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 o f Podospora anserina was demonstrated to be able to integrate into the high molecular weight mitochondrial DNA (mtDNA). Hybridization analys is and densitometric evaluation of the mitochondrial genome isolated f rom cultures of different ages revealed that the mtDNA is highly stabl e during the whole life span of the mutant. In addition, and in sharp contrast to the situation in certain senescence-prone Neurospora strai ns, the mutated P. anserina mtDNA molecules containing integrated plas mid copies are not suppressive to wild-type genomes. As demonstrated b y hybridization and polymerase chain reaction (PCR) analysis, the prop ortion of mtDNA molecules affected by the integration of pAL2-1 fluctu ates between 10% and 50%. Comparative sequence analysis of free and in tegrated plasmid copies revealed four differences within the terminal inverted repeats (TIRs). These point mutations are not caused by the i ntegration event since they occur subsequent to integration and at var ious ages. Interestingly, both repeats contain identical sequences ind icating that the mechanism involved in the maintenance of perfect TIRs is active on both free and integrated plasmid copies. Finally, in rec iprocal crosses between AL2 and the wildtype strain A, some abnormal p rogeny 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-li ved 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.