Transcription of a mitochondrial plasmid during senescence in Podospora anserina.

In the ascomycete fungus Podospora anserina, cellular senescence is characterized by the excision, circularization, and amplification of specific segments of the non-senescent mitochondrial genome. During senescence, various plasmids can be found in the mitochondria, and different senescent events produce different plasmid populations. In this paper we have examined the transcriptional activity of one mitochondrial plasmid (α-sen DNA) and have contrasted this with the non-senescent mitochondrial genome of rapidly (A(+)) and slowly (s(+)) senescing races. In non-senescent and senescent mitochondria we observe two RNAs which are homologous to α-sen DNA and to the parental locus on the native genome. These are 2.4 and 2.5 kb long and have different 5' ends while overlapping throughout most of their lengths. They may represent different transcripts for α-sen DNA and the parental genome and indicate that excision of the plasmid begins 450 bp from the 5' end of the genomic coding sequence. Transcription of the α-sen DNA plasmid appears to be active in both senescent and in non-senescent mycelia.