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I am studying the mitochondrial genome and have read that some contain introns. However, these introns code for proteins. I cannot really understand this. Could someone tell me what intron in which mitochondria genome this refers to?

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  • $\begingroup$ Can you please share the exact sentence and the reference to the paper/textbook mentioning this? I tend to think that introns are by definition the portion of genes that does not cod for proteins (although some exception may apply, such as intron retention). I would therefore be curios to investigate the issue. $\endgroup$ May 28 '20 at 9:22
  • $\begingroup$ unfortunately my references are in Greek, so not helpful I guess. if i translate it says sth like that: ORFs inside introns of group I and II. ORFs usually encode for GIY-YIG or LAGLI-DADG endonucleases $\endgroup$
    – marilu
    May 28 '20 at 9:45
  • $\begingroup$ Pehaps the mitochondrial proteins are encoded (with introns) in the nuclear genome, and then transported to the mitochondria? $\endgroup$ May 28 '20 at 12:49
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Mitochondrial genomes differ greatly in size, coding potential and even whether they are circular or linear. Mammalian mitochondrial DNA is small (11–28 kbp) and intronless. However the mitochondria of certain other organisms range up to 1000 kbp in size.

Certain sponges (demosponges) with large mitochondrial genomes contain type I introns and type II introns. Although introns were initially thought to have no other function than to separate exons, the introns of certain nuclear genes have been found themselves to contain genes. This turns out also to be true of some of the mitochondrial introns. To quote from then introduction to a paper I found through an internet search:

Most Group I introns encode homing endonuclease genes (HEG) and/or maturase of the LAGIDADG † family, while most Group II introns encode a reverse transcriptase (RT).

These introns are found in genes such as that encoding cytochrome oxidase subunit 1 (COI).


† This appears to be a misprint for the LAGLIDAD mutases, which are also endonucleases, and are involved in splicing the introns in which they reside.

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