal pattern which permitted the identification of four distinct classes. One of the translatable mRNAs was induced in less than 2 min, while the others showed lag periods of 5, 10 or 20 min from the beginning of illumination. The white collar mutants, wc-1 and wc-2, which do not display any of the blue-light-induced physiological effects tested until now were found to be defective for the photoinduction of translatable mRNAs.

X-ray-induced specific-locus mutations in the ad-3 region of two-component heterokaryons of Neurospora crassa. IV. Irreparable mutants of genotype ad-3A and ad-3B result from multilocus deletion and an unexpectedly high frequency of multiple-locus mutations

The induction of specific-locus mutations in the ad-3 region of Neurospora crassa after X-irradiation was studied in a two-component heterokaryon to determine: (1) the ratio of reparable ad-3 mutants (presumed gene/point mutations, designated ad-3R) to irreparable ad-3 mutants (presumed multilocus deletions, designated ad-3IR), and (2) the induction kinetics of each class (Webber and de Serres, 1965). More extensive genetic tests made subsequently (de Serres, 1989a) on the 832 X-ray-induced specific-locus mutations recovered in those experiments showed that unexpected high frequencies of reparable and irreparable ad-3 mutants are actually multiple-locus mutants that have additional, but separate, sites of recessive lethal (RLCL) damage in the immediately adjacent genetic regions (designated ad-3R + RLCL or ad-3IR + RLCL). The frequencies of these X-ray-induced multiple-locus mutants in the ad-3 region are orders of magnitude higher than expected on the basis of target theory (where the frequency of the double mutant is expected to be the product of the frequencies of each single mutant) and classical models of chromosome structure during interphase (de Serres, 1989a). In the present paper, a random sample of 832 X-ray-induced ad-3 mutants of genotype ad-3A or ad-3B that are irreparable have been subjected to more extensive genetic fine-structure analysis. These experiments were designed to determine the extent of the functional inactivation in individual mutants in the ad-3 and immediately adjacent genetic regions in mutants classified as presumptive multilocus deletions or multiple-locus mutations. These experiments have shown that in Neurospora crassa most X-ray-induced irreparable mutants of genotype ad-3A or ad-3B map as a series of overlapping multilocus deletions. Among the 29 irreparable mutants of genotype ad-3A, there are 16 different subgroups of complementation patterns; and among the 63 irreparable mutants of genotype ad-3B, there are also 16 different subgroups. In addition, mutants classified as presumptive multiple-locus mutants result from a variety of separate, but closely linked, sites of genetic damage.(ABSTRACT TRUNCATED AT 400 WORDS)

X-ray-induced specific locus mutations in the ad-3 region of two-component heterokaryons of Neurospora crassa. III. Genetic fine structure analysis of the ad-3 and immediately adjacent genetic regions by means of complementation tests

Genetic fine structure analysis of the ad-3 and immediately adjacent genetic regions was made by means of complementation tests on all possible pairwise combinations of 50 X-ray-induced irreparable adenine-3 mutants (designated ad-3IR). All mutants were induced in either heterokaryon 11 or heterokaryon 12 of Neurospora crassa, 2-component heterokaryons heterozygous for mutants at the 3 closely linked loci ad-3A and ad-3B and nic-2 (nicotinamide-requiring) located about 5.0 map units distal to ad-3B. The complementation tests involved mutants of the following genotypes: 15 ad-3A, 27 ad-3B, 7 ad-3A ad-3B nic-2 and 1 ad-3B nic-2. To facilitate mapping, 5 additional strains (each consisting of a gene/point mutation at the ad-3A or ad-3B locus and a separate site of closely linked recessive lethal damage in the immediately adjacent regions [designated ad-3R + RLCL]) were also included. The data from these complementation tests showed that the majority (46/50) of X-ray-induced irreparable ad-3 mutants mapped as a series of overlapping multilocus deletions that extend both proximally and distally into the immediately adjacent genetic regions, as well as into the 'X' region (a region of unknown, but essential function) between ad-3A and ad-3B. The remaining mutants (4/50) were found to result from a series of closely linked, but separate, mutations (designated multilocus mutations) of the type ad-3IR + RLCL, different from those found in previous studies (de Serres, 1968; de Serres and Brockman, 1968). The data from the present complementation tests have expanded the process of genetic fine structure mapping of the ad-3 and immediately adjacent regions (de Serres, 1968) and defined the presence of the following 11 genetic loci: (a) 4 loci (with either known [i.e. col-1t] or unknown [i.e. unknA]) function proximal to ad-3A: unknA, unknB, col-1t, and col-2t, (b) 4 loci in the 'X' region: unknC, unknD, unknE, and unknF, (c) 2 loci distal to ad-3B: unknG, col-3t, and (d) 1 locus distal to nic-2: unknH.

X-ray-induced specific-locus mutations in the ad-3 region of two-component heterokaryons of Neurospora crassa. II. More extensive genetic tests reveal an unexpectedly high frequency of multiple-locus mutations

More extensive genetic tests have been performed on a series of 832 X-ray-induced specific-locus mutations in the ad-3 region of a 2-component heterokaryon (H-12) of Neurospora crassa, reported earlier (Webber and de Serres 1965). Using new tester strains and techniques for performing large-scale genetic tests (heterokaryon, dikaryon and trikaryon) to characterize ad-3 mutants induced in 2-component heterokaryons, new data have been obtained on this sample of X-ray-induced ad-3 mutants. These new data show that unexpectedly high frequencies of both single-locus (gene/point) mutations and multilocus deletions in the ad-3 region have additional, but separate, sites of recessive lethal (RLCL) damage in the immediately adjacent genetic regions. The frequencies of these X-ray-induced multiple-locus mutants in the ad-3 region are orders of magnitude higher than expected on the basis of target theory and classical models of chromosome structure during interphase. Current models of interphase chromosome structure in higher eukaryotes as revealed by chromosome "painting" offer a possible explanation of the Neurospora data.

Repair-resistant mutation in Neurospora

Chronic UV treatment produces severalfold fewer mutations in Neurospora conidia than does the same total dose of acute UV. Experiments were designed to determine the conditions required for chronic UV mutagenesis. Measurement of the coincidence frequency for two independent mutations revealed the existence of a subset of cells which are mutable by chronic UV. Analysis of forward mutation at the mtr locus showed that the genetic alterations produced by chronic UV were virtually all point mutants, even though the assay system could detect alterations or deletions extending into neighboring genes. A significant fraction of the mutants produced by acute UV were multigenic deletions. The size of the dose-rate effect (acute UV mutation frequency divided by chronic UV mutation frequency) was compared for several different mutation assay systems. Forward mutations (recessive lethals and mtr) gave values ranging from four to nine. For events which were restricted to specific molecular sites (specific reversions and nonsense suppressor mutations), there was a wider range of dose-rate ratios. This suggests that chronic UV mutation may be restricted to certain molecular sequences or configurations.

Gamma-ray-sensitive mutants of Neurospora crassa with characteristics analogous to ataxia telangiectasia cell lines

Well characterized gamma-ray sensitive mutants of the fungus Neurospora crassa have been screened for characteristics analogous to those of cell lines derived from humans with the genetic disease, ataxia telangiectasia (AT). Two Neurospora mutants, uvs-6 and mus-9, show the AT cell line characteristics of gamma-ray and bleomycin sensitivity, and little or no repression of DNA synthesis following treatment with these agents. Normal human or Neurospora cells show an extensive biphasic DNA synthesis repression (to 50% of control) and when DNA synthesis is analyzed by alkaline sucrose gradient centrifugation, repression of DNA synthesis by low doses of gamma-radiation occurs primarily in low molecular weight (MW) DNA pieces in both organisms. In AT cells and the uvs-6 mutant, no repression in synthesis of low or higher MW DNA is seen at low doses, while the mus-9 mutant shows little repression of high MW DNA, but an intermediate level of low MW DNA synthesis. Both mutants have been shown previously to have an increased level of spontaneous chromosome instability as do AT lines. The uvs-6 and mus-9 mutations are known to be due to two different genes in two different epistatic groups. These results demonstrate that AT-like cellular characteristics can arise from defects in at least two and probably any of several genes, and that lower eukaryotes such as Neurospora can provide an inexpensive and useful model for AT while avoiding the problems inherent in using transformed cell lines.

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.

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.

UV-induced recessive lethals in uvs strains of Neurospora which are deficient in UV mutagenesis

The frequencies of spontaneous and UV-induced recessive lethal mutations were compared for UV-sensitive and wild-type heterokaryons of Neurospora crassa. These heterokaryons were homokaryotic either for one of two alleles of uvs-3, or for uvs-6 or uvs+. For uvs-3, which is known to have mutator effects, spontaneous recessive lethals were found to be 4-6 times more frequent than observed in uvs+. After correction for clonal distribution of spontaneous mutants, an observed 2-fold increase for uvs-6 was not statistically significant and may have been due to chance occurrence of a few large clones of mutants. Treatment with low doses of UV (50-200 J/m2) produced very similar overall rates of increase for recessive lethals in uvs and uvs+ heterokaryons. This means, that in contrast to results obtained when mutation to ad-3 was measured, both uvs-3 alleles showed highly significant increases for recessive lethals when treated with UV. It is proposed that certain types of UV damage may be processed into recessive lethal mutations by an alternate mechanism from that responsible for viable 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.

Analysis of lethal events induced by ultraviolet in a heterokaryon of Neurospora

The relative frequencies of heterokaryons and the two kinds of homokaryons have been scored among colonies from conidia harvested from a heterokaryon and treated with UV, in order to determine which kinds of lethal mutations were induced. Recessive lethal mutations were scored directly. The pattern of surviving types indicated that recessive lethals and mitotic lethals (events destroying whole nuclei) occurred with similar frequencies. But the absolute frequency of these mutations was not sufficient to account for the observed kill, suggesting that dominant lethals and/or cytoplasmic lethals were also induced at a similar rate.

Comparison of ultraviolet and blacklight for the induction of nutritional independence at two loci in Neurospora crassa

The inactivating effects of both near ultraviolet (blacklight) and shortwave ultraviolet (UV) have been investigated for two auxotrophic strains of Neurospora crassa. The two strain were indistinguishable with respect to their sensitivity to inactivation by blacklight, but differed in their sensitivity to UV (DEF congruent to 1.3-1.6 at 0.1 survival). The strains each carried an allele of different genes previously demonstrated as being capable of mutation in response to UV. The results confirm the mutability of these alleles, but reveal that within the population densities investigated mutations to nutritional independence by blacklight were undetectable.

Enrichment for auxotrophic and heat-sensitive mutants of Neurospora crassa by tritium suicide

Tritium suicide is shown to be an effective technique for mutant enrichment in Neurospora crassa. When mutagenized conidia were labelled to a high specific radioactivity either with a tritiated amino acid mixture or with [5-3H]uridine at a non-permissive temperature and stored at 4 degrees C to accumulate decays, there was a 13-15 fold enrichment for temperature-sensitive mutants relative to the original mutagenized cultures. For a wild type culture of Neurospora crassa labelled with [5-3H]uridine at 35 degrees C the probability of cell killing per tritium decay was calculated to be 3.64 X 10(-5).

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.

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.

The modifying effects of strain and age on the mutagenic specificity of ultraviolet light in Neurospora crassa

Two derivatives of K3/17 ad-3A 38701; inos 37401 of Neurospora crassa are described which show opposite specific reversional responses to UV. Both derivatives carry the same two auxotrophic alleles and appear to differ only in a single gene which influences the pattern of mutagen specificity. The differences between the derivatives only develop after the cultures have been aged for two to four weeks. Various possible explanations are considered.

The manifestation of chromosome rearrangements in unordered asci of Neurospora

Rapid, effective techniques have been developed for detecting and characterizing chromosome aberrations in Neurospora by visual inspection of ascospores and asci. Rearrangements that are detectable by the presence of deficient, nonblack ascospores in test crosses make up 5 to 10% of survivors after UV doses giving 10-55% survival. Over 135 rearrangements have been diagnosed by classifying unordered asci according to numbers of defective spores. (These include 15 originally identified or analyzed by other workers.) About 100 reciprocal translocations (RT's) have been confirmed and mapped genetically, involving all combinations of the seven chromosomes. Thirty-three other rearrangements generate viable nontandem duplications in meiosis. These consist of insertional translocations (IT's) (15 confirmed), and of rearrangements that involve a chromosome tip (10 translocations and 3 pericentric inversions). No inversion has been found that does not include the centromere. A reciprocal translocation was found within one population in nature. When pairs of RT's that involve the same two chromosome arms were intercrossed, viable duplications were produced if the breakpoints overlapped in such a way that pairing resembled that of insertional translocations (27 combinations).-The rapid analytical technique depends on the following. Deficiency ascospores are usually nonblack (W: "white") and inviable, while nondeficient ascospores, even those that include duplications, are black (B) and viable. Thus RT's typically produce 50% black spores, and IT's 75% black. Asci are shot spontaneously from ripe perithecia, and can be collected in large numbers as groups of eight ascospores representing unordered tetrads, which fall into five classes: 8B:0W; 6B:2W, 4B:4W, 2B:6B, 0B:8W. In isosequential crosses, 90-95% of tetrads are 8:0. When a rearrangement is heterozygous, the frequencies of tetrad classes are diagnostic of the type of rearrangement, and provide information also on the positions of break points. With RT's, 8:0 (alternate centromere segregation) = 0:8 (adjacent-1), 4:4's require interstitial crossing over in a centromere-break point interval, and no 6:2's or 2:6's are expected. With IT's, duplications are viable, 8:0 = 4:4, 6:2's are from interstitial crossing over, 0:8's or 2:6's are rare. Tetrads from RT's that involve a chromosome tip resemble those from IT's, as do tetrads from intercrosses between partially overlapping RT's that involve identical chromosome arms.-Because viable duplications and other aneuploid derivatives regularly occur among the offspring of rearrangements such as insertional translocations, care must be taken in selecting stocks, and original strains should be kept for reference.

Unstable diploids of Neurospora and a model for their somatic behavior

When ascospores from crosses of certain strains were germinated under conditions selective for heterozygosity of complementing markers on one linkage group, a portion of the resulting colonies were also heterozygous for unselected markers on other chromosomes, implying multiple disomy. The frequency of disomy and the pattern of marker homozygosity are consistent with most or all multiple disomics having originated as complete diploids following nondisjunction at meiosis I. The production of diploid ascospores in these strains is apparently under polygenic control. The diploids are highly unstable and do not differ from n+1 disomics in rates and mechanisms of haploidization and mitotic crossing over.

Genetics of 5-fluorodeoxyuridine-resistant mutants of Neurospora crassa

Mutants resistant to inhibitory concentrations of 5-fluorodeoxyuridine (FdUrd) were induced by ultraviolet light. Resistance was demonstrated by growth-tube experiments and by plating efficiencies in the presence and in the absence of FdUrd. Linkage studies involving five mutants revealed the existence of at least two loci conferring resistance to the inhibitor. These loci, designated fdu-1 and fdu-2, were assigned to Linkage Groups VII and IV, respectively. Both resistance genes are recessive to their sensitive alleles in heterokaryons. Resistance to FdUrd is stable both in the presence and in the absence of the inhibitor. Possible mechanisms of resistance to FdUrd are discussed.

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.

Sexuality in Neurospora crassa. I. Mutations to male sterility

Mutations giving rise to sexual sterility were induced in Neurospora crassa macroconidia by ultraviolet-light irradiation. Thirty mutants were isolated on the basis of their male sterility in crosses with a wild-type strain. When used as the male parent these mutants exhibited a wide spectrum of sexual behaviour patterns ranging from the production of only small brown protoperithecia (complete male sterility) to the production of large and normally pigmented perithecia but with an undeveloped ostiole and very few if any spores. For many of the mutants the behaviour pattern is different when the strain is used as the female parent. Segregation data reveal that none of these mutants represent mutations of the mating-type locus. These findings suggest that the sexual development cycle is blocked at various stages in the different mutant strains. All attempts to restore fertility by supplying various additives to the medium or by varying the incubation time and temperature were unsuccessful. Conidial viability tests carried out on many of the strains revealed no abnormality in this respect. The aberrant segregation patterns exhibited by many of the mutants are discussed.