The 3′ Ends of Mature Transcripts Are Generated by a Processosome Complex in Fission Yeast Mitochondria

In this article, we report on the genetic analysis of the Schizosaccharomyces pombe open reading frames SPCC1322.01 and SPAC637.11, respectively, which encode proteins that are similar to the exoribonuclease Dss1p and the RNA helicase Suv3p, respectively, forming the mitochondrial degradosome of Saccharomyces cerevisiae. While the helicase Suv3p is exchangeable between S. cerevisiae and S. pombe, the functions of Dss1p and the putative fission yeast RNase protein are specific for each species. Unlike S. cerevisiae mutants lacking a functional degradosome, the major defect of fission yeast knock-out strains is their inability to perform downstream processing of transcripts. In addition, the lack of pah1 results in instability of mitochondrial RNA ends. Overexpression of par1 and pah1 has no significant effect on the steady-state levels of mitochondrial RNAs. The Pet127p-stimulated RNA degradation activity is independent of Par1p/Pah1p in fission yeast mitochondria. The results presented herein indicate that both fission yeast proteins play only a minor role (if at all) in mitochondrial RNA degradation. We assume that the RNA-degrading function was taken over by other enzymes in fission yeast mitochondria, while the former degradosome proteins were recruited to new cellular pathways, for example, RNA processing in fission yeast (as discussed in this article) or mitochondrial DNA replication, apoptosis, or chromatin maintenance in eukaryotes, during evolution.