The presence of mitochondria and related organelles in every studied eukaryote supports the view that mitochondria are essential cellular components. Here, we report the genome sequence of a microbial eukaryote, the oxymonad Monocercomonoides sp., which revealed that this organism lacks all hallmark mitochondrial proteins. Crucially, the mitochondrial iron-sulfur cluster assembly pathway, thought to be conserved in virtually all eukaryotic cells, has been replaced by a cytosolic sulfur mobilization system (SUF) acquired by lateral gene transfer from bacteria. In the context of eukaryotic phylogeny, our data suggest that Monocercomonoides is not primitively amitochondrial but has lost the mitochondrion secondarily. This is the first example of a eukaryote lacking any form of a mitochondrion, demonstrating that this organelle is not absolutely essential for the viability of a eukaryotic cell.

Department of Biochemistry and Molecular Biology Dalhousie University Halifax NS B3H 4R2 Canada

Department of Biology and Ecology University of Ostrava Ostrava 710 00 Czech Republic

Department of Cell Biology University of Alberta Edmonton AB T6G 2H7 Canada

Department of Parasitology Charles University Prague Prague 12843 Czech Republic

Department of Parasitology Charles University Prague Prague 12843 Czech Republic; Department of Molecular Phylogenetics and Evolution University of Warsaw Warsaw 00478 Poland

Institute of Molecular Genetics Academy of Sciences of the Czech Republic Prague 14220 Czech Republic

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Curr Biol. 2016 May 23;26(10):R410-2. doi: 10.1016/j.cub.2016.04.032. PubMed

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