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the mitochondrial iron-sulfur cluster assembly pathway, thought to be conserved in virtually all eukaryotic cells, has been replaced by a cytosolic sulfur mobilization system (SUF) acquired by lateral gene transfer from bacteria.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.

The discovery that the protist Monocercomonoides exilis completely lacks mitochondria demonstrates that these organelles are not absolutely essential to eukaryotic cells. However, the degree to which the metabolism and cellular systems of this organism have adapted to the loss of mitochondria is unknown. Here, we report an extensive analysis of the M. exilis genome to address this question. Unexpectedly, we find that M. exilis genome structure and content is similar in complexity to other eukaryotes and less “reduced” than genomes of some other protists from the Metamonada group to which it belongs. Furthermore, the predicted cytoskeletal systems, the organization of endomembrane systems, and biosynthetic pathways also display canonical eukaryotic complexity. The only apparent preadaptation that permitted the loss of mitochondria was the acquisition of the SUF system for Fe–S cluster assembly and the loss of glycine cleavage system. Changes in other systems, including in amino acid metabolism and oxidative stress response, were coincident with the loss of mitochondria but are likely adaptations to the microaerophilic and endobiotic niche rather than the mitochondrial loss per se. Apart from the lack of mitochondria and peroxisomes, we show that M. exilis is a fully elaborated eukaryotic cell that is a promising model system in which eukaryotic cell biology can be investigated in the absence of mitochondria.

All eukaryotic organisms contain mitochondria or organelles that evolved from the same endosymbiotic event like classical mitochondria. Organisms inhabiting low oxygen environments often contain mitochondrial derivates known as hydrogenosomes, mitosomes or neutrally as mitochondrion-like organelles. The detailed investigation has shown unexpected evolutionary plasticity in the biochemistry and protein composition of these organelles in various protists. We investigated the mitochondrion-like organelle in Trimastix pyriformis, a free-living member of one of the three lineages of anaerobic group Metamonada. Using 454 sequencing we have obtained 7 037 contigs from its transcriptome and on the basis of sequence homology and presence of N-terminal extensions we have selected contigs coding for proteins that putatively function in the organelle. Together with the results of a previous transcriptome survey, the list now consists of 23 proteins – mostly enzymes involved in amino acid metabolism, transporters and maturases of proteins and transporters of metabolites. We have no evidence of the production of ATP in the mitochondrion-like organelle of Trimastix but we have obtained experimental evidence for the presence of enzymes of the glycine cleavage system (GCS), which is part of amino acid metabolism. Using homologous antibody we have shown that H-protein of GCS localizes into vesicles in the cell of Trimastix. When overexpressed in yeast, H- and P-protein of GCS and cpn60 were transported into mitochondrion. In case of H-protein we have demonstrated that the first 16 amino acids are necessary for this transport. Glycine cleavage system is at the moment the only experimentally localized pathway in the mitochondrial derivate of Trimastix pyriformis.

The oxymonad Monocercomonoides exilis was recently reported to be the first eukaryote that has completely lost the mitochondrial compartment. It was proposed that an important prerequisite for such a radical evolutionary step was the acquisition of the SUF Fe–S cluster assembly pathway from prokaryotes, making the mitochondrial ISC pathway dispensable. We have investigated genomic and transcriptomic data from six oxymonad species and their relatives, composing the group Preaxostyla (Metamonada, Excavata), for the presence and absence of enzymes involved in Fe–S cluster biosynthesis. None possesses enzymes of mitochondrial ISC pathway and all apparently possess the SUF pathway, composed of SufB, C, D, S, and U proteins, altogether suggesting that the transition from ISC to SUF preceded their last common ancestor. Interestingly, we observed that SufDSU were fused in all three oxymonad genomes, and in the genome of Paratrimastix pyriformis. The donor of the SUF genes is not clear from phylogenetic analyses, but the enzyme composition of the pathway and the presence of SufDSU fusion suggests Firmicutes, Thermotogae, Spirochaetes, Proteobacteria, or Chloroflexi as donors. The inventory of the downstream CIA pathway enzymes is consistent with that of closely related species that retain ISC, indicating that the switch from ISC to SUF did not markedly affect the downstream process of maturation of cytosolic and nuclear Fe–S proteins.

Until recently, mitochondria were considered essential organelles impossible to truly lose in a lineage. This view changed in 2016, with the report that the oxymonadMonocercomonoides exilis, was the first known eukaryote without any mitochondrion. Questions remain, however, about whether this extends to the entire lineage and how this transition took place. Oxymonadida are a group of gut endobionts of insects, reptiles, and mammals. They are housed in the Preaxostyla (Metamonada), a protistan group that also contains free-living flagellates of generaTrimastixandParatrimastix. These latter two taxa harbor conspicuous mitochondrion-related organelles (MROs), while no mitochondria were reported for any oxymonad. Here we report genomic data sets of two Preaxostyla representatives, the free-livingParatrimastix pyriformisand the oxymonadBlattamonas nauphoetae. We note thatP. pyriformispossesses a set of unique or ancestral features among metamonads or eukaryotes, e.g.,p-cresol synthesis, UFMylation system, NAD+synthesis, selenium volatilization, or mercury methylation, demonstrating the biochemical versatility of this protist lineage. We performed thorough comparisons among all available genomic and transcriptomic data of Preaxostyla to corroborate both the absence of MRO in Oxymonadida and the nature of MROs present in other Preaxostyla and to decipher the evolutionary transition towards amitochondriality and endobiosis. Our results provide insights into the metabolic and endomembrane evolution, but most strikingly the data confirm the complete loss of mitochondria and every protein that has ever participated in the mitochondrion function for all three oxymonad species (M. exilis,B. nauphoetae, andStreblomastix strix) extending the amitochondriate status to the whole Oxymonadida.

Monocercomonoides exilis is the first eukaryotic organism described as a complete amitochondriate, yet it shares common features with heterotrophic anaerobic/microaerophilic protists, some of which bear divergent mitochondrion-related organelles or MROs. It has been postulated that the retention of these organelles stems from their involvement in the assembly of essential cytosolic and nuclear FeS proteins, whose maturation requires the evolutionarily conserved mitochondrial ISC and cytosolic CIA machineries. The amitochondriate M. exilis lacks genes encoding the ISC machinery yet contains a bacteria-derived SUF system (MeSuf), composed of the cysteine desulphurase SufS fused to SufD and SufU, as well as the FeS scaffolding components MeSufB and MeSufC. Here, we show that expression of the M. exilis SUF genes, either individually or in tandem, can restore the maturation of the FeS protein IscR in the Escherichia coli double mutants of ΔsufS ΔiscS and ΔsufB ΔiscUA. In vivo and in vitro studies indicate that purified MeSufB, MeSufC and MeSufDSU proteins interact suggesting that they act as a complex in the protist. MeSufBC can undergo conformational changes in the presence of ATP and assemble FeS clusters under anaerobic conditions in presence and absence of ATP in vitro. Altogether, these results indicate that the dynamically interacting MeSufDSUBC proteins may function as an FeS cluster assembly complex in M. exilis thereby being capable of replacing the organelle-enclosed ISC system of canonical eukaryotes.

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