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Iron hydrogenases and the evolution of anaerobic eukaryotes



Iron hydrogenases and the evolution of anaerobic eukaryotes



Molecular Biology and Evolution 17(11): 1695-1709



Hydrogenases, oxygen-sensitive enzymes that can make hydrogen gas, are key to the function of hydrogen-producing organelles (hydrogenosomes), which occur in anaerobic protozoa scattered throughout the eukaryotic tree. Hydrogenases also play a central role in the hydrogen and syntrophic hypotheses for eukaryogenesis. Here, we show that sequences related to iron-only hydrogenases ((Fe) hydrogenases) are more widely distributed among eukaryotes than reports of hydrogen production have suggested. Genes encoding small proteins which contain conserved structural features unique to (Fe) hydrogenases were identified on all well-surveyed aerobic eukaryote genomes. Longer sequences encoding (Fe) hydrogenases also occur in the anaerobic eukaryotes Entamoeba histolytica and Spironucleus barkhanus, both of which lack hydrogenosomes. We also identified a new (Fe) hydrogenase sequence from Trichomonas vaginalis, bringing the total of (Fe) hydrogenases reported for this organism to three, all of which may function within its hydrogenosomes. Phylogenetic analysis and hypothesis testing using likelihood ratio tests and parametric bootstrapping suggest that the (Fe) hydrogenases in anaerobic eukaryotes are not monophyletic. Iron-only hydrogenases from Entamoeba, Spironucleus, and Trichomonas are plausibly monophyletic, consistent with the hypothesis that a gene for (Fe) hydrogenase was already present on the genome of the common, perhaps also anaerobic, ancestor of these phylogenetically distinct eukaryotes. Trees where the (Fe) hydrogenase from the hydrogenosomal ciliate Nyctotherus was constrained to be monophyletic with the other eukaryote sequences were rejected using a likelihood ratio test of monophyly. In most analyses, the Nyctotherus sequence formed a sister group with a (Fe) hydrogenase on the genome of the eubacterium Desulfovibrio vulgaris. Thus, it is possible that Nyctotherus obtained its hydrogenosomal (Fe) hydrogenase from a different source from Trichomonas for its hydrogenosomes. We find no support for the hypothesis that components of the Nyctotherus (Fe) hydrogenase fusion protein derive from the mitochondrial respiratory chain.

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Accession: 010885215

Download citation: RISBibTeXText

PMID: 11070057

DOI: 10.1093/oxfordjournals.molbev.a026268



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