Cytosolic ribosomal mutations that abolish accumulation of circular intron in the mitochondria without preventing senescence of Podospora anserina

The C3HC type zinc-finger protein (ZFC3) interacting with Lon/MAP1 is important for mitochondrial gene regulation, infection hypha development and longevity of Magnaporthe oryzae

Background

The rice blast is a typical fungal disease caused by Magnaporthe oryzae, and the mitochondrial ATP-dependent Lon protease (MAP1) has been proven to be involved in blast development. We previously screened a C3HC type Zinc-finger domain protein (ZFC3), which is interacted with MAP1. The purpose of this research was to study the biological function of ZFC3 protein in M. oryzae.

Results

We first confirmed that the ZFC3-RFP fusion protein is localized within the mitochondria. The deleted mutant strains of ZFC3 (∆ZFC3) showed the enhanced expression level of mtATP6, particularly mtATP8, and almost unchanged nATP9. ΔZFC3 produces more conidia and more tolerance to multiple stressors. The knock-out strain shows more melanin accumulation suggests the susceptibility to aging. ΔZFC3 displays faster early-stage hypha infiltration involved in MAP1-mediated pathogenicity in host rice.

Conclusion

These results support the view that ZFC3 is a key regulator involved in gene regulation, stress response, cell wall integrity, longevity, conidiation, infection hypha development and MAP1-mediated pathogenicity in M. oryzae.

Maintaining two mating types: structure of the mating type locus and its role in heterokaryosis in Podospora anserina

Pseudo-homothallism is a reproductive strategy elected by some fungi producing heterokaryotic sexual spores containing genetically different but sexually compatible nuclei. This lifestyle appears as a compromise between true homothallism (self-fertility with predominant inbreeding) and complete heterothallism (with exclusive outcrossing). However, pseudohomothallic species face the problem of maintaining heterokaryotic mycelia to fully benefit from this lifestyle, as homokaryons are self-sterile. Here, we report on the structure of chromosome 1 in mat+ and mat- isolates of strain S of the pseudohomothallic fungus Podospora anserina. Chromosome 1 contains either one of the mat+ and mat- mating types of P. anserina, which is mostly found in nature as a mat+/mat- heterokaryotic mycelium harboring sexually compatible nuclei. We identified a "mat" region ∼0.8 Mb long, devoid of meiotic recombination and containing the mating-type idiomorphs, which is a candidate to be involved in the maintenance of the heterokaryotic state, since the S mat+ and S mat- strains have different physiology that may enable hybrid-vigor-like phenomena in the heterokaryons. The mat region contains 229 coding sequences. A total of 687 polymorphisms were detected between the S mat+ and S mat- chromosomes. Importantly, the mat region is colinear between both chromosomes, which calls for an original mechanism of recombination inhibition. Microarray analyses revealed that 10% of the P. anserina genes have different transcriptional profiles in S mat+ and S mat-, in line with their different phenotypes. Finally, we show that the heterokaryotic state is faithfully maintained during mycelium growth of P. anserina, yet mat+/mat+ and mat-/mat- heterokaryons are as stable as mat+/mat- ones, evidencing a maintenance of heterokaryosis that does not rely on fitness-enhancing complementation between the S mat+ and S mat- strains.

Podospora anserina: a model organism to study mechanisms of healthy ageing

The filamentous ascomycete Podospora anserina has been extensively studied as an experimental ageing model for more than 50 years. As a result, a huge body of data has been accumulated and various molecular pathways have been identified as part of a molecular network involved in the control of ageing and life span. The aim of this review is to summarize data on P. anserina ageing, including aspects like respiration, cellular copper homeostasis, mitochondrial DNA (mtDNA) stability/instability, mitochondrial dynamics, apoptosis, translation efficiency and pathways directed against oxidative stress. It becomes clear that manipulation of several of these pathways bears the potential to extend the healthy period of time, the health span, within the life time of the fungus. Here we put special attention on recent work aimed to identify and characterize this type of long-lived P. anserina mutants. The study of the molecular pathways which are modified in these mutants can be expected to provide important clues for the elucidation of the mechanistic basis of this type of 'healthy ageing' at the organism level.

The genome sequence of the model ascomycete fungus Podospora anserina

A 10X draft sequence of Podospora anserina genome shows highly dynamic evolution since its divergence from Neurospora crassa.

Background

The dung-inhabiting ascomycete fungus Podospora anserina is a model used to study various aspects of eukaryotic and fungal biology, such as ageing, prions and sexual development.

Results

We present a 10X draft sequence of P. anserina genome, linked to the sequences of a large expressed sequence tag collection. Similar to higher eukaryotes, the P. anserina transcription/splicing machinery generates numerous non-conventional transcripts. Comparison of the P. anserina genome and orthologous gene set with the one of its close relatives, Neurospora crassa, shows that synteny is poorly conserved, the main result of evolution being gene shuffling in the same chromosome. The P. anserina genome contains fewer repeated sequences and has evolved new genes by duplication since its separation from N. crassa, despite the presence of the repeat induced point mutation mechanism that mutates duplicated sequences. We also provide evidence that frequent gene loss took place in the lineages leading to P. anserina and N. crassa. P. anserina contains a large and highly specialized set of genes involved in utilization of natural carbon sources commonly found in its natural biotope. It includes genes potentially involved in lignin degradation and efficient cellulose breakdown.

Conclusion

The features of the P. anserina genome indicate a highly dynamic evolution since the divergence of P. anserina and N. crassa, leading to the ability of the former to use specific complex carbon sources that match its needs in its natural biotope.

Integration of a pAL2-1 homologous mitochondrial plasmid associated with life span extension in Podospora anserina

We isolated and characterized a novel spontaneous longevity mutant of Podospora anserina strain Wa32 carrying one of the pAL2-1 homologous mitochondrial plasmids. This mutant is at least ten fold longer-lived than the wild type, and is hence a formal suppressor of both the regular and the 'plasmid-based' senescence process. We show that the longevity trait is maternally inherited and coincides with the presence of a copy of the plasmid integrated in the 5' UTR of the mitochondrial Complex I genes nd2 and nd3. This mutation is associated with complex alterations in the respiratory chain, including a dispensable induction of the alternative oxidase. It is also associated with a stabilization of the mitochondrial chromosome and a reduction of the overall cellular level of reactive oxygen species.

Impact of ROS on ageing of two fungal model systems: Saccharomyces cerevisiae and Podospora anserina

To provide a foundation for the development of effective interventions to counteract various age-related diseases in humans, ageing processes have been extensively studied in various model organisms and systems. However, the mechanisms underlying ageing are still not unravelled in detail in any system including rather simple organisms. In this article, we review some of the molecular mechanisms that were found to affect ageing in two fungal models, the unicellular ascomycete Saccharomyces cerevisiae and the filamentous ascomycete Podospora anserina. A selection of issues like retrograde response, genomic instability, caloric restriction, mtDNA reorganisation and apoptosis is presented and discussed with special emphasis on the role reactive oxygen species (ROS) play in these diverse molecular pathways.

Mitochondrial metabolism and aging in the filamentous fungus Podospora anserina

The filamentous fungus Podospora anserina has a limited lifespan. In this organism, aging is systematically associated to mitochondrial DNA instability. We recently provided evidence that the respiratory function is a key determinant of its lifespan. Loss of function of the cytochrome pathway leads to the compensatory induction of an alternative oxidase, to a decreased production of reactive oxygen species and to a striking increase in lifespan. These changes are associated to the stabilization of the mitochondrial DNA. Here we review and discuss the links between these different parameters and their implication in the control of lifespan. Since we demonstrated the central role of mitochondrial metabolism in aging, the same relationship has been evidenced in several model systems from yeast to mice, confirming the usefulness of simple organisms as P. anserina for studying lifespan regulation.

Biogenesis and replication of small plasmid-like derivatives of the mitochondrial DNA in Neurospora crassa

For reasons that are not obvious, sets of related, small, plasmid-like elements appear spontaneously and become amplified in the mitochondria of some cytochrome-deficient and/or UV-sensitive mutants of Neurospora crassa. These plasmid-like DNAs are multimeric series of circular molecules, each consisting of a finite number of identical tandem repeats of a relatively short mtDNA-derived nucleotide sequence (monomer). The plasmid-like elements that have been characterized in this study consist of monomers that vary in length from 125 to 296 base pairs, depending on the strain of origin. Each monomer includes a GC-rich palindrome that is followed by the promoter and a short section of the 5' terminal region of the mitochondrial large-subunit rRNA gene (rnl). Analyses of the nucleotide sequences of variants of this group of elements indicates that they are not generated by intra-molecular recombination, but are the result of single- or double-strand DNA breaks that are produced by a mismatch or base excision repair process. These elements do not appear to contain a defined origin of replication, but replicate by a recombination-dependent rolling-circle mechanism. One- and two-dimensional gel electrophoresis of the plasmid-like element derived Hind III and Pst I fragments combined with S1 nuclease treatments suggest that the intergenic GC-rich palindromes, which are ubiquitous in the mtDNA Neurospora, could be replication fork pausing points.

Asexual transmission, non-suppressiveness and meiotic extinction of small plasmid-like derivatives of the mitochondrial DNA in Neurospora crassa

For reasons that are not obvious, sets of related plasmid-like elements that consist of short segments of DNA that overlap the 5' terminal region of the mitochondrial large-subunit rRNA gene sometimes appear spontaneously and become amplified in the mitochondria of some cytochrome-deficient and/or UV-sensitive mutants of Neurospora crassa. These elements are transmitted efficiently through hyphal anastomoses and appear to invade the mitochondria of recipient strains, but they do not cause senescence and at best cause only slight deficiencies in cytochromes a and b even though they are transcribed copiously. Hence, the small elements are not suppressive and, unlike large deletion derivatives of the mitochondrial chromosome, do not displace normal mtDNA molecules in vegetatively propagated mycelia. Unlike the mitochondrial chromosome, large plasmid-like mtDNA derivatives and true mitochondrial plasmids, the small plasmid-like mtDNA derivatives are rarely transmitted sexually even though they persist without selection in very high copy numbers in vegetative cells. The high copy numbers and high stability of these elements in vegetatively propagated cultures suggests that their monomers contain all the features required for their replication and transmission in the hyphae and conidia of Neurospora. However, the mt-rnl-derived molecules appear to lack a sequence or attribute required for the maintenance or transmission of mitochondrial genetic elements at some stage of the sexual reproductive cycle, including ascospore maturation and germination.

Intramolecular recombination and deletions in mitochondrial DNA of senescent, a nuclear-gene mutant of Neurospora crassa exhibiting “death” phenotype

In Neurospora crassa, a nuclear-gene mutant, senescent, derived from a phenotypically normal wild isolate of Neurospora intermedia exhibits a 'death' phenotype. Regardless of the composition of the culture medium, the mycelium ceases to grow in 2-6 subcultures at 26 degrees C and 1 or 2 subcultures at 34 degrees C. Senescence of vegetative mycelium is associated with deficiencies in cytochromes aa3 and b and reduced oxygen uptake. The restriction fragment analysis of mitochondrial DNA from senescing mycelia showed deletions and gross sequence rearrangements. Analysis of mitochondrial DNA of (sen + sen+) heterokaryons constructed with "excess" sen cytoplasm suggested correlation between mtDNA deletions and senescence. Three novel sen-specific EcoRI fragments of sizes 3.6, 3.9, and 4.4 kb were cloned, sequenced, and analyzed. Nucleotide sequences of the sen-specific EcoRI fragments suggested that deletions were a consequence of intramolecular recombination between EcoRI-5 and -10 and/or between EcoRI-8 and -10. The recombination junctions were close to stretches of GC-rich-PstI palindromic sequences that potentially form stable hairpin structures and might facilitate recombination between homologous repeats as short as 6-10 bp. These observations suggest that the wild-type (sen+) allele encodes a factor that protects the mitochondrial genome from undergoing intramolecular recombination and deletions. In this respect sen+ (linkage group V) has a function similar to nd+ (linkage group I) and the two gene products probably have mutually exclusive roles in suppressing cruciform-associated and homologous recombination, respectively, thus safeguarding mitochondrial genome integrity. The sen+ allele most likely codes for a factor involved in recombination, repair or replication of the mitochondrial genome, or a transcription factor that regulates the expression of genes affiliated with mitochondrial DNA metabolism.

Two copies of mthmg1, encoding a novel mitochondrial HMG-like protein, delay accumulation of mitochondrial DNA deletions in Podospora anserina

In the filamentous fungus Podospora anserina, two degenerative processes which result in growth arrest are associated with mitochondrial genome (mitochondrial DNA [mtDNA]) instability. Senescence is correlated with mtDNA rearrangements and amplification of specific regions (senDNAs). Premature death syndrome is characterized by the accumulation of specific mtDNA deletions. This accumulation is due to indirect effects of the AS1-4 mutation, which alters a cytosolic ribosomal protein gene. The mthmg1 gene has been identified as a double-copy suppressor of premature death. It greatly delays premature death and the accumulation of deletions when it is present in two copies in an ASI-4 context. The duplication of mthmg1 has no significant effect on the wild-type life span or on senDNA patterns. In anAS1+ context, deletion of the mthmg1 gene alters germination, growth, and fertility and reduces the life span. The deltamthmg1 senescent strains display a particular senDNA pattern. This deletion is lethal in an AS1-4 context. According to its physical properties (very basic protein with putative mitochondrial targeting sequence and HMG-type DNA-binding domains) and the cellular localization of an mtHMG1-green fluorescent protein fusion, mtHMG1 appears to be a mitochondrial protein possibly associated with mtDNA. It is noteworthy that it is the first example of a protein combining the two DNA-binding domains, AT-hook motif and HMG-1 boxes. It may be involved in the stability and/or transmission of the mitochondrial genome. To date, no structural homologues have been found in other organisms. However, mtHMG1 displays functional similarities with the Saccharomyces cerevisiae mitochondrial HMG-box protein Abf2.

Genes, mitochondria and aging in filamentous fungi

Fungi are eukaryotic microorganisms studied in various areas of general and applied biology. A few species were among the first systems in which specific aspects of aging were addressed experimentally. Various factors, both environmental and genetic, were found to affect lifespan and aging. Mitochondrial pathways play a paramount role. Since mitochondria are semiautonomous organelles and depend on both nuclear as well as mitochondrial genes, mitochondrial-nuclear interactions are of major relevance. As a main generator of reactive oxygen species (ROS), mitochondria are prone to molecular damage. However, cells can cope with the negative effects of ROS utilizing different scavenging systems and, once defects became manifested, by repair of damaged molecules. Both, lowering ROS generation and increasing mitochondrial "caretaker" systems bear great potential to interfere with natural aging processes.

Aging in fungi: role of mitochondria in Podospora anserina

In experimental gerontology, there is a long tradition in the use of both unicellular and filamentous species of fungi. In the last three decades, biochemical, genetic and molecular approaches have proved very fruitful in elucidating different aspects of ageing. It was shown that various genes and molecular pathways are involved in life span control. The oxygenic energy metabolism plays a central role. During mitochondrial energy transduction, reactive oxygen species (ROS) are generated as by-products. These molecules are able to damage all cellular compounds leading to cellular dysfunctions. Within certain limits, however, cells are able to cope with ROS-related problems. First, ROS scavengers can be induced which are effective in lowering the molecular burden of ROS on cellular functions. Second, if damage occurs, specific repair mechanisms and the general turnover of affected molecules can maintain cellular functions. Finally, if damage of essential components is too severe, cells may induce specific pathways to compensate for the corresponding impairments. A coordinated interaction between different cellular compartments is involved in these processes. In this review I shall concentrate on the ageing in the filamentous ascomycete Podospora anserina. It is clear that both environmental as well as genetic traits are involved in the control of life span and that mitochondrial-nuclear interactions play a paramount role.

Mitochondrial functions and aging

In the filamentous ascomycete Podospora anserina mitochondria play a major role in lifespan control. Since the function of these organelles depends on a large number of individual components it is no surprise that a complex network of interacting branches of individual molecular pathways is involved in this process. Recently, the nuclear encoded transcription factor GRISEA was found to significantly affect mitochondrial functions. GRISEA is involved in the control of cellular copper homeostasis. Most importantly, the high affinity uptake of copper from the environment is controlled by this transcription factor. Once copper has entered the cell, it becomes distributed to different compartments and different target molecules. This process depends on a group of proteins, termed copper chaperones. PaCOX17, a homologue of the yeast copper chaperone yCOX17, appears to be involved in copper delivery to mitochondria. Most importantly, the metal is crucial for the assembly and the function of complex IV of the respiratory chain. However, although P. anserina is an obligate aerobe and therefore depends on mitochondrial energy transduction, impairments in the copper delivery pathway are not lethal. This is due to the induction of a molecular back-up system able to compensate for deficiencies in complex IV. The system utilizes an alternative oxidase (PaAOX) which uses iron instead of copper as a cofactor. The alternative respiratory pathway is characterized by a decreased ATP generation but, most significantly, also a decrease in the production of reactive oxygen species. Consequently, molecular damage is reduced which contributes to an increased lifespan of this type of mutant. In addition, modifications in the availability of cellular copper have other relevant consequences. Most significantly, the characteristic age-related rearrangements occurring in the mitochondrial DNA of wild-type strains of P. anserina were found to be dependent on the availability of copper.

Genome quality control: RIP (repeat-induced point mutation) comes to Podospora

RIP (repeat-induced point mutation) is a silencing process discovered in Neurospora crassa and so far clearly established only in this species as a currently occurring process. RIP acts premeiotically on duplicated sequences, resulting in C-G to T-A mutations, with a striking preference for CpA/TpG dinucleotides. In Podospora anserina, an RIP-like event was observed after several rounds of sexual reproduction in a strain with a 40 kb tandem duplication resulting from homologous integration of a cosmid in the mating-type region. The 9 kb sequenced show 106 C-G to T-A transitions, with 80% of the replaced cytosines located in CpA dinucleotides. This led to the alteration of at least six genes, two of which were unidentified. This RIP-like event extended to single-copy genes between the two members of the repeat. The overall data show that the silencing process is strikingly similar to a light form of RIP, unaccompanied by C-methylation. Interestingly, the N. crassa zeta-eta sequence, which acts as a potent de novo C-methylation RIP signal in this species, is weakly methylated when introduced into P. anserina. These results demonstrate that RIP, at least in light forms, can occur beyond N. crassa.

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.

Metabolism and aging in the filamentous fungus Podospora anserina

In Podospora anserina, lifespan is under the control of environmental and genetic factors. Both suggest an important impact of metabolism on lifespan and aging. Environmental changes of temperature, of the carbon source in the growth medium, or the addition of specific inhibitors to the growth medium are some of the investigated factors. Genetic approaches underscore the significance of metabolism. In particular, the mitochondrial electron transport plays a major role. As a by-product of a cytochrome oxidase (COX) dependent energy transduction, reactive oxygen species (ROS) are generated and lead to damage of cellular biomolecules. Damaged mitochondria, compromised at complex IV (COX) of the respiratory chain, signal to the nucleus and induce a nuclear gene, PaAox, encoding an alternative oxidase (AOX). This pathway resembles the retrograde response that, at least in yeast, is induced by dysfunctional mitochondria. ROS generation is lowered when electrons are transferred via an alternative pathway utilizing the AOX. As a consequence, lifespan of the corresponding strains is increased. Cellular copper levels were found to play a significant role not only in the generation of ROS but also have an impact on the cytoplasmic and the mitochondrial superoxide dismutase (SOD). In addition, copper is involved in the control of mitochondrial DNA rearrangements and affects the ability of the system to remodel damaged mitochondria. All these different components and pathways are part of the complex molecular network involved in lifespan control of this aging model.

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.

Cellular copper homeostasis, mitochondrial DNA instabilities, and lifespan control in the filamentous fungus Podospora anserina

In the fungal aging model Podospora anserina, lifespan is controlled by mitochondrial and nuclear genetic traits. Different nuclear genes are known to affect the integrity of the mitochondrial DNA (mtDNA). One gene of this type is Grisea encoding a copper-modulated transcription factor involved in the control of cellular copper homeostasis. The characterization of a long-lived mutant with a loss-of-function mutation in this gene revealed that the last step in the pathway, homologous recombination, leading to the characteristic age-related mtDNA reorganizations is copper-dependent. In growing parts of the culture, the stabilization of the mtDNA has an important impact on the biogenesis of functional mitochondria, on their capacity to remodel damaged respiratory chains and on longevity.

Role of Mitochondrial DNA in the Senescence and Hypovirulence of Fungi and Potential for Plant Disease Control

The unique coenocytic anatomy of the mycelia of the filamentous fungi and the formation of anastomoses between hyphae from different mycelia enable the intracellular accumulation and infectious transmission of plasmids and mutant mitochondrial DNAs (mtDNAs) that cause senescence. For reasons that are not fully apparent, mitochondria that are rendered dysfunctional by so-called "suppressive" mtDNA mutations proliferate rapidly in growing cells and gradually displace organelles that contain wild-type mtDNA molecules and are functional. The consequence of this process is senescence and death if the suppressive mtDNA contains a lethal mutation. Suppressive mtDNA mutations and mitochondrial plasmids can elicit cytoplasmically transmissible "mitochondrial hypovirulence" syndromes in at least some of the phytopathogenic fungi. In the chestnut-blight fungus Cryphonectria parasitica, the pattern of asexual transmission of mutant mtDNAs and mitochondrial plasmids resembles the pattern of "infectious" transmission displayed by the attenuating virus that is most commonly used for the biological control of this fungus. At least some of the attenuating mitochondrial hypovirulence factors are inherited maternally in crosses, whereas the viruses are not transmitted sexually. The natural control of blight in an isolated stand of chestnut trees has resulted from the invasion of the local population of C. parasitica by a senescence-inducing mutant mtDNA. Moreover, a mitochondrial plasmid, pCRY1, attenuates at least some virulent strains of C. parasitica, suggesting that such factors could be applied to control plant diseases caused by fungi.

Maintenance and integrity of the mitochondrial genome: a plethora of nuclear genes in the budding yeast

Instability of the mitochondrial genome (mtDNA) is a general problem from yeasts to humans. However, its genetic control is not well documented except in the yeast Saccharomyces cerevisiae. From the discovery, 50 years ago, of the petite mutants by Ephrussi and his coworkers, it has been shown that more than 100 nuclear genes directly or indirectly influence the fate of the rho(+) mtDNA. It is not surprising that mutations in genes involved in mtDNA metabolism (replication, repair, and recombination) can cause a complete loss of mtDNA (rho(0) petites) and/or lead to truncated forms (rho(-)) of this genome. However, most loss-of-function mutations which increase yeast mtDNA instability act indirectly: they lie in genes controlling functions as diverse as mitochondrial translation, ATP synthase, iron homeostasis, fatty acid metabolism, mitochondrial morphology, and so on. In a few cases it has been shown that gene overexpression increases the levels of petite mutants. Mutations in other genes are lethal in the absence of a functional mtDNA and thus convert this petite-positive yeast into a petite-negative form: petite cells cannot be recovered in these genetic contexts. Most of the data are explained if one assumes that the maintenance of the rho(+) genome depends on a centromere-like structure dispensable for the maintenance of rho(-) mtDNA and/or the function of mitochondrially encoded ATP synthase subunits, especially ATP6. In fact, the real challenge for the next 50 years will be to assemble the pieces of this puzzle by using yeast and to use complementary models, especially in strict aerobes.

A causal link between respiration and senescence in Podospora anserina

Senescence, a progressive degenerative process leading to age-related increase in mortality, is found in most eukaryotes. However, the molecular events underlying aging remain largely unknown. Understanding how longevity is regulated is a fundamental problem. Here we demonstrate that the respiratory function is a key factor that contributes to shortening lifespan of the filamentous fungus Podospora anserina. In this organism, senescence is systematically associated with mitochondrial DNA instabilities. We show that inactivation of the nuclear COX5 gene encoding subunit V of the cytochrome c oxidase complex leads to the exclusive use of the alternative respiratory pathway and to a decrease in production of reactive oxygen species. This inactivation results in a striking increase of longevity associated with stabilization of the mitochondrial chromosome. Moreover, accumulation of several senescence-specific mitochondrial DNA molecules is prevented in this nuclear mutant. These findings provide direct evidence of a causal link between mitochondrial metabolism and longevity in Podospora anserina.

Senescent: a new Neurospora crassa nuclear gene mutant derived from nature exhibits mitochondrial abnormalities and a "death" phenotype

Fungi are capable of potentially unlimited growth. We resolved nuclear types from multinuclear mycelium of a phenotypically normal wild isolate of the fungus Neurospora intermedia by plating its uninucleate microconidia and obtained a strain which, unlike the "parent" strain, exhibited clonal senescence in subcultures. The mutant gene, senescent, was introgressed into N. crassa and mapped four map units to the right of the his-1 locus on linkage group VR. senescent is the first nuclear gene mutant of Neurospora derived from nature that shows the death phenotype. Death of the sen mutant occurred faster at 34 degrees C than at 22 or 26 degrees C. Measurements of oxygen uptake of conidia using respiratory inhibitors and the spectrophotometric analyses of mitochondrial cytochromes showed that in sen cultures grown at 34 degrees C, cytochromes b and aa(3) were present but cytochrome c was absent. By contrast at 26 degrees C, cytochromes b and c were present but cytochrome aa(3) was diminished in the late subcultures. This suggested that the sen mutation does not affect the potential to produce functional cytochromes. The deficiency of the respiratory chain cytochromes may not be the cause of death of the sen mutant because the cytochrome c and aa(3) mutants of N. crassa are capable of sustained growth whereas sen is not. Possible explanations for the observations are discussed.

Mitochondrial-nuclear interactions and lifespan control in fungi

In fungi, mitochondrial-nuclear interactions are part of a complex molecular network involved in the control of aging processes. The generation of reactive oxygen species at the mitochondrial respiratory chain plays a major role in this network. Mitochondrial DNA instabilities, which are under the control of nuclear genes, affect the generation of reactive oxygen species and modulate the rate of aging. As mitochondria become dysfunctional, they transduce signals to the nucleus and induce the expression of a set of nuclear genes, a process termed retrograde regulation. Molecular data are emerging which suggest that retrograde regulation is involved in lifespan control.

Mitochondrial group II introns, cytochrome c oxidase, and senescence in Podospora anserina

Podospora anserina is a filamentous fungus with a limited life span. It expresses a degenerative syndrome called senescence, which is always associated with the accumulation of circular molecules (senDNAs) containing specific regions of the mitochondrial chromosome. A mobile group II intron (alpha) has been thought to play a prominent role in this syndrome. Intron alpha is the first intron of the cytochrome c oxidase subunit I gene (COX1). Mitochondrial mutants that escape the senescence process are missing this intron, as well as the first exon of the COX1 gene. We describe here the first mutant of P. anserina that has the alpha sequence precisely deleted and whose cytochrome c oxidase activity is identical to that of wild-type cells. The integration site of the intron is slightly modified, and this change prevents efficient homing of intron alpha. We show here that this mutant displays a senescence syndrome similar to that of the wild type and that its life span is increased about twofold. The introduction of a related group II intron into the mitochondrial genome of the mutant does not restore the wild-type life span. These data clearly demonstrate that intron alpha is not the specific senescence factor but rather an accelerator or amplifier of the senescence process. They emphasize the role that intron alpha plays in the instability of the mitochondrial chromosome and the link between this instability and longevity. Our results strongly support the idea that in Podospora, "immortality" can be acquired not by the absence of intron alpha but rather by the lack of active cytochrome c oxidase.

What triggers senescence in Podospora anserina?

Senescence of Podospora anserina is triggered by a cytoplasmic and infectious factor (the determinant of senescence) and is always correlated with mitochondrial DNA modifications, especially with the accumulation of small circular subgenomic DNA molecules, the senDNAs. Several observations have suggested that the senDNAs could be the cytoplasmic and infectious determinant. However, we show here (1) that senDNA molecules can be transferred to a young culture without the cotransmission of the determinant of senescence and (2) that the determinant of senescence does not segregate as a mitochondrial DNA mutation. Overall, our data strongly argue that amplification of senDNA molecules in the mitochondria is not an intrinsic property of these small DNA molecules. They question the nature of the actual determinant of senescence.

Non-conventional infectious elements in filamentous fungi

Old data (most often in French) described phenomena involving non-conventional infectious factors in filamentous fungi. Recently, it was shown that two yeast cytoplasmic determinants are similar to known mammalian prions, in that their different states are attributed to conformational changes of normal cellular proteins. In the light of this discovery, fungal elements are now being reconsidered. This review presents four elements that affect vegetative incompatibility, conidiogenesis, morphology and cell growth. Recently, one element has been shown to be a prion analogue. The status of the others is not clear. We consider the view that non-conventional inheritance might be initiated by the appearance, in the cytoplasm, of a metabolite or a macromolecule whose production involves a positive regulatory loop.

Aging in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae divides asymmetrically, giving rise to a mother cell and a smaller daughter cell. Individual mother cells produce a finite number of daughter cells before senescing, undergoing characteristic changes as they age such as a slower cell cycle and sterility. The average life span is fixed for a given strain, implying that yeast aging has a strong genetic component. Genes that determine yeast longevity have highlighted the importance of such processes as cAMP metabolism, epigenetic silencing, and genome stability. The recent finding that yeast aging is caused, in part, by the accumulation of circular rDNA molecules has unified many seemingly disparate observations.

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.

Mutations in genes encoding the mitochondrial outer membrane proteins Tom70 and Mdm10 of Podospora anserina modify the spectrum of mitochondrial DNA rearrangements associated with cellular death

Tom70 and Mdm10 are mitochondrial outer membrane proteins. Tom70 is implicated in the import of proteins from the cytosol into the mitochondria in Saccharomyces cerevisiae and Neurospora crassa. Mdm10 is involved in the morphology and distribution of mitochondria in S. cerevisiae. Here we report on the characterization of the genes encoding these proteins in the filamentous fungus Podospora anserina. The two genes were previously genetically identified through a systematic search for nuclear suppressors of a degenerative process displayed by the AS1-4 mutant. The PaTom70 protein shows 80% identity with its N. crassa homolog. The PaMdm10 protein displays 35.9% identity with its S. cerevisiae homolog, and cytological analyses show that the PaMDM10-1 mutant exhibits giant mitochondria, as does the S. cerevisiae mdm10-1 mutant. Mutations in PaTOM70 and PaMDM10 result in the accumulation of specific deleted mitochondrial genomes during the senescence process of the fungus. The phenotypic properties of the single- and double-mutant strains suggest a functional relationship between the Tom70 and Mdm10 proteins. These data emphasize the role of the mitochondrial outer membrane in the stability of the mitochondrial genome in an obligate aerobe, probably through the import process.

Mitochondrial DNA rearrangements of Podospora anserina are under the control of the nuclear gene grisea

Podospora anserina is a filamentous fungus with a limited life span. Life span is controlled by nuclear and extranuclear genetic traits. Herein we report the nature of four alterations in the nuclear gene grisea that lead to an altered morphology, a defect in the formation of female gametangia, and an increased life span. Three sequence changes are located in the 5' upstream region of the grisea ORF. One mutation is a G --> A transition at the 5' splice site of the single intron of the gene, leading to a RNA splicing defect. This loss-of-function affects the amplification of the first intron of the mitochondrial cytochrome c oxidase subunit I gene (COI) and the specific mitochondrial DNA rearrangements that occur during senescence of wild-type strains. Our results indicate that the nuclear gene grisea is part of a molecular machinery involved in the control of mitochondrial DNA reorganizations. These DNA instabilities accelerate but are not a prerequisite for the aging of P. anserina cultures.