In the filamentous fungus Podospora anserina, the association of two nuclear genes inevitably leads to a "premature death" phenotype consisting of an early end of vegetative growth a few days after ascospore germination. Mycelia showing this phenotype contain a mitochondrial chromosome that always bears the same deletion. One of the break points is exactly at the 5' splice site of a particular mitochondrial intron, suggesting that the deletion event could result from molecular mechanisms also involved in intron mobility.One of the nuclear genes involved in triggering this site-specific event belongs to the mating-type minus haplotype; the other is a mutant allele of a gene encoding a cytosolic ribosomal protein.In obligate aerobes, rearrangements of mitochondrial DNA (mtDNA) are responsible for various deleterious symptoms, such as maternally inherited male sterility in plants (1), several mycelial degenerative phenomena in fungi (2), and, as described more recently in humans, numerous neuromuscular (3) or hematological (4) diseases. In some cases, nuclear genes have been shown to control the mtDNA rearrangements (5, 6). The molecular mechanisms producing these rearrangements and the role played by nuclear-encoded proteins remain unclear in all cases.In the filamentous fungus Podospora anserina, vegetative growth is limited by a maternally inherited syndrome, called senescence, ending in cessation of mycelial elongation and apical cell death (7,8). In this species, senescence is clearly associated with mtDNA rearrangements and most probably is caused by them. 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. For instance, the life-span of our reference strains differs according to their mating type: the process of senescence is delayed in mat+ strains compared with matones. However the differences, although significant, are not striking (see Table 1). Moreover the functions encoded by these genes are still unknown.The work presented here follows the observation that, in Podospora, mutations in several genes involved in the control of translational accuracy have an effect on the life-span (A. Raynal, personal communication; Table 1). As a first step, we focused our attention on the AS1-4 mutation. This particular allele confers a very long life-span to mat+ mycelia, while, when associated with mat-, it leads systematically to the premature death phenotype, consisting of an early end of mycelial growth a few days after ascospore germination.We showed that mycelia presenting this phenotype were in all cases heteroplasmic and that they systematically contained the same ...