02676nas a2200313   4500008004100000245005400041210005100095300000900146490000800155520141200163653011201575653025901687100001401946700002101960700002601981700002102007700001502028700001802043700002102061700003202082700002002114700002602134700002002160700001802180700001702198700001902215700002202234856010602256       2005                        eng d00aAn anaerobic mitochondrion that produces hydrogen0 aanaerobic mitochondrion that produces hydrogen  a74-90 v4343 aHydrogenosomes are organelles that produce ATP and hydrogen, and are found in various unrelated eukaryotes, such as anaerobic flagellates, chytridiomycete fungi and ciliates. Although all of these organelles generate hydrogen, the hydrogenosomes from these organisms are structurally and metabolically quite different, just like mitochondria where large differences also exist. These differences have led to a continuing debate about the evolutionary origin of hydrogenosomes. Here we show that the hydrogenosomes of the anaerobic ciliate Nyctotherus ovalis, which thrives in the hindgut of cockroaches, have retained a rudimentary genome encoding components of a mitochondrial electron transport chain. Phylogenetic analyses reveal that those proteins cluster with their homologues from aerobic ciliates. In addition, several nucleus-encoded components of the mitochondrial proteome, such as pyruvate dehydrogenase and complex II, were identified. The N. ovalis hydrogenosome is sensitive to inhibitors of mitochondrial complex I and produces succinate as a major metabolic end product–biochemical traits typical of anaerobic mitochondria. The production of hydrogen, together with the presence of a genome encoding respiratory chain components, and biochemical features characteristic of anaerobic mitochondria, identify the N. ovalis organelle as a missing link between mitochondria and hydrogenosomes.10a*Anaerobiosis Animals Ciliophora/*cytology/genetics/*metabolism/ultrastructure Cockroaches/parasitology DNA10aMitochondrial/genetics Electron Transport Electron Transport Complex I/antagonists & inhibitors/metabolism Genome Glucose/metabolism Hydrogen/*metabolism Mitochondria/enzymology/genetics/*metabolism/ultrastructure Molecular Sequence Data Open Reading Fra1 aBoxma, B.1 ade Graaf, R., M.1 avan der Staay, G., W.1 avan Alen, T., A.1 aRicard, G.1 aGabaldón, T.1 avan Hoek, A., H.1 avan der Staay, S., Y. Moon-1 aKoopman, W., J.1 avan Hellemond, J., J.1 aTielens, A., G.1 aFriedrich, T.1 aVeenhuis, M.1 aHuynen, M., A.1 aHackstein, J., H.  uhttp://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Citation&list_uids=15744302