Allele specific, gene unspecific suppressors in Aspergillus nidulans

In Aspergillus nidulans the suppressors suaA and suaC code for release factors eRF1 and eRF3 and suaD codes for a glutamine tRNA

In Aspergillus nidulans, after extensive mutagenesis, a collection of mutants was obtained and four suppressor loci were identified genetically that could suppress mutations in putative chain termination mutations in different genes. Suppressor mutations in suaB and suaD have a similar restricted spectrum of suppression and suaB111 was previously shown to be an alteration in the anticodon of a gln tRNA. We have shown that like suaB, a suaD suppressor has a mutation in the anticodon of another gln tRNA allowing suppression of UAG mutations. Mutations in suaA and suaC had a broad spectrum of suppression. Four suaA mutations result in alterations in the coding region of the eukaryotic release factor, eRF1, and another suaA mutation has a mutation in the upstream region of eRF1 that prevents splicing of the first intron within the 5'UTR. Epitope tagging of eRF1 in this mutant results in 20% of the level of eRF1 compared to the wild-type. Two mutations in suaC result in alterations in the eukaryotic release factor, eRF3. This is the first description in Aspergillus nidulans of an alteration in eRF3 leading to suppression of chain termination mutations.

Lethal and mutagenic effects of ion beams and γ-rays in Aspergillus oryzae

Aspergillus oryzae is a fungus that is used widely in traditional Japanese fermentation industries. In this study, the lethal and mutagenic effects of different linear energy transfer (LET) radiation in freeze-dried conidia of A. oryzae were investigated. The lethal effect, which was evaluated by a 90% lethal dose, was dependent on the LET value of the ionizing radiation. The most lethal ionizing radiation among that tested was (12)C(5+) ion beams with an LET of 121keV/μm. The (12)C(5+) ion beams had a 3.6-times higher lethal effect than low-LET (0.2keV/μm) γ-rays. The mutagenic effect was evaluated by the frequency of selenate resistant mutants. (12)C(6+) ion beams with an LET of 86keV/μm were the most effective in inducing selenate resistance. The mutant frequency following exposure to (12)C(6+) ion beams increased with an increase in dose and reached 3.47×10(-3) at 700Gy. In the dose range from 0 to 700Gy, (12)C(5+) ion beams were the second most effective in inducing selenate resistance, the mutant frequency of which reached a maximum peak (1.67×10(-3)) at 400Gy. To elucidate the characteristics of mutation induced by ionizing radiation, mutations in the sulphate permease gene (sB) and ATP sulfurylase gene (sC) loci, the loss of function of which results in a selenate resistant phenotype, were compared between (12)C(5+) ion beams and γ-rays. We detected all types of transversions and transitions. For frameshifts, the frequency of a +1 frameshift was the highest in all cases. Although the incidence of deletions >2bp was generally low, deletions >20bp were characteristic for (12)C(5+) ion beams. γ-rays had a tendency to generate mutants carrying a multitude of mutations in the same locus. Both forms of radiation also induced genome-wide large-scale mutations including chromosome rearrangements and large deletions. These results provide new basic insights into the mutation breeding of A. oryzae using ionizing radiation.

Phenotypes of mutations in the 5'-UTR of a limiting transcription factor in Aspergillus nidulans can be accounted for by translational inhibition and leaky scanning

The uaY gene encodes the transcriptional activator of purine catabolism genes in Aspergillus nidulans. uaY12 results in strongly defective growth on purines as nitrogen sources and in strongly diminished transcription of UaY-regulated genes. This mutation introduces an ATG codon 64 bp upstream of the uaY ATG, generating a 68-codon open reading frame (uORFA), overlapping with the uaY ORF. uaY12 revertants fall into three categories: i. The majority eliminate the aberrant ATG. The growth and transcriptional phenotypes of these revertants are identical to those of the wild type. i. Two revertants create a stop codon in frame with the uaY12 aberrant ATG, shortening the length of the uORFA, thus uORFA no longer overlaps the uaY ORF. The latter are partial suppressors of the uaY12 mutation, while chain termination suppressors, in turn, suppress this novel phenotype. iii. Two partial suppressors are unlinked to uaY. These two mutations result in a pleiotropic phenotype usually associated with ribosomal proteins. We hypothesize that uORFA strongly diminishes translation of the uaY ORF and that revertants negate this effect by a number of different mechanisms. The first-AUG rule and the phenomena of translational inhibition and leaky scanning provide a coherent explanation of the results presented in this article.

Suppression of the acuH13 and acuH31 nonsense mutations in the carnitine/acylcarnitine translocase (acuH) gene of Aspergillus nidulans by the G265S substitution in the domain 2 of the release factor eRF1

A search for suppressors of the carnitine/acylcarnitine translocase (CACT) deficiency in Aspergillus nidulans permitted the identification of the suaE7 mutation, mapping at a new translational suppressor (suaE) gene. The suaE gene is essential in A. nidulans and encodes the eukaryotic release factor 1 (eRF1). The suaE7 mutation suppresses two acuH alleles (acuH13 and acuH31), both carrying nonsense mutations in the CACT encoding gene that involve the replacement of a CAG (Gln) codon with a premature TAG stop codon. In contrast, the suaE7 gene does not suppress the acuH20 amber nonsense mutation involving a TGG-->TAG change. The phenotype associated to the suaE7 mutation strictly resembles that of mutants at the suaA and suaC genes, two translational suppressor genes previously identified, suggesting that their gene products might functionally interact in translation termination. Sequencing of the suaE7 gene allowed the identification of a mutation in the domain 2 of the omnipotent class-1 eukaryotic release factor involving the Gly265Ser substitution in the A. nidulans eRF1. This mutation creates a structural context unfavourable for normal eRF binding that allows the misreading of stop codons by natural suppressor tRNAs, such as the tRNAs(Gln). Structural analysis using molecular modelling of A. nidulans eRF1 domain 2 bearing the G265S substitution and computer simulation results suggest that this mutation might impair the necessary conformational changes in the eRF1 to optimally recognize the stop codon and simultaneously interact with the peptidyl transferase centre of the 60S ribosomal subunit.

Convergent evolution of hydroxylation mechanisms in the fungal kingdom: molybdenum cofactor-independent hydroxylation of xanthine via α-ketoglutarate-dependent dioxygenases

The xanthine oxidases and dehydrogenases are among the most conserved enzymes in all living kingdoms. They contain the molybdopterin cofactor Moco. We show here that in the fungi, in addition to xanthine dehydrogenase, a completely different enzyme is able to catalyse the oxidation of xanthine to uric acid. In Aspergillus nidulans this enzyme is coded by the xanA gene. We have cloned the xanA gene and determined its sequence. A deletion of the gene has the same phenotype as the previously known xanA1 miss-sense mutation. Homologues of xanA exist only in the fungal kingdom. We have inactivated the cognate gene of Schizosaccharomyces pombe and this results in strongly impaired xanthine utilization as a nitrogen source. We have shown that the Neurospora crassa homologue is functionally equivalent to xanA. The enzyme coded by xanA is an alpha-ketoglutarate- and Fe(II)-dependent dioxygenase which shares a number of properties with other enzymes of this group. This work shows that only in the fungal kingdom, an alternative mechanism of xanthine oxidation, not involving Moco, has evolved using the dioxygenase scaffold.

Chromatin rearrangements in the prnD-prnB bidirectional promoter: dependence on transcription factors

The prnD-prnB intergenic region regulates the divergent transcription of the genes encoding proline oxidase and the major proline transporter. Eight nucleosomes are positioned in this region. Upon induction, the positioning of these nucleosomes is lost. This process depends on the specific transcriptional activator PrnA but not on the general GATA factor AreA. Induction of prnB but not prnD can be elicited by amino acid starvation. A specific nucleosomal pattern in the prnB proximal region is associated with this process. Under conditions of induction by proline, metabolite repression depends on the presence of both repressing carbon (glucose) and nitrogen (ammonium) sources. Under these repressing conditions, partial nucleosomal positioning is observed. This depends on the CreA repressor's binding to two specific cis-acting sites. Three conditions (induction by the defective PrnA80 protein, induction by amino acid starvation, and induction in the presence of an activated CreA) result in similar low transcriptional activation. Each results in a different nucleosome pattern, which argues strongly for a specific effect of each signal on nucleosome positioning. Experiments with trichostatin A suggest that both default nucleosome positioning and partial positioning under induced-repressed conditions depend on deacetylated histones.

Relationships between the ethanol utilization (alc) pathway and unrelated catabolic pathways in Aspergillus nidulans

The ethanol utilization pathway in Aspergillus nidulans is a model system, which has been thoroughly elucidated at the biochemical, genetic and molecular levels. Three main elements are involved: (a) high level expression of the positively autoregulated activator AlcR; (b) the strong promoters of the structural genes for alcohol dehydrogenase (alcA) and aldehyde dehydrogenase (aldA); and (c) powerful activation of AlcR by the physiological inducer, acetaldehyde, produced from growth substrates such as ethanol and l-threonine. We have previously characterized the chemical features of direct inducers of the alc regulon. These studies allowed us to predict which type of carbonyl compounds might induce the system. In this study we have determined that catabolism of different amino acids, such as L-valine, L-isoleucine, L-arginine and L-proline, produces aldehydes that are either not accumulated or fail to induce the alc system. On the other hand, catabolism of D-galacturonic acid and putrescine, during which aldehydes are transiently accumulated, gives rise to induction of the alc genes. We show that the formation of a direct inducer from carboxylic esters does not depend on alcA-encoded alcohol dehydrogenase I or on AlcR, and suggest that a cytochrome P450 might be responsible for the initial formation of a physiological aldehyde inducer.

Ethanol catabolism in Aspergillus nidulans: a model system for studying gene regulation

This article reviews our knowledge of the ethanol utilization pathway (alc system) in the hyphal fungus Aspergillus nidulans. We discuss the progress made over the past decade in elucidating the two regulatory circuits controlling ethanol catabolism at the level of transcription, specific induction, and carbon catabolite repression, and show how their interplay modulates the utilization of nutrient carbon sources. The mechanisms featuring in this regulation are presented and their modes of action are discussed: First, AlcR, the transcriptional activator, which demonstrates quite remarkable structural features and an original mode of action; second, the physiological inducer acetaldehyde, whose intracellular accumulation induces the alc genes and thereby a catabolic flux while avoiding intoxification; third, CreA, the transcriptional repressor mediating carbon catabolite repression in A. nidulans, which acts in different ways on the various alc genes; Fourth, the promoters of the structural genes for alcohol dehydrogenase (alcA) and aldehyde dehydrogenase (aldA) and the regulatory alcR gene, which exhibit exceptional strength compared to other genes of the respective classes. alc gene expression depends on the number and localization of regulatory cis-acting elements and on the particular interaction between the two regulator proteins, AlcR and CreA, binding to them. All these characteristics make the ethanol regulon a suitable system for induced expression of heterologous protein in filamentous fungi.

Regulation of the aldehyde dehydrogenase gene (aldA) and its role in the control of the coinducer level necessary for induction of the ethanol utilization pathway in Aspergillus nidulans

Expression of the structural genes for alcohol and aldehyde dehydrogenase, alcA and aldA, respectively, enables the fungus Aspergillus nidulans to grow on ethanol. The pathway-specific transcriptional activator AlcR mediates the induction of ethanol catabolism in the presence of a coinducing compound. Ethanol catabolism is further subject to negative control mediated by the general carbon catabolite repressor CreA. Here we show that, in contrast to alcA and alcR, the aldA gene is not directly subject to CreA repression. A single cis-acting element mediates AlcR activation of aldA. Furthermore, we show that the induction of the alc gene system is linked to in situ aldehyde dehydrogenase activity. In aldA loss-of-function mutants, the alc genes are induced under normally noninducing conditions. This pseudo-constitutive expression correlates with the nature of the mutations, suggesting that this feature is caused by an intracellular accumulation of a coinducing compound. Conversely, constitutive overexpression of aldA results in suppression of induction in the presence of ethanol. This shows unambiguously that acetaldehyde is the sole physiological inducer of ethanol catabolism. We hypothesize that the intracellular acetaldehyde concentration is the critical factor governing the induction of the alc gene system.

The cloning and sequencing of the alcB gene, coding for alcohol dehydrogenase II, in Aspergillus nidulans

Alcohol dehydrogenase II (ADH II, structural gene alcB) was purified from a strain H1035, biA1; alcE1; alc500 alcD1, which produces 100-times more ADH II activity than the alcAalcR deletion strain (alc500). Antibodies were raised against this ADH, and were used to screen a cDNA library in lambda gt11. We have isolated the gene for an ADH which is over-expressed in H1035, and which we believe to be the alcB gene: cDNA and genomic clones were sequenced. The sequence contains three introns and encodes a protein of 367 amino acids. This protein shows a clear level of identity to a range of alcohol dehydrogenases, but is no more closely related to the ADH I and ADH III previously described in A. nidulans than to the ADHs of S. pombe and S. cerevisiae. The significance of consensus sequences found in the 5' region of the gene is discussed in relation to the regulation of the gene.

The basal level of transcription of the alc genes in the ethanol regulon in Aspergillus nidulans is controlled both by the specific transactivator AlcR and the general carbon catabolite repressor CreA

In the A. nidulans ethanol utilization pathway, specific induction is mediated by the transactivator AlcR which is subject to strong positive autogenous regulation and activates the transcription of the two structural genes alcA and aldA. Carbon catabolite repression is mediated by CreA which represses directly the transacting gene alcR and the two structural genes. We show here that the basal expression of the alcR and alcA genes is also controlled by the two regulatory circuits, positively by the transactivator AlcR and negatively by the repressor CreA, the aldA gene being subject only to the control of the CreA repressor.

Specific binding sites in the alcR and alcA promoters of the ethanol regulon for the CREA repressor mediating carbon catabolite repression in Aspergillus nidulans

The CREA repressor responsible for carbon catabolite repression in Aspergillus nidulans represses the transcription of the ethanol regulon. The N-terminal part of the CREA protein encompassing the two zinc fingers (C2H2 class family) and an alanine-rich region was expressed in Escherichia coli as a fusion protein with glutathione-S-transferase. Our results show that CREA is a DNA-binding protein able to bind to the promoters of both the specific trans-acting gene, alcR, and of the structural gene, alcA, encoding the alcohol dehydrogenase I. DNase I protection footprinting experiments revealed several specific binding sites in the alcR and in the alcA promoters having the consensus sequence 5'-G/CPyGGGG-3'. The disruption of one of these CREA-binding sites in the alcR promoter overlapping the induction target for the trans-activator ALCR results in a partially derepressed alc phenotype and derepressed alcR transcription, showing that this binding site is functional in vivo. Our data suggest that CREA represses the ethanol regulon by a double lock mechanism repressing both the trans-acting gene, alcR, and the structural gene, alcA.

An intragenic map of the brlA locus of Aspergillus nidulans

We have constructed an intragenic map for the Aspergillus nidulans brlA gene, mutants in which are distinguishable by visual criteria only. Most of the leaky phenotype mutants map near the right (3') end. The gene shows distinct recombinational polarity consistent with recombination initiation at the promoter (centromere-proximal) end of the gene. brlA12 and brlA20 mutants gave abnormal DNA restriction patterns consistent with the III; VIII and VI; VIII translocations, respectively, determined by haploidization.

Hygromycin- and paromomycin-resistant mutants of Aspergillus nidulans alter translational fidelity

Mutants of Aspergillus nidulans resistant to the aminoglycoside antibiotics paromomycin and hygromycin B have been isolated and their growth characteristics are described here. Most paromomycin mutants were cross-resistant to hygromycin and geneticin. All the hygromycin-resistant mutants were slightly cross-resistant to geneticin. Out of the 15 mutants tested 14 had drug-resistant ribosomes in vitro and all 12 of those investigated further had reduced levels of translational misreading. Five new loci have been found--parA on linkage group I, hygA on III, hygB on IV, hygC on V, hygD on VI and parB on VIII. This increases, to at least 12, the number of translational fidelity loci in A. nidulans.

Antisuppressor mutations reduce misreading of the genetic code in Aspergillus nidulans

Antisuppressor mutations were isolated in a strain containing the omnipotent suppressor suaC109. The antisuppressors reduce the activity of translational suppressors in vivo and counteract most aspects of the pleiotropic phenotype associated with the suaC and the suaA suppressor mutations. Using an homologous system for cell-free translation, we have measured translational accuracy in two antisuppressor strains with the genotype suaC109 and either the asuB11 or the asuD14 antisuppressor mutation. Ribosomes from antisuppressor mutants have higher levels of translation accuracy than those from the suppressor strain (suaC109, asu+). Mistranslation levels depended solely on the source of the sucrose-cleaned ribosomes. However, the increased accuracy associated with sucrose-cleaned ribosomes from antisuppressor strains can be nullified by salt-washing, suggesting that the component responsible can be washed off.

Alcohol dehydrogenase III in Aspergillus nidulans is anaerobically induced and post-transcriptionally regulated

An alcohol dehydrogenase was shown to be induced in Aspergillus nidulans by periods of anaerobic stress. This alcohol dehydrogenase was shown to correspond to the previously described cryptic enzyme, alcohol dehydrogenase III (McKnight et al. 1985), by analysis of a mutation in the structural gene of alcohol dehydrogenase III, alcC, created by gene disruption. Survival tests on agar plates showed that this enzyme is required for long-term survival under anaerobic conditions. Northern blot analysis and gene fusion studies showed that the expression of the alcC gene is regulated at both the transcriptional and translational levels. Thus there are mechanisms in this filamentous fungus allowing survival under anaerobic stress that are similar to those described in higher plants.

The ethanol regulon in Aspergillus nidulans: characterization and sequence of the positive regulatory gene alcR

The regulatory gene, alcR, of Aspergillus nidulans, encodes a protein that induces the expression of the alcA and aldA genes. The alcR gene is inducible, autoregulated, and subject to carbon catabolite repression. We report the complete nucleotide sequence of the alcR gene and its 5' and 3' non-coding regions. In the 5' flanking region of the alcR gene, several repeats and inverted repeats were found, and small sequence similarities were also found with the 5' flanking regions of the alcA and aldA genes. One intron of small size interrupts the open reading frame. The start point of transcription was mapped 50 nucleotides upstream from the putative start codon, and a sequence CAATG was found 5' to the polyadenylation site of the transcript that could play a role in selection of the polyadenylation site. The putative alcR-encoded protein was identified in vivo as an inducible polypeptide of 96 kDa in a transformant carrying multiple copies of the alcR gene.

Ammonium ion sensitivity is a ribosomal phenotype associated with suppressor mutations in the suaC gene of Aspergillus nidulans

Ammonium ions are selectively toxic to strains containing mutations in the suaC gene which can mutate to a suppressor phenotype. This phenotype is associated with increased ribosomal misreading in vitro (Zamir and Martinelli 1987) and altered ribosomal proteins (Harvey and Martinelli 1983). Such ammonium-sensitivity is a feature of both strong and weak suppressor alleles, and segregates with suppressor ability in crosses. Suppressor mutations in the suaB and suaD genes are not affected, nor are those in suaA, another ribosomal suppressor gene. Thus, the ammonium-effect is locus specific. Mutations which act as anti-suppressors (asu-) of suppressor suaC109 also partially reverse the ammonium ion sensitivity associated with this mutation. This effect is in line with their restoration of other aspects of the pleiotropic phenotype to normal. The cations, lithium and rubidium, mimick the effects of ammonium ions. Only ribosomes from suaC strains are sensitive to the presence of NH+4 ions in vitro.

Differences in the regulation of aldehyde dehydrogenase genes in Aspergillus niger and Aspergillus nidulans

In order to study mechanisms of gene regulation in A. niger, and to compare these to similar systems in A. nidulans, a gene encoding an aldehyde dehydrogenase enzyme has been cloned. In wild-type strains of A. niger the gene shows expression which is regulated by induction and repression. Levels of induction by various compounds and the extent of repression under various growth conditions differs from that seen for the A. nidulans aldA gene. Unlike the A. nidulans aldA gene, the A. niger gene has both carbon catabolite repressible and nonrepressible induction mechanisms. Studies of heterologous expression of the A. niger gene in A. nidulans have shown that its expression is regulated by the alcR gene product of A. nidulans.

Every ribosomal suppressor mutation in Aspergillus nidulans has a unique and highly pleiotropic phenotype

18 suppressors of alcR125 have been selected in Aspergillus nidulans. They have been located in genes as follows: 12 in suaA, 1 in suaB and 5 in suaC. Suppressors have been examined to see whether their phenotype is diagnostic for their genotype. Several new traits are described: conidial viability, cycloheximide resistance, fertility, suppression of niaD500, niaD501 and fwA1. These tests, added to those already in use, provide a battery of tests suitable for assigning suppressor mutations to physiological type (tRNA or ribosomal), and in one case to a specific gene since only suaA mutations suppressed fwA1. A very broad range of phenotypes was associated with suppressors such that every mutation had a unique phenotype. This indicates that the ribosomal suppressor mutations are in genes which code directly for ribosomal proteins, rather than genes which code for modifying enzymes.

Suppressors suaC109 and suaA101 of Aspergillus nidulans alter the ribosomal phenotype in vitro

A new homologous, cell-free system for protein synthesis has been devised for use with ribosomes and elongation factors from Aspergillus nidulans. Ribosome preparations from strains with either the suaA101 or suaC109 mutations have a higher misreading ratio (non-cognate:cognate amino acid incorporation) in the presence of hygromycin than controls. They can be classed as fidelity mutants. These results also prove that the mutations must be in genes coding for ribosomal proteins or enzymes which modify ribosomal proteins post-translationally. Alternatively, the genes could code for translation factors.

Regulation of alcR, the positive regulatory gene of the ethanol utilization regulon of Aspergillus nidulans

The alcR positive control gene is necessary for the expression of both alcA (coding for alcohol dehydrogenase ADH I), and aldA (coding for aldehyde dehydrogenase, AldDH) in Aspergillus nidulans. Using a cloned alcR probe and Northern blots analysis we show that: (1) alcR itself is inducible; (2) alcR inducibility depends on the expression of the alcR gene itself; and (3) alcR is subject to carbon catabolite repression and its expression is controlled by the negatively acting creA wide specificity gene. The repression of alcR is sufficient to explain the carbon catabolite repression of ADH I and AldDH.

Antisuppressor mutations in Aspergillus nidulans: cold-resistant revertants of suppressor suaC109

Cold-resistant revertants of the cold-sensitive, ribosomal suppressorsuaC109have been isolated, with a view to obtaining mutations in new ribosomal protein genes. Many revertants had reduced suppressor activity and were classified as antisuppressor mutants. Both intragenic and extragenic reversions were found. In seven strains the extragenic reversion to cold resistance segregated with the antisuppressor phenotype, and these were designatedasumutations. Three of the fiveasugenes, C, B and D were mapped to linkage groups, I, II and V respectively. The antisuppressors are not gene-specific, although they mainly antagonize the activity of ribosomal suppressors. The antisuppressors altered all aspects of the phenotype of suppressorsuaC109including sensitivity to aminoglycoside antibiotics, and are therefore thought to be mutations in ribosomal protein genes.

Genomic clones of Aspergillus nidulans containing alcA, the structural gene for alcohol dehydrogenase and alcR, a regulatory gene for ethanol metabolism

Our aim was to obtain from Aspergillus nidulans a genomic bank and then clone a region we expected from earlier genetic mapping to contain two closely linked genes, alcA, the structural gene for alcohol dehydrogenase (ADH) and alcR, a positive trans-acting regulatory gene for ethanol metabolism. The expression of alcA is repressed by carbon catabolites. A genomic restriction fragment characteristic of the alcA-alcR region was identified, cloned in pBR322, and used to select from a genomic bank in lambda EMBL3A three overlapping clones covering 24 kb of DNA. Southern genomic analysis of wild-type, alcA and alcR mutants showed that the mutants contained extra DNA at sites near the center of the cloned DNA and are close together, as expected for alcA and alcR. Transcription from the cloned DNA and hybridization with a clone carrying the Saccharomyces cerevisiae gene for ADHI (ADC1) are both confined to the alcA-alcR region. At least one of several species of mature mRNA is about 1 kb, the size required to code for ADH. For all species, carbon catabolite repression overrides control by induction. The overall characteristics of transcription, hybridization to ADC1 and earlier work suggest that alcA consists of a number of exons and/or that the alcA-alcR region represents a cluster of alcA-related genes or sequences.

The product of the regulatory gene of the proline catabolism gene cluster of Aspergillus nidulans is a positive-acting protein

Eight new deletion mutations in the prn gene cluster involved in L-proline catabolism in Aspergillus nidulans have been characterised and mapped. Three of these are located within prnA, the regulatory gene mediating proline induction, and confirm the positive nature of the action of the prnA product. In addition, four prnA- alleles which are phenotypically suppressible by aminoglycoside antibiotics have been identified. Of these four phenotypically suppressible prnA- mutations, two have been tested for suppression by translational suppressors. Both are genotypically suppressible, showing that the prnA product must be a protein.

Cloning and characterization of the ethanol utilization regulon in Aspergillus nidulans

In Aspergillus nidulans alcohol dehydrogenase (ADH) I and aldehyde dehydrogenase (AldDH) are co-inducible by acetaldehyde (Pateman et al., 1983; Sealy-Lewis and Lockington, 1984) and subject to carbon catabolite repression. The structural genes alcA and aldA are unlinked, but alcA is closely linked to the positive control gene alcR. We have obtained cDNA clones of alcA and aldA and genomic clones comprising alcA and alcR. The location of these genes in a genomic clone carrying a 13-kb insert was determined by subcloning and subsequent transformation of previously characterised point mutants. We have characterised at the physical level some large deletions encompassing both linked genes. We have shown that induction affects the level of RNA hybridisible with alcA and aldA probes. Mutations in the regulatory gene alcR, result in non-inducibility of RNA hybridisible with either probe. Thus the induction process is possibly at the level of transcription. Analogous experiments suggest that carbon catabolite repression of alcohol dehydrogenase I is equally at the level of transcription.

Evidence for a nonsense mutation at the niaD locus of Aspergillus nidulans

Two mutations at theniaDlocus (structural gene for nitrate reductase apo-protein) are genotypically suppressible. Both mutations result in loss of nitrate reductase enzyme activity and cross reacting material and are non complementing, nonleaky and highly revertible. They have the properties of nonsense mutations. This implies that some of the allele specific suppressors, which act on these and alleles at several other loci, are nonsense suppressors.

Isolation of genomic clones containing the amdS gene of Aspergillus nidulans and their use in the analysis of structural and regulatory mutations

Previous analysis of the amdS gene of Aspergillus nidulans has identified multiple regulatory circuits mediated by trans-acting regulatory genes, cis-acting mutations have been identified and shown to specifically affect individual regulatory circuits. Fine-structure genetic mapping of the amdS regions showed that these cis-acting mutations occur in a complex controlling region adjacent to the amdS structural gene. The amdS gene was cloned by differential hybridization, using cDNA probes derived from a high-level-producing strain and from a strain with a large amdS deletion mutation. RNA blotting experiments showed that a single RNA species of 1,600 to 1,700 base pairs is transcribed from the amdS gene. DNA blotting experiments on a large number of amdS mutant strains, including deletions and translocations, allowed the genetic and physical maps of the gene to be correlated. The controlling region of the gene is situated at the 5' end of the gene and the direction of transcription is toward the centromere of chromosome III. The regulatory mutations in the controlling region were found to be due to small-scale alterations in the DNA rather than to large-scale rearrangements resulting in gene fusions.

Isolation of genomic clones containing the amdS gene of Aspergillus nidulans and their use in the analysis of structural and regulatory mutations

Previous analysis of the amdS gene of Aspergillus nidulans has identified multiple regulatory circuits mediated by trans-acting regulatory genes, cis-acting mutations have been identified and shown to specifically affect individual regulatory circuits. Fine-structure genetic mapping of the amdS regions showed that these cis-acting mutations occur in a complex controlling region adjacent to the amdS structural gene. The amdS gene was cloned by differential hybridization, using cDNA probes derived from a high-level-producing strain and from a strain with a large amdS deletion mutation. RNA blotting experiments showed that a single RNA species of 1,600 to 1,700 base pairs is transcribed from the amdS gene. DNA blotting experiments on a large number of amdS mutant strains, including deletions and translocations, allowed the genetic and physical maps of the gene to be correlated. The controlling region of the gene is situated at the 5' end of the gene and the direction of transcription is toward the centromere of chromosome III. The regulatory mutations in the controlling region were found to be due to small-scale alterations in the DNA rather than to large-scale rearrangements resulting in gene fusions.

Diagnosis of nonsense mutations in Aspergillus nidulans

Three genotypically suppressible alleles, a1X4, alcA125, and niaD500, are phenotypically suppressed by aminoglycoside antibiotics. Unsuppressible alleles at these loci are unaffected as are known missense mutations at the yA and gdhA loci. This is consistent with the premise that the suppressible mutations are nonsense and that this highly-allele-specific phenotypic suppression can be used to distinguish nonsense from missense mutations of Aspergillus nidulans. Paromomycin and tobramycin are recommended for screening unknown mutations.

Search for ribosomal mutants in Podospora anserina: genetic analysis of cold-sensitive mutants

Twenty-four cold-sensitive (prototrophic) mutants were isolated after UV mutagenesis of protoplasts of the fungusPodospora anserina. Genetic analysis of these mutants was performed in order to detect those among them which were most likely to be impaired in translational fidelity. The 24 mutations belonged to 24 different genes. One half of the mutants were pleiotropic and displayed an altered phenotype: growth rate at the permissive temperature, germination of the spores, fertility and/or sporulation. Nine mutants differed from wild-type in their resistance levels to cycloheximide, trichodermin and/or paromomycin. Several mutations were linked to known ribosomal loci. Two mutations behaved like informational antisuppressors: one is allelic to the previously describedAs3gene and the other one defines a new antisuppressor gene,AS6.