Ribosomal suppressors and antisuppressors in Podospora anserina: altered susceptibility to paromomycin and relationships between genetic and phenotypic suppression

Rab-GDI complex dissociation factor expressed through translational frameshifting in filamentous ascomycetes

In the model fungus Podospora anserina, the PaYIP3 gene encoding the orthologue of the Saccharomyces cerevisiae YIP3 Rab-GDI complex dissociation factor expresses two polypeptides, one of which, the long form, is produced through a programmed translation frameshift. Inactivation of PaYIP3 results in slightly delayed growth associated with modification in repartition of fruiting body on the thallus, along with reduced ascospore production on wood. Long and short forms of PaYIP3 are expressed in the mycelium, while only the short form appears expressed in the maturing fruiting body (perithecium). The frameshift has been conserved over the evolution of the Pezizomycotina, lasting for over 400 million years, suggesting that it has an important role in the wild.

Suppression of mitochondrial DNA instability of autosomal dominant forms of progressive external ophthalmoplegia-associated ANT1 mutations in Podospora anserina

Maintenance and expression of mitochondrial DNA (mtDNA) are essential for the cell and the organism. In humans, several mutations in the adenine nucleotide translocase gene ANT1 are associated with multiple mtDNA deletions and autosomal dominant forms of progressive external ophthalmoplegia (adPEO). The mechanisms underlying the mtDNA instability are still obscure. A current hypothesis proposes that these pathogenic mutations primarily uncouple the mitochondrial inner membrane, which secondarily causes mtDNA instability. Here we show that the three adPEO-associated mutations equivalent to A114P, L98P, and V289M introduced into the Podospora anserina ANT1 ortholog dominantly cause severe growth defects, decreased reactive oxygen species production (ROS), decreased mitochondrial inner membrane potential (Deltapsi), and accumulation of large-scale mtDNA deletions leading to premature death. Interestingly, we show that, at least for the adPEO-type M106P and A121P mutant alleles, the associated mtDNA instability cannot be attributed only to a reduced membrane potential or to an increased ROS level since it can be suppressed without restoration of the Deltapsi or modification of the ROS production. Suppression of mtDNA instability due to the M106P and A121P mutations was obtained by an allele of the rmp1 gene involved in nucleo-mitochondrial cross- talk and also by an allele of the AS1 gene encoding a cytosolic ribosomal protein. In contrast, the mtDNA instability caused by the S296M mutation was not suppressed by these alleles.

eEF1A Controls ascospore differentiation through elevated accuracy, but controls longevity and fruiting body formation through another mechanism in Podospora anserina

Antisuppressor mutations in the eEF1A gene of Podospora anserina were previously shown to impair ascospore formation, to drastically increase life span, and to permit the development of the Crippled Growth degenerative process. Here, we show that eEF1A controls ascospore formation through accuracy level maintenance. Examination of antisuppressor mutant perithecia reveals two main cytological defects, mislocalization of spindle and nuclei and nuclear death. Antisuppression levels are shown to be highly dependent upon both the mutation site and the suppressor used, precluding any correlation between antisuppression efficiency and severity of the sporulation impairment. Nevertheless, severity of ascospore differentiation defect is correlated with resistance to paromomycin. We also show that eEF1A controls fruiting body formation and longevity through a mechanism(s) different from accuracy control. In vivo, GFP tagging of the protein in a way that partly retains its function confirmed earlier cytological observation; i.e., this factor is mainly diffuse within the cytosol, but may transiently accumulate within nuclei or in defined regions of the cytoplasm. These data emphasize the fact that the translation apparatus exerts a global regulatory control over cell physiology and that eEF1A is one of the key factors involved in this monitoring.

Propagation of a novel cytoplasmic, infectious and deleterious determinant is controlled by translational accuracy in Podospora anserina

Some mutant strains of the filamentous fungus Podospora anserina spontaneously present a growth impairment, which has been called Crippled Growth (CG). CG is caused by a cytoplasmic and infectious factor, C. C is efficiently transmitted during mitosis but is not transmitted to the progeny after sexual crosses. C is induced by stationary phase and cured by various means, most of which stress the cells. Translational accuracy is shown to tightly regulate the propagation of C during the active growth period, because its propagation in dividing hyphae is restricted to cells that display an increased translational accuracy. However, induction of C in stationary phase proceeds independently from the translational accuracy status of the strain. CG does not seem to be accompanied by mitochondrial DNA modifications, although C activates the action of the Determinant of Senescence, another cytoplasmic and infectious element, which causes a disorganization of the mitochondrial genome. In addition, presence of C drastically modifies the spectrum of the mitochondrial DNA rearrangements in AS6-5 mat- cultures during Senescence. C seems to belong to the growing list of unconventional genetic elements. The biological significance of such elements is discussed.

Identification of the genes encoding the cytosolic translation release factors from Podospora anserina and analysis of their role during the life cycle

In an attempt to decipher their role in the life history and senescence process of the filamentous fungus Podospora anserina, we have cloned the su1 and su2 genes, previously identified as implicated in cytosolic translation fidelity. We show that these genes are the equivalents of the SUP35 and SUP45 genes of Saccharomyces cerevisiae, which encode the cytosolic translation termination factors eRF3 and eRF1, respectively. Mutations in these genes that suppress nonsense mutations may lead to drastic mycelium morphology changes and sexual impairment but have little effect on life span. Deletion of su1, coding for the P. anserina eRF3, is lethal. Diminution of its expression leads to a nonsense suppressor phenotype whereas its overexpression leads to an antisuppressor phenotype. P. anserina eRF3 presents an N-terminal region structurally related to the yeast eRF3 one. Deletion of the N-terminal region of P. anserina eRF3 does not cause any vegetative alteration; especially life span is not changed. However, it promotes a reproductive impairment. Contrary to what happens in S. cerevisiae, deletion of the N terminus of the protein promotes a nonsense suppressor phenotype. Genetic analysis suggests that this domain of eRF3 acts in P. anserina as a cis-activator of the C-terminal portion and is required for proper reproduction.

Escape from Premature Death Due to Nuclear Mutations in Podospora anserina: Repeal versus Respite

Premature death has been defined as a growth stoppage linked to the accumulation of specific deletions of the mitochondrial genome (mtDNA) in Podospora anserina. This occurs only in strains carrying the AS1-4 mutation which lies in a gene encoding a cytosolic ribosomal protein. Here we describe the isolation and genetic characterization of 10 nuclear mutations which either delay the appearance of this syndrome (respite from premature death) or cause a switch to the classical senescence process (repeal of premature death). These mutations lie in at least six genes. Some cause defects at the levels of ascospore germination, growth rates, and/or sensitivity toward inhibitors of protein syntheses. All modify the onset of senescence in wild-type (AS1+) strains. The role played by these genes is discussed with respect to the control of diseases due to mtDNA rearrangements in filamentous fungi. Copyright 1998 Academic Press.

Cloning, sequencing, and transgenic expression of Podospora curvicolla and Sordaria macrospora eEF1A genes: relationship between cytosolic translation and longevity in filamentous fungi

We have cloned and sequenced the gene encoding the translation elongation factor eEF1A from two filamentous fungi, Podospora curvicolla and Sordaria macrospora. These fungi are close relatives of Podospora anserina and also show senescence syndromes. Comparison of the sequences of the deduced proteins with that of P. anserina reveals that the three proteins differ in several positions. Replacement of the P. anserina gene by either of the two exogenous genes does not entail any modification in P. anserina physiology; the longevity of the fungus is not affected. No alteration of in vivo translational accuracy was detected; however, the exogenous proteins nonetheless promoted a modification of the resistance to the aminoglycoside antibiotic paromomycin. These data suggest that optimization of life span between these closely related fungi has likely not been performed during evolution through modifications of eEF1A activity, despite the fact that mutations in this factor can drastically affect longevity.

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.

Identification of two genes controlling kasugamycin resistance in the filamentous fungus Podospora anserina

We have investigated the effect of the ribosome-targeted antibiotic kasugamycin (ksg) inPodospora anserina. While ksg inhibits both growth and sporulation, it has a stronger inhibitory effect on the sporulation process. It was previously reported that sporulation ofPodosporacould be impaired when ribosomes translate with a too high accuracy, and since ksg was demonstrated to increase the ribosomal accuracy inE. coli, we wondered whether it would act similarly inPodospora. As a first approach we have isolated two mutations at different loci,Ks1andKs2, that increase the resistance to ksg at the level of both growth and sporulation. InterestinglyKs1−1also confers a decreased resistance to paromomycin, which is a mistranslation inducer. Characterization ofKs1−1andKs2−1mutants suggests that they could be ribosomal mutants.

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.

Altered sensitivity of protein synthesis to paromomycin in extracts from aging human diploid fibroblasts

Age-related differences in the effects of paromomycin (Pm) on protein synthesis have been investigated in translation reactions with extracts derived from young and old human diploid fibroblasts. Translation products from reactions directed by endogenous or exogenous mRNA were analyzed by polyacrylamide gel electrophoresis and fluorography. The exogenous mRNA lacked codons for cysteine, and therefore cysteine incorporation into translation products represented translational error. This laboratory has previously used this assay to show that the basal translational error level in the absence of Pm increases in extracts from old fibroblasts. In this report, Pm stimulated the misincorporation of cysteine by 6-7 fold over cysteine misincorporation levels in the absence of Pm. This degree of Pm stimulation was similar in extracts from young and old fibroblasts. However, other results showed quantitative differences in the responses to Pm between young and old cell extracts. Old cell extracts were less sensitive to the stimulation of the rate of protein synthesis, and more sensitive to the inhibition of protein synthesis, by Pm. It is proposed that aging human diploid fibroblasts contain altered ribosomes which react differently with Pm.

Inherent resistance of HeLa cell derivatives to paromomycin

The human tumor-derived cell line HeLa S3 and nuclear and mitochondrial gene mutants derived from it are resistant to the aminoglycoside antibiotic, paromomycin (PAR). Other carcinoma-derived cells, SV40-transformed cells, and four human diploid fibroblast cell lines are all sensitive to PAR. Sensitivity is dependent on cell density, and at cell numbers greater than 400/cm2 sensitive cells will proliferate in PAR. The resistance to PAR is inherited in a dominant manner in cell-to-cell fusion hybrids, but is not transferred in cytoplast-to-cell fusions. PAR resistance is therefore encoded by a nuclear gene(s). Resistance to PAR is not caused by changes in the response of mitochondrial or cytoplasmic protein synthesis to PAR in vitro. The uptake of PAR is similar in resistant and sensitive cells, and dimethyl sulfoxide does not render resistant cells more sensitive. Thus, HeLa cell PAR resistance is unlike previously reported ribosomal mutations and may derive from differences in the intracellular metabolism of PAR.

Genetics of translational fidelity in Podospora anserina: are all the genes involved in this ribosomal function identified?

Su12-1 and su12-2 are two ribosomal suppressor mutations previously described in the fungus Podospora anserina. Revertants were isolated on the criteria of either improved growth at 27 degrees C (for su12-1) or suppression of the paromomycin hypersensitivity (for su12-2). Among 45 mutations lying outside the su12 locus, only one was found which defines a new antisuppressor locus, AS9. About 3/4 of these mutations are antisuppressor mutations localized in the previously identified AS6 and AS7 genes. While the AS6 mutations harbour diverse phenotypes, all the mutations lying in the AS7 gene lead to the same phenotypic alterations. In addition, two new su3 mutations were obtained and shown to display an antisuppressor effect on su12-1.

Ribosomal suppressors in Podospora anserina: evidence for two new loci by means of a new screening procedure

I describe here a new screening procedure to isolate ribosomal suppressors in Podospora anserina. I have used the sporulation defect displayed by an antisuppressor mutation AS7–2. The revertants able to sporulate are due to either true reversions or external mutations. The mutations which restore most efficiently the sporulation show all the properties of ribosomal suppressors and are localized in two new suppressor loci su11 and su12.

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.

Does translational ambiguity increase during cell differentiation?

Several observations made in the fungus Podospora anserina suggest that translational ambiguity may increase, and possibly must increase, at specific stages of the life cycle. Such changes in the properties of the translational apparatus seem to occur as well in the yeast S. cerevisiae and in the alga C. reinhardii. A slight increase of the misreading level would allow readthrough or frameshifting necessary to synthesise regulatory proteins in low amount at key points of cellular differentiation.

Mutations relieving hypersensitivity to paromomycin caused by ribosomal suppressors in Podospora anserina

In the fungusPodospora anserina, mutations were selected which relieved the hypersensitivity to paromomycin caused by four suppressors assumed to be ribosomal ambiguity mutations (su1–31,su1–49,su1–60,su2–5). Our first purpose was to isolate new antisuppressor mutations and in fact a new antisuppressor gene,AS7was uncovered. TheAS7–1mutant displays a pleiotropic phenotype and particularly a sporulation defect. On the other hand, a newsu1mutant was obtained which acts as a suppressor and also as an antisuppressor: it can specifically reduce the suppressor effect of certainsu2mutations. This property of somesu1andsu2mutations was already known. Apart from these mutations probably involved in the control of translational fidelity, six mutations conferring cross-resistance to paromomycin and neomycin were isolated. While four of them are localized in thePm1andPm2loci previously identified, the two others define a new gene which controls paromomycin and neomycin resistance,Pm3. Strains carrying thePm3–1allele are sensitive to temperature at the level of growth and sporulation. The three last mutations which were obtained confer no mutant phenotype when separated from thesu1background. They are closely linked to thesu2locus.

Analysis of revertants of a ribosomal mutation in Podospora anserina: evidence for new ribosomal mutations which confer hypersensitivity to paromomycin

This paper describes the analysis of cold-resistant revertants of a cold-sensitive mutant. Pm1-1 is a ribosomal mutation screened for its paromomycin resistance. Suppression of its cold sensitivity occurs with two kinds of external mutations localized in two different loci. One of them, PmB, is assumed to be a ribosomal gene. PmB mutations confer hypersensitivity to paromomycin in vivo as well as in vitro in a cell-free protein synthesis system.

Mutations affecting translational fidelity in the eucaryote Podospora anserina: characterization of two ribosomal restrictive mutations

Fifty-nine mutations that restrict suppressor efficiency were selected in the fungus Podospora anserina using four different screening methods. Previous genetic analysis has shown that these antisuppressors lie in six loci and that they could be similar to ribosomal restrictive mutations known in Escherichia coli. The present study deals with the response of two of them, AS1-1 and AS6-1, to paromomycin and low temperature both in vivo and in vitro. The data demonstrate that ribosomes of the mutant and double-mutant strains are equally resistant to the ambiguity effect of paromomycin. These data are the first demonstration of mutations that increase translational fidelity in eucaryotic organism.