Hybrid mitochondrial plasmids from senescence suppressor isolates of Neurospora intermedia

We analyzed several natural suppressor isolates of the pKalilo-based fungal senescence syndrome of Neurospora intermedia. The pKalilo plasmid did not increase in titer in these isolates. Nor did it show integration "de novo." In at least two of the senescence suppressor isolates, pKalilo had formed stable recombinants with other mitochondrial elements. pKalilo/mtDNA recombination junctions were complete and appeared to have been formed via a nonhomologous recombination mechanism. Further analysis revealed that pKalilo had recombined a novel, 2.6-kb cryptic mitochondrial retroplasmid, similar to the mitochondrial retroplasmid pTHR1 from Trichoderma harzianum and retroplasmids of the "Varkud" homology group. The recombinant molecules consisted of pKalilo, the novel element, and short intervening stretches of mtDNA. The latter stretches clearly corresponded to "in vivo" mitochondrial cDNA, suggesting that the molecules had formed via the action of a template-switching reverse transcriptase. We discuss how different types of mitochondrial plasmids interact and how their detrimental effect on the host may be suppressed.