Mitochondria are thought to have originated as free-living prokaryotes. Mitochondria organelles have small circular genomes with substantial structural and genetic similarity to bacteria. Contrary to the prevailing concept of intronless mitochondria, here we present evidence that mitochondrial RNA transcripts (mtRNA) are not limited to policystronic molecules, but also processed as nuclei-like transcripts that are differentially spliced and expressed in a cell-type specific manner. The presence of canonical splice sites in the mtRNA introns and of core components of the nuclei-encoded spliceosome machinery within the mitochondrial organelle suggest that nuclei-encoded spliceosome can mediate splicing of mtRNA.
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