RT Journal Article SR Electronic T1 A functional bacterial-derived restriction modification system in the mitochondrion of a heterotrophic protist JF bioRxiv FD Cold Spring Harbor Laboratory SP 2021.02.01.429123 DO 10.1101/2021.02.01.429123 A1 Milner, David S. A1 Wideman, Jeremy G. A1 Stairs, Courtney W. A1 Dunn, Cory D. A1 Richards, Thomas A. YR 2021 UL http://biorxiv.org/content/early/2021/02/01/2021.02.01.429123.abstract AB The overarching trend in mitochondrial evolution is functional streamlining coupled with gene loss; therefore, gene acquisition by mitochondria is considered to be exceedingly rare. Selfish elements in the form of self-splicing introns occur in many organellar genomes, but the wider diversity of selfish elements, and how they persist in organellar genomes, has not been explored. In the mitochondrial genome of a marine heterotrophic katablepharid protist, we identify a functional type II restriction modification system originating from a horizontal gene transfer event involving bacteria related to flavobacteria. This restriction modification system consists of an HpaII-like endonuclease and a cognate cytosine methyltransferase. We demonstrate that these proteins are functional by heterologous expression in both bacterial and eukaryotic cells. These results suggest that toxin-antitoxin selfish elements, such as restriction modification systems, could be co-opted by eukaryotic genomes to drive uniparental organellar inheritance.