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In the ascomycete Podospora anserina strain ageing (senescence) is caused by a mitochondrial plasmid. In juvenile mycelia it is an integral part of the mtDNA and becomes liberated during ageing. The nucleotide sequence of this plasmid and of its flanking regions was determined. It consists of 2,539 by and contains an un identified reading frame (URF) originating in the adjacent mtDNA upstream of excision point 1. Within the URF a putative 48 by autonomously replicating sequence (ars) was identified. At both excision sites of the plasmid there are two short nonidentical interrupted palindromes and a few base pairs apart from these palindromes, both upstream and downstream, two short inverted repeats are localised. The experimental data make it evident that the mt plasmid is an intron of the cytochrome c oxidase gene (subunit I) which may be excised at the DNA level and thus become the mobile infective agent causing senescence. The concept of this mobile intron and current hypotheses concerning the relationship between introns and transposons are stressed.

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The existence of plasmid like DNA was demonstrated in senescent mycelia of Podospora anserina (strain s) by biophysical and electronmicroscopic methods. According to their contour length of about 1.4 and 2.7 micron respectively the molecular weight for the monomer is in the range of 3.10(6).

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