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PubMed1999

Prions in Saccharomyces and Podospora spp.: protein-based inheritance.

Genetic evidence showed two non-Mendelian genetic elements of Saccharomyces cerevisiae, called [URE3] and [PSI], to be prions of Ure2p and Sup35p, respectively. [URE3] makes cells derepressed for nitrogen catabolism, while [PSI] elevates the efficiency of weak suppressor tRNAs. The same approach led to identification of the non-Mendelian element [Het-s] of the filamentous fungus Podospora anserina, as a prion of the het-s protein. The prion form of the het-s protein is required for heterokaryon incompatibility, a normal fungal function, suggesting that other normal cellular functions may be controlled by prions. [URE3] and [PSI] involve a self-propagating aggregation of Ure2p and Sup35p, respectively. In vitro, Ure2p and Sup35p form amyloid, a filamentous protein structure, high in beta-sheet with a characteristic green birefringent staining by the dye Congo Red. Amyloid deposits are a cardinal feature of Alzheimer's disease, non-insulin-dependent diabetes mellitus, the transmissible spongiform encephalopathies, and many other diseases. The prion domain of Ure2p consists of Asn-rich residues 1 to 80, but two nonoverlapping fragments of the molecule can, when overproduced, induce the de nova appearance of [URE3]. The prion domain of Sup35 consists of residues 1 to 114, also rich in Asn and Gln residues. While runs of Asn and Gln are important for [URE3] and [PSI], no such structures are found in PrP or the Het-s protein. Either elevated or depressed levels of the chaperone Hsp104 interfere with propagation of [PSI]. Both [URE3] and [PSI] are cured by growth of cells in millimolar guanidine HCl. [URE3] is also cured by overexpression of fragments of Ure2p or fusion proteins including parts of Ure2p.

Amino Acid Sequence↗
PubMed2000

In vivo labelling of functional ribosomes reveals spatial regulation during starvation in Podospora anserina.

BACKGROUND: To date, in eukaryotes, ribosomal protein expression is known to be regulated at the transcriptional and/or translational levels. But other forms of regulation may be possible. RESULTS: Here, we report the successful tagging of functional ribosomal particles with a S7-GFP chimaeric protein, making it possible to observe in vivo ribosome dynamics in the filamentous fungus Podospora anserina. Microscopic observations revealed a novel kind of ribosomal protein regulation during the passage between cell growth and stationary phases, with a transient accumulation of ribosomal proteins and/or ribosome subunits in the nucleus, possibly the nucleolus, being observed at the beginning of stationary phase. CONCLUSION: Nuclear sequestration can be another level of ribosomal protein regulation in eukaryotic cells. This may contribute to the regulation of cell growth and division.

Cell Cycle↗
PubMed2004

Incomplete penetrance and variable expressivity of a growth defect as a consequence of knocking out two K(+) transporters in the euascomycete fungus Podospora anserina.

We describe an example of incomplete penetrance and variable expressivity in the filamentous fungus Podospora anserina, two genetic properties classically associated with mutations in more complex organisms, such as green plants and animals. We show that the knockouts of two TRK-related K(+) transporters of this ascomycete present variability in their phenotype that cannot be attributed to fluctuations of the genetic background or the environment. Thalli of the knockout strains derived from independent monokaryotic ascospores or from a single monokaryotic ascospore and cultivated under standard growth conditions may or may not present impaired growth. When impaired, thalli exhibit a range of phenotypes. Environmental conditions control expressivity to a large extent and penetrance to a low extent. Restoration of functional potassium transport by heterologous expression of K(+) transporters from Neurospora crassa abolishes or strongly diminishes the growth impairment. These data show that incomplete penetrance and variable expressivity can be an intrinsic property of a single Mendelian loss-of-function mutation. They also show that such variability in the expression of a mutant phenotype can be promoted by a phenomenon not obviously related to the well-known chromatin structure modifications, i.e., potassium transport. They provide a framework to understand human channelopathies with similar properties.

Base Sequence↗
PubMed2004

PaASK1, a mitogen-activated protein kinase kinase kinase that controls cell degeneration and cell differentiation in Podospora anserina.

MAPKKK are kinases involved in cell signaling. In fungi, these kinases are known to regulate development, pathogenicity, and the sensing of external conditions. We show here that Podospora anserina strains mutated in PaASK1, a MAPKKK of the MEK family, are impaired in the development of crippled growth, a cell degeneration process caused by C, a nonconventional infectious element. They also display defects in mycelium pigmentation, differentiation of aerial hyphae, and making of fruiting bodies, three hallmarks of cell differentiation during stationary phase in P. anserina. Overexpression of PaASK1 results in exacerbation of crippled growth. PaASK1 is a large protein of 1832 amino acids with several domains, including a region rich in proline and a 60-amino-acid-long polyglutamine stretch. Deletion analysis reveals that the polyglutamine stretch is dispensable for PaASK1 activity, whereas the region that contains the prolines is essential but insufficient to promote full activity. We discuss a model based on the hysteresis of a signal transduction cascade to account for the role of PaASK1 in both cell degeneration and stationary-phase cell differentiation.

Amino Acid Sequence↗
PubMed1978

[Electron microscopy of mitochondrial DNA in Podospora anserina and the presence of a multimeric range of circular DNA molecules from senescent cultures].

Mitochondrial DNA from young cultures of race s of Podospora anserina was isolated. Its density in Cesium chloride density equilibrium gradients was 1.694 g/cc. Examination by the electron microscope revealed that ca 1% of this DNA consisted of circles, 31 micrometer in contour length; the remaining DNA was composed of linear molecules ranging in length from 2 to 33 micrometer. In DNA of similar density obtained from senescent cultures of the same race s, about 11% of the molecules consisted of a multimeric set of circles ranging in size from 0.9 to 15 micrometer, with most being in the 1.8 and 2.7 micrometer classes. The similarity of these DNA molecules with the mitochondrial DNA from rho(-) yeast mutants is discussed.

Ascomycota↗
PubMed1987

A novel family of mitochondrial plasmids associated with longevity mutants of Podospora anserina.

We have identified a novel family of plasmids, each containing very short monomeric units, in Podospora anserina longevity mutants. These plasmids, termed small mitochondrial DNAs (sMt-DNAs), are derived from a highly ordered 368-base pair region of the mitochondrial genome. A total of five direct repeat sequences and seven significant regions of dyad symmetry (i.e. palindromes) were found within a 434-base pair mitochondrial sequence, which includes this 368-base pair region. Mitochondrial DNA rearrangements accompany the formation of these small plasmids indicating their derivation from a plastic region of the mitochondrial genome. A possible relationship between the direct repeat sequences, the palindromic regions, and the excision process is discussed.

Ascomycota↗
PubMed1986

Different alterations of the ribosomal protein S7 lead to opposite effects on translational fidelity in the fungus Podospora anserina.

In the fungus Podospora anserina, the su12-1 mutation was previously found to decrease translational accuracy and to alter the ribosomal protein S7. The mutant protein is more basic than the wild type. Among the revertants of the two ribosomal mutations su12-1 and su12-2, 29 contained a second mutation very closely linked to su12. Biochemical analysis of these revertants by functional poly(U) tests and electrophoretical study of the ribosomes led to two conclusions. First, some revertant strains contain new mutant forms of S7. This suggests that su12 is the structural gene for the ribosomal protein S7. Second, the su12-2 revertants display antisuppressor properties in vivo and in vitro (i.e. increased translational accuracy). The electrophoretical patterns of their ribosomal proteins show new, more acidic, forms of S7. Therefore, su12 can be mutated towards either a lower or a greater translational accuracy corresponding to two opposite modifications of the global charge of the ribosomal protein S7. A more acidic form than wild type leads to increased accuracy and a more basic form to decreased accuracy.

Ascomycota↗
PubMed1979

Isolation and partial characterization of the multiple forms of deoxyribonucleic acid-dependent ribonucleic acid polymerase in the fungus Podospora anserina.

Three DNA-dependent RNA polymerases have been isolated and partially purified from the mycelium of the fungus Podospora anserina. Separated by DEAE-Sephadex chromatography, they have been designated RNA polymerases I, II, and III according to their order of elution. Their catalytic properties and alpha-amanitin sensitivity are in agreement with those of the homologous enzymes found in other eukaryotic organisms. The three enzymes exhibit rather sharp monophasic ammonium sulfate dependence with optima which are, respectively, 0.035 M, 0.050 M, and 0.075 M. Enzyme I has the largest Mn2+/Mg2+ activity ratio, shows a marked preference for native DNA, and is insensitive to alpha-amanitin. Enzyme III uses poly(dA-dT) in preference to native DNA as template and is only partially sensitive to alpha-amanitin. Enzyme II is sensitive to alpha-amanitin, but high concentrations of the toxin are required for inhibition compared to other eukaryotic class II enzymes. Three similar RNA polymerases with comparable levels of activity were found in the temperature-dependent VR strain when cellular incompatibility, leading to a rapid cessation of RNA synthesis, was induced.

Amanitins↗
PubMed1983

Immunological studies of ribosomal mutants in the fungus Podospora anserina.

In the fungus Podospora anserina, two mutants with ribosomes apparently lacking one ribosomal protein were investigated using immunological techniques to determine whether these proteins were indeed missing. Antibodies directed against the wild type ribosomal proteins apparently absent in the mutants were raised from two-dimensional gel spots containing these proteins. These sera were used to detect cross-reacting proteins in the mutants. Immunological results have revealed in both mutants the presence of strongly altered ribosomal proteins which co-migrate with other wild type proteins on two-dimensional gel. The molecular weight of mutant proteins differs at least 25% from that of homologous wild type proteins. Furthermore, we observed that the growth of the mutants and the activity of their ribosomes are very reduced in both single mutants containing the altered proteins.

Ascomycota↗
PubMed1980

Altered ribosomal proteins in mutants of the fungus Podospora anserina.

In the fungus Podospora anserina, a double mutant strain which combines two ribosomal mutations exhibits a marked sensitivity to NH4+ acetate. Mutations which suppress this phenotype were isolated. Two mutants were found to have an electrophoretically altered ribosomal protein. In one mutant, protein S15 from the small subunit has a reduced charge; in the other, the 60 S protein L16 is shorter than the homologous wild type protein. Both mutant and wild type proteins are present in heterokaryotic strains combining mutant and wild type nuclei. We therefore propose that the alteration of S15 and L16 results from mutations in the structural genes for these proteins.

Ammonia↗
PubMed1994

The S12 ribosomal protein of Podospora anserina belongs to the S19 bacterial family and controls the mitochondrial genome integrity through cytoplasmic translation.

In the filamentous fungus Podospora anserina, two nuclear genes are involved in the premature death syndrome associated with a site-specific deletion of the mitochondrial DNA: a mutant allele of the AS1 gene, encoding the cytoplasmic ribosomal protein S12, and an uncharacterized gene closely linked to the mating-type locus. We describe here the cloning and the sequencing of the wild-type and two mutant alleles of the AS1 gene. The P. anserina S12 protein belongs to the bacterial S19 ribosomal protein family and shows 72% identity with the S15 human ribosomal protein. Transformation experiments have shown that the AS1-4 mutation itself is responsible for the premature death phenotype and that it corresponds to a Gly to Asp change in the highly conserved COOH-terminal part of the protein. Use of antibodies directed against S12 did not permit detection of the mutant ribosomal protein inside the mitochondria. However, cross-reactions were observed with at least one mitochondrial ribosomal protein displaying a higher molecular weight than S12. The mitochondrial protein does not seem to be a by-product of the AS1 gene but is more likely the mitochondrial homologue of S12. These results strongly suggest that the mutant S12 protein acts indirectly to promote the mitochondrial deletion, via the cytoplasmic translation.

Amino Acid Sequence↗
PubMed1997

[Genetic analysis of two cellular degenerations in filamentous fungus Podospora anserina].

The filamentous fungus Podopsora anserina presents an unavoidable arrest of vegetative growth (Senescence) determined by a cytoplasmic and infectious factor. Senescence is correlated with a disorganization of the mitochondrial DNA. This disorganization is caused by an event which is not the appearance of the first defective DNA molecules. These ones are generated constitutively and their accumulation during Senescence requires the presence of an additional factor. Life span of the strains is under nuclear and cytoplasmic genetic control. At least 600 nuclear genes influence longevity. Our analysis focuses on the role of the genes involved in cytosolic translation, since mutations in these genes seem to display the most drastic effects on longevity but also on the structure of the defective mitochondrial DNA molecules that accumulate during Senescence. We have detected in some Podospora anserina mutant strains (permissive strains) the presence of a novel cytoplasmic and infectious determinant that entails an easily discernible phenotype associated with a severe growth alteration (Crippled Growth). This growth alteration is not associated with mitochondrial DNA modifications. Only the strains that have an increased translational accuracy present Crippled Growth. However, the Crippled Growth Determinant is found in all the strains during the stationary phase; it is eliminated from the non permissive strains during the exit of the stationary phase. The mutants, that have an increased translational accuracy, probably lack a factor which is needed to eliminate the determinant when cells enter the growth phase.

Ascomycota↗