Mitochondrion of oxymonads

Oxymonads are among the last larger eukaryotic groups in which mitochondrion-like organelles were not detected. Among almost one million transcriptome sequences of oxymonad Monocercomonoides we found sequences for only three proteins commonly associated with mitochondrion-like organelles - pyruvate:ferredoxin oxidoreductase (PFO), [FeFe]hydrogenase and pyridine:nucleotide transhydrogenase (PNT). Out of these tree proteins PNT is most closely associated with mitochondrion-like organelles. Although the transcriptome data set is quite large we did not detect any other mitochondrial proteins including proteins for synthesis of FeS clusters and proteins associated with mitochondrial protein import. In the phylogenetic tree, PNT from Monocercomonoides formed a clade with sequences of other eukaryotes. This suggests that PNT of Monocercomonoides is likely of mitochondrial origin; however, the possibility that Monocercomonoides acquired PNT by lateral gene transfer cannot be excluded. We tried to localize PNT in the cell of Monocercomonoides by immunofluorescent microscopy but the results were difficult to interpret. We could not prove the presence of mitochondrion-like organelle in Monocercomonoides. If there is any such organelle, it is highly probable that this organelle is very reduced because we did not find..