Despite mtDNA polymorphism the mobile intron (plDNA) of the COI gene is present in ten different races of Podospora anserina

Identification of autophagy as a longevity-assurance mechanism in the aging model Podospora anserina

The filamentous ascomycete Podospora anserina is a well-established aging model in which a variety of different pathways, including those involved in the control of respiration, ROS generation and scavenging, DNA maintenance, proteostasis, mitochondrial dynamics, and programmed cell death have previously been demonstrated to affect aging and life span. Here we address a potential role of autophagy. We provide data demonstrating high basal autophagy levels even in strains cultivated under noninduced conditions. By monitoring an N-terminal fusion of EGFP to the fungal LC3 homolog PaATG8 over the lifetime of the fungus on medium with and without nitrogen supplementation, respectively, we identified a significant increase of GFP puncta in older and in nitrogen-starved cultures suggesting an induction of autophagy during aging. This conclusion is supported by the demonstration of an age-related and autophagy-dependent degradation of a PaSOD1-GFP reporter protein. The deletion of Paatg1, which leads to the lack of the PaATG1 serine/threonine kinase active in early stages of autophagy induction, impairs ascospore germination and development and shortens life span. Under nitrogen-depleted conditions, life span of the wild type is increased almost 4-fold. In contrast, this effect is annihilated in the Paatg1 deletion strain, suggesting that the ability to induce autophagy is beneficial for this fungus. Collectively, our data identify autophagy as a longevity-assurance mechanism in P. anserina and as another surveillance pathway in the complex network of pathways affecting aging and development. These findings provide perspectives for the elucidation of the mechanisms involved in the regulation of individual pathways and their interactions.

Assessing organismal aging in the filamentous fungus Podospora anserina

Podospora anserina is an extensively studied model organism to unravel the mechanism of organismal aging. This filamentous fungus is short-lived and accessible to experimentation. Aging and lifespan are controlled by genetic and environmental traits and, in this model, have a strong mitochondrial etiology. Here, we describe methods and protocols to manipulate and study the aging process in P. anserina at different levels including biochemistry, cell biology, genetics, and physiology.

Alternative oxidase dependent respiration leads to an increased mitochondrial content in two long-lived mutants of the aging model Podospora anserina

The retrograde response constitutes an important signalling pathway from mitochondria to the nucleus which induces several genes to allow compensation of mitochondrial impairments. In the filamentous ascomycete Podospora anserina, an example for such a response is the induction of a nuclear-encoded and iron-dependent alternative oxidase (AOX) occurring when cytochrome-c oxidase (COX) dependent respiration is affected. Several long-lived mutants are known which predominantly or exclusively respire via AOX. Here we show that two AOX-utilising mutants, grisea and PaCox17::ble, are able to compensate partially for lowered OXPHOS efficiency resulting from AOX-dependent respiration by increasing mitochondrial content. At the physiological level this is demonstrated by an elevated oxygen consumption and increased heat production. However, in the two mutants, ATP levels do not reach WT levels. Interestingly, mutant PaCox17::ble is characterized by a highly increased release of the reactive oxygen species (ROS) hydrogen peroxide. Both grisea and PaCox17::ble contain elevated levels of mitochondrial proteins involved in quality control, i. e. LON protease and the molecular chaperone HSP60. Taken together, our work demonstrates that AOX-dependent respiration in two mutants of the ageing model P. anserina is linked to a novel mechanism involved in the retrograde response pathway, mitochondrial biogenesis, which might also play an important role for cellular maintenance in other organisms.

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.

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.

Impact of a disruption of a pathway delivering copper to mitochondria on Podospora anserina metabolism and life span

A global depletion of cellular copper as the result of a deficiency in high-affinity copper uptake was previously shown to affect the phenotype and life span of the filamentous fungus Podospora anserina. We report here the construction of a strain in which the delivery of copper to complex IV of the mitochondrial respiratory chain is affected. This strain, PaCox17::ble, is a PaCox17-null mutant that does not synthesize the molecular chaperone targeting copper to cytochrome c oxidase subunit II. PaCox17::ble is characterized by a decreased growth rate, a reduction in aerial hyphae formation, reduced female fertility, and a dramatic increase in life span. The mutant respires via a cyanide-resistant alternative pathway, displays superoxide dismutase (SOD) activity profiles significantly differing from those of the wild-type strain and is characterized by a stabilization of the mitochondrial DNA. Collectively, the presented data define individual components of a molecular network effective in life span modulation and copper as an element with a dual effect. As a cofactor of complex IV of the respiratory chain, it is indirectly involved in the generation of reactive oxygen species (ROS) and thereby plays a life span-limiting role. In contrast, Cu/Zn SOD as a ROS-scavenging enzyme lowers molecular damage and thus positively affects life span. Such considerations explain the reported differences in life span of independent mutants and spread more light on the delicate tuning of the molecular network influencing biological ageing.

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.

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.

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.

Optional introns in mitochondrial DNA of Podospora anserina are the primary source of observed size polymorphisms

The significant differences in mitochondrial genome size among seven races (B, E, M, T, U, W, and Y) of Podospora anserina have been found to be primarily due to the presence and/or absence of introns, including four introns not previously known to be optional. Information from physical mapping of races M and T, and sequence data from races A and s, was used to identify regions likely to contain insertions or deletions, which were then characterized using PCR and sequence analysis. Newly confirmed optional introns are the first intron of the large ribosomal RNA (LSUr1), the single intron of NADH dehydrogenase subunit 3 (ND3i1), the single intron in ATPase subunit 6 (ATPase6), and the fifth intron of cytochrome oxidase subunit I (COIi5). We have also found that race M exists in two forms as determined by mitochondrial DNA. These results bring to nine (including races A and s) the number of races characterized by mitochondrial intron content with a total of six known optional introns and one optional insertion. Eight of the nine races contain a distinct set of introns, providing a more reliable means for identification and comparison. The identification of optional mitochondrial introns in P. anserina may have evolutionary implications regarding the transfer and/or mobility of these introns.

Horizontal transfer of a mitochondrial plasmid

Direct evidence for horizontal transfer of a mitochondrial plasmid from the discomycete Ascobolus immersus to the pyrenomycete Podospora anserina is presented. Southern blot hybridisation analysis, polymerase chain reaction (PCR) amplification, and DNA sequencing demonstrate transmission of a linear plasmid upon hyphal contact. DNA extraction from isolated organelles indicates a mitochondrial localisation for the plasmid in P. anserina. This is the first report of horizontal gene transfer among unrelated fungi. These results have important evolutionary implications for plasmid propagation in fungi.

Evidence for a life span-prolonging effect of a linear plasmid in a longevity mutant of Podospora anserina

The linear mitochondrial plasmid pAL2-1 of the long-lived mutant AL2 of Podospora anserina was demonstrated to be able to integrate into the high molecular weight mitochondrial DNA (mtDNA). Hybridization analysis and densitometric evaluation of the mitochondrial genome isolated from cultures of different ages revealed that the mtDNA is highly stable during the whole life span of the mutant. In addition, and in sharp contrast to the situation in certain senescence-prone Neurospora strains, the mutated P. anserina mtDNA molecules containing integrated plasmid copies are not suppressive to wild-type genomes. As demonstrated by hybridization and polymerase chain reaction (PCR) analysis, the proportion of mtDNA molecules affected by the integration of pAL2-1 fluctuates between 10% and 50%. Comparative sequence analysis of free and integrated plasmid copies revealed four differences within the terminal inverted repeats (TIRs). These point mutations are not caused by the integration event since they occur subsequent to integration and at various ages. Interestingly, both repeats contain identical sequences indicating that the mechanism involved in the maintenance of perfect TIRs is active on both free and integrated plasmid copies. Finally, in reciprocal crosses between AL2 and the wild-type strain A, some abnormal progeny were obtained. One group of strains did not contain detectable amounts of plasmid pAL2-1, although the mtDNA was clearly of the type found in the long-lived mutant AL2. These strains exhibited a short-lived phenotype. In contrast, one strain was selected that was found to contain wild-type A-specific mitochondrial genomes and traces of pAL2-1. This strain was characterized by an increased life span. Altogether these data suggest that the linear plasmid pAL2-1 is involved in the expression of longevity in mutant AL2.

Three mitochondrial unassigned open reading frames of Podospora anserina represent remnants of a viral-type RNA polymerase gene

The mitochondrial DNA of Podospora anserina is complex, consisting of a characteristic set of genes with a large number of introns and a substantial amount of sequence of unknown function and origin. In addition, as indicated by various types of reorganization, this genome is highly flexible. Here we report the identification of three unassigned mitochondrial open reading frames (ORF P', ORF Q', ORF 11) as remnants of a rearranged viral-type RNA polymerase gene. These ORFs are not transcribed and may be derived from the integration of a linear plasmid of the type recently identified in a mutant of P. anserina.

The role of mitochondrial DNA rearrangements in aging and human diseases

Instabilities and point mutations of the high molecular weight mitochondrial DNA (mtDNA) were shown to be correlated with various degenerative processes in both lower eukaryotes as well as in mammals. In filamentous fungi, circular and linear plasmids were demonstrated to be involved in mtDNA rearrangements and in the genetic control of senescence. In addition, in these eukaryotic microorganisms, which have proved to be ideal model systems in experimental gerontology, a number of nuclear genes were identified controlling the stability of the mitochondrial genome. Although the mitochondrial genome of mammals, including humans, appears to be quite stable in comparison to other species, mtDNA instabilities of the type described in fungi were observed in mitochondria of patients with different mitochondrial degenerative disorders (CPEO, KSS, Pearson syndrome, LHON, MERRF, MELAS). It was later demonstrated that such mtDNA rearrangements appear to accumulate progressively during aging in human subjects. These data suggest that instabilities of the mitochondrial genome may play an important role in the control of life span not only in lower eukaryotes, but also in humans.

The complete DNA sequence of the mitochondrial genome of Podospora anserina

The complete 94,192 bp sequence of the mitochondrial genome from race s of Podospora anserina is presented (1 kb = 10(3) base pairs). Three regions unique to race A are also presented bringing the size of this genome to 100,314 bp. Race s contains 31 group I introns (33 in race A) and 2 group II introns (3 in race A). Analysis shows that the group I introns can be categorized according to families both with regard to secondary structure and their open reading frames. All identified genes are transcribed from the same strand. Except for the lack of ATPase 9, the Podospora genome contains the same genes as its fungal counterparts, N. crassa and A. nidulans. About 20% of the genome has not yet been identified. DNA sequence studies of several excision-amplification plasmids demonstrate a common feature to be the presence of short repeated sequences at both termini with a prevalence of GGCGCAAGCTC.

DNA sequence analysis of the mitochondrial ND4L-ND5 gene complex from Podospora anserina. Duplication of the ND4L gene within its intron

A 15 kb region of the 100 kb mitochondrial genome of Podospora anserina has been mapped and sequenced (1 kb = 10(3) base-pairs). The genes for ND4L and ND5 are identified as contiguous genes with overlapping termination and initiation codons. In race A (101 kb) the gene for ND4L (4.3 kb) has a gene duplication within an intron including a second subgroup IC intron. Race s (95 kb) lacks this second gene complex. Each intron has the identical 5' exon boundary. Secondary structure analysis showed that the closest relative of the second intron is the first intron itself. The open reading frames of the two introns are also closely related to each other as well as to their counterpart in the ND4L gene of Neurospora crassa. The 9.9 kb ND5 gene starts immediately at the termination codon of ND4L and is split by two group IB introns, one group IC intron and one group II intron. The group II intron is closely related to other group II introns although its open reading frame sequence similarity with retroviral reverse transcriptase appears to be more divergent. The similarities in secondary structure and open reading frames for these six introns are discussed.

DNA sequence analysis of the 24.5 kilobase pair cytochrome oxidase subunit I mitochondrial gene from Podospora anserina: a gene with sixteen introns

The DNA sequence of a 26.7 Kilobase pair (10(3) base pairs = 1 Kb) region of the mitochondrial genomes of races s and A from Podospora anserina was determined. Within this region, the 24.5 Kb cytochrome oxidase subunit I gene was located and its exon sequences determined by computer analysis comparisons with other fungal genes. The Podospora COI gene was interrupted by two group II introns (one in race s) and fourteen group I introns ranging in size from about 2.2 Kb to 404 bp. Earlier studies on secondary structure analysis, as well as comparison of their open reading frames (ORFs), showed that the two group II introns were closely related. The fourteen group I introns were representatives of three subgroupings (IB, C and a new category, subgroup ID). Two of these group I introns were separated by just a single exon codon. The analysis of all these introns is discussed in comparison with other fungal introns as well as with the known Podospora anserina introns.

DNA sequence analysis of the apocytochrome b gene of Podospora anserina: a new family of intronic open reading frame

The 5,969 bp (base pair) DNA sequence of the apocytochrome b mitochondrial (mt) gene of race A Podospora anserina was located in a 8.5 Kbp region. This gene contained a 2,499 bp subgroup IB and a 1,306 bp subgroup ID intron as well as a 990 bp subgroup IB intron which is present in race A but not race s. The large subgroup IB intron and the race A specific IB intron both contained potential alternate splice sites which brought their open reading frames into phase with their upstream exon sequences. All three introns were compared with regard to their secondary structures and open reading frames to the other 30 group I introns in Podospora anserina, as well as to other fungal introns. We detected a new family of intronic ORFs comprising seven P. anserina introns, several N. crassa introns, as well as the T4td bacteriophage intron. Sequence similarities to intron-encoded endonucleases were noteworthy. The DNA sequences reported here and in the accompanying paper complete the analysis of race s and race A mitochondrial DNA.

DNA sequence, structure, and phylogenetic relationship of the small subunit rRNA coding region of mitochondrial DNA from Podospora anserina

DNA sequence analysis and the localization of the 5' and 3' termini by S1 mapping have shown that the mitochondrial (mt) small subunit rRNA coding region from Podospora anserina is 1980 bp in length. The analogous coding region for mt rRNA is 1962 bp in maize, 1686 bp in Saccharomyces cerevisiae, and 956 bp in mammals, whereas its counterpart in Escherichia coli is 1542 bp. The P. anserina mt 16S-like rRNA is 400 bases longer than that from E. coli, but can be folded into a similar secondary structure. The additional bases appear to be clustered at specific locations, including extensions at the 5' and 3' termini. Comparison with secondary structure diagrams of 16S-like RNAs from several organisms allowed us to specify highly conserved and variable regions of this gene. Phylogenetic tree construction indicated that this gene is grouped with other mitochondrial genes, but most closely, as expected, with the fungal mitochondrial genes.

DNA sequence and secondary structures of the large subunit rRNA coding regions and its two class I introns of mitochondrial DNA from Podospora anserina

DNA sequence analysis has shown that the gene coding for the mitochondrial (mt) large subunit ribosomal RNA (rRNA) from Podospora anserina is interrupted by two class I introns. The coding region for the large subunit rRNA itself is 3715 bp and the two introns are 1544 (r1) and 2404 (r2) bp in length. Secondary structure models for the large subunit rRNA were constructed and compared with the equivalent structure from Escherichia coli 23S rRNA. The two structures were remarkably similar despite an 800-base difference in length. The additional bases in the P. anserina rRNA appear to be mostly in unstructured regions in the 3' part of the RNA. Secondary structure models for the two introns show striking similarities with each other as well as with the intron models from the equivalent introns in Saccharomyces cerevisiae, Neurospora crassa, and Aspergillus nidulans. The long open reading frames in each intron are different from each other, however, and the nucleotide sequence similarity diverges as it proceeds away from the core structure. Each intron is located within regions of the large subunit rRNA gene that are highly conserved in both sequence and structure. Computer analysis showed that the open reading frame for intron r1 contained a common maturase-like polypeptide. The open reading frames of intron r2 appeared to be chimeric, displaying high sequence similarity with the open reading frames in the r1 and ATPase 6 introns of N. crassa.

A mobile group I intron in the nuclear rDNA of Physarum polycephalum

We have shown that a strain-specific group I intron (intron 3) in the nuclear extrachromosomal rDNA or Physarum polycephalum is a mobile element. Shortly after mating of amoebae from intron-lacking and intron-containing strains, intron 3 transposes in a site-specific manner into all available recipient molecules. The transposition appears to occur by gene conversion, as evidence by the co-conversion of adjacent sequences and by double strand breakage observed in some of the recipient rDNA molecules. We infer that the double strand break is induced by an endonuclease encoded by intron 3, since in vitro transcription and translation of the cloned intron leads to the synthesis of an enzymatically active, site-specific nuclease. This is the first demonstration of the transposition of a nuclear intron in an experimental setting, and provides a rare example of a protein encoded by an RNA polymerase I transcript.

Mitochondrial DNA rearrangements are correlated with a delayed amplification of the mobile intron (plDNA) in a long-lived mutant of Podospora anserina

A new long-lived mutant of Podospora anserina has been isolated and characterized. Its longevity is maternally inherited as revealed by reciprocal crosses. A molecular analysis resulted in the identification of an amplified DNA species (designated pAL2-1) with homology to mitochondrial DNA (mtDNA). The presence of this DNA species is correlated with mtDNA rearrangements and a delayed amplification of the mobile intron (plDNA).

Sequence analysis of mitochondrial DNA from Podospora anserina. Pervasiveness of a class I intron in three separate genes

A 48 kb region of the 95 kb mitochondrial genome of Podospora anserina has been mapped and sequenced (1 kb = 10(3) base-pairs). The DNA sequence of the genes for ND2, 3, 4, ATPase 6 and URFC are presented here. As in Neurospora crassa, the ND2 and 3 genes consist of a unit separated by one TAA stop codon. ND3, 4 and ATPase 6 are interrupted by class I introns. All three introns are remarkably similar in the C-domain of their secondary structure, sufficient enough to designate them as new subgroup, class IC introns. The open reading frames of the ND3 and 4 introns bear a high sequence similarity to the open reading frame of the class IB introns of ATPase 6 from N. crassa and ND1 from Neurospora intermedia Varkud. We also show that the tRNA Met-2 gene is duplicated and is involved in a recombinational event. The 5' region of URFC is also duplicated but no involvement of this gene with recombination or formation of plasmids is known. The evolutionary significance of the similarities of intron secondary structures and open reading frames of the ND3, 4 and ATPase 6 genes is discussed, including the possible separate evolution of structural and coding sequences.

DNA sequence and organization of the mitochondrial ND1 gene from Podospora anserina: analysis of alternate splice sites

Earlier, we reported that the ND1 mitochondrial gene of Podospora anserina is mosaic, containing at least three class I introns. We have now completed the sequence of the ND1 gene and have determined that it contains four class I introns of 1,820, 2,631, 2,256 and 2,597 bp with the entire gene complex containing 10,505 bp, only 1,101 of which are exon sequences. Introns 1 and 3 appear to be related in that their open reading frames (ORFS) exhibit extensive amino acid sequence similarity and like the URFN sequence from Neurospora crassa have multiple sequence repetitions. Introns 2 and 4 are similar in that both appear to be mosaic introns. Where intron 2 has many short ORFS, intron 4 has two, 391 and 262 aa respectively. The first ORF has some patch work sequence similarity with one of the intron 2 ORFs but the second ORF is strikingly similar to the single intron ORF in the ND1 gene of N. crassa. Just upstream of the sequences necessary to form the central core of the P. anserina intron 4 secondary structure, there is a 17 bp sequence which is an exact replica of the exon sequence abutting the 5' flank of the 1,118 bp N. crassa ND1 intron. Secondary structure analysis suggests that the 2,597 bp intron 4 can fold as an entity but a similar structure can be constructed just from an 1,130 bp portion by utilizing the 17 bp element as an alternate splice site. Detailed structural analysis suggests that intron 4 (as well as the single ND1 intron from N. crassa) can utilize helical configurations which bring the downstream open reading frame into juxtaposition with the exon sequences.

Podospora anserina does not senesce when serially passaged in liquid culture

A procedure was developed for the prolonged growth of the ascomycete fungus Podospora anserina in liquid culture to determine the effects of such growth on the senescence phenotype. Senescence in P. anserina, which is maternally inherited and associated with the excision and amplification of specific mitochondrial plasmids, occurs when this species is grown on solid medium. In two independent experiments no evidence of senescence was observed as mycelia were serially passaged in liquid culture. Further, when separable mycelial masses, termed puff balls, from the liquid cultures were plated on solid medium, a significant increase in their average longevity was observed. The apparent immortality of P. anserina in liquid culture was not dependent upon mitochondrial DNA rearrangements, nor was it affected by the presence of a previously described senescence plasmid, alpha senDNA. Evidence was obtained indicating that growth in liquid culture exerts selective pressure to maintain the wild-type mitochondrial genome.

An additional class II intron with homology to reverse transcriptase in rapidly senescing Podospora anserina

Senescence in Podospora anserina is maternally inherited and the parameters of senescence are race specific. We have compared the restriction enzyme fragment maps of race A, the most rapidly senescing race, with race s and have found three inserts in race A which are not present in race s mitochondrial DNA. Fragment A was mapped and found to be located downstream of the so-called alpha senDNA, a class II intron, near the 5' end of the COI gene, separated from alpha senDNA by two class I introns. DNA sequence analysis showed that fragment A is also a class II intron, but with only 10% DNA sequence homology to alpha senDNA. Like alpha senDNA, intron A contains significant amino acid homology with known reverse transcriptases. The importance of this additional class II intron in the mitochondrial genome with the relative rate of senescence in race A is discussed.