Variable DNA splicing sites of a mitochondrial intron: relationship to the senescence process in Podospora

Abstract

The unavoidable phenomenon of senescence in Podospora was previously shown to be correlated with the presence of a senescence‐specific DNA originating from amplification of some regions of the mitochondrial chromosome. The most frequently amplified region (α) corresponds to the first intron of the gene coding for subunit one of cytochrome oxidase. Eleven long‐lived mitochondrial mutants were isolated. Here we report sequencing experiments that show that three of them are deleted for most of intron α and for a few base pairs belonging to the upstream adjacent exon. We also report an analysis of the residual mitochondrial DNA associated with amplification of senescence‐specific DNA α which allows us to identify, in senescent cultures, mitochondrial chromosomes lacking sequence α. These results taken together suggest that excision of intron α from the mitochondrial DNA occurs systematically during the aging process in Podospora. They furthermore provide the first example of inaccurate intron excision at the DNA level.

A site-specific deletion in mitochondrial DNA of Podospora is under the control of nuclear genes.

“…3). This explains the phenotype of the revertant mycelium in most points similar to that of other mitochondrial mutants of Podospora, which were shown to have a part of the CO] gene deleted (16,18). More interestingly, the reversion of the premature death phenotype is correlated with loss of the wild-type upstream exon-intron a junction, which is one of the target sequences for the deletion.…”

“…Preparation, purification, cloning, restriction analyses, and DNADNA hybridization of mtDNA were done by standard methods as described (9,16,18 …”

“…More precisely, it has been shown that short mtDNA sequences are amplified as circular multimeric DNA molecules in senescent cultures (9)(10)(11)(12)(13)(14). It was shown that the most frequently amplified sequence corresponds exactly to a mitochondrial intron (15), intron a, and that most of the mutations allowing mycelia to escape senescence are rearrangements in intron a (12,(16)(17)(18). It has been known for a long time (19) that the nuclear genome controls the life-span of the fungus.…”

A Podospora anserina longevity mutant with a temperature-sensitive phenotype for senescence.

“…It has been shown by others that independent nuclear mutations, each with moderate effects on longevity, can act synergistically to produce strains with markedly increased longevities (8). It has also been demonstrated that mitochondrial rearrangements, many of which include the region from which a senDNA is derived, can significantly increase longevity (2,7,13). Therefore, the TS1 mutant may contain more than one genetic alteration, and it is possible that the continuousgrowth phenotype is separable from the temperaturesensitive phenotype.…”

“…A number of observations have supported this hypothesis. Several long-lived mutant strains, isolated as outgrowths from senescent mycelia of the s race, have been reported to lack the mitochondrial sequence from which a senDNA is derived (2,7,13). Furthermore, a subset of these mutants lack the freely replicating plasmid (26,27), as does a double nuclear mutant gr viv (23).…”