Mitochondrial plasmids of Neurospora: integration into mitochondrial DNA and evidence for reverse transcription in mitochondria.

The Mauriceville (3.6 kb) and Varkud (3.8 kb) mitochondrial plasmids of Neurospora are closely related, closed circular DNAs whose nucleotide sequences and genetic organization suggest relationships to mitochondrial introns and retrotransposons. Here we isolated mutants whose growth is impaired as a result of malevolent behavior of these plasmids. All 12 mutants contain variant plasmids that are suppressive relative to mtDNA, and ten also contain defective mtDNAs. All the suppressive plasmids contain small insertions, generally including a mitochondrial tRNA sequence, at or near the major 5' RNA start site. The structure of the suppressive plasmids suggests that they were generated via an RNA intermediate and a reverse transcription step. At least three of the mutants contain defective mtDNAs into which mitochondrial plasmid sequences have integrated. Sequences at the plasmid-mtDNA junctions are also consistent with integration via an RNA intermediate.