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introns

Spliceosomal introns that need additional splicing machinery in order to be cut out are characteristic of eukaryotes. Mitochondria do not possess the genes for splicing machinery in their genome and also not contain spliceosomal introns, but they do contain other types of introns that are self-splicing. It is therefore interesting to see whether the mitochondrial genome contains introns of what type and at which position. The nuclear genes that enocode mitochondrial proteins often contain spliceosomal introns, which is expected when mitochondrial genes are derived from nuclear genes but lost their introns at transferral.

  • In plants, many group I mitochondrial introns are found at exactly the same position as their fungal counterpart (Cox1 essential gene, see here), in line with a nuclear origin of the mitochondrial DNA followed by a loss in most lineages. This is actually quite strong evidence for the nuclear origin since in my opinion it is unlikely that 1000s of introns have found their way towards specific insertion sites in another organelle.
  • In Chlamydomonas, the mitochondrial genome lacks two important genes, cox2 and cox3, which are instead encoded by the nuclear genome. These genes contain multiple eukaryotic spliceosomal introns as well as a mitochondrial targeting sequence and could therefore represent the original, premitochondrial genes that were later transferred to the mitochondria (here). A subunit (#6) of the ATPase in Chlamydomonas is also nuclear-encoded and contains spliceosomal introns and a mitochondrial target sequence (here).
  • By comparing nuclear-encoded (!) cytoplasmic ribosomal proteins (CRPs) and mitochondrial ribosomal proteins (MRPs), it was show that both proteins contained conserved intron positions, indicative for their common descent and eukaryotic origin (here). The CRPs and MRPs amongst themselves share also many intron positions, showing that the mitochondrial proteins that are nuclear-encoded show a common origin between all eukaryotes (here).