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

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 (alpha) 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 alpha 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 alpha which allows us to identify, in senescent cultures, mitochondrial chromosomes lacking sequence alpha. These results taken together suggest that excision of intron alpha 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.