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Eukaryotic Cell doi:10.1128/EC.00251-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Hydrogen Production by Termite-Gut protists: Characterization of Iron Hydrogenases of Parabasalian Symbionts of the Termite Coptotermes formosanus

Environmental Molecular Biology Laboratory, RIKEN, Wako, Saitama 351-0198, Japan, and Division of Biotechnology, Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, Yamanashi 400-8510, Japan

^* To whom correspondence should be addressed. Email: mohkuma{at}riken.jp.

	Abstract

Cellulolytic flagellated protists in the gut of termites produce molecular hydrogen (H₂) that is emitted by the termites; however, little is known about physiology and biochemistry of H₂ production from cellulose in the gut symbiotic protists due to their formidable unculturability. In order to understand the molecular basis for H₂ production, we here identified two genes encoding proteins homologous to iron-only hydrogenase (Fe-hydrogenase) in Pseudotrichonympha grassii, a large cellulolytic symbiont in the phylum Parabasalia, in the gut of the termite Coptotermes formosanus. The two Fe-hydrogenases were phylogenetically distinct and had different N-terminal accessory domains. The long-form protein represented a unique phylogenetic lineage among eukaryotic Fe-hydrogenases, whereas the short-form was monophyletic with those of other parabasalids. The active recombinant enzymes of these two Fe-hydrogenases were successfully obtained without their specific auxiliary maturases. Although they differed in their extent of specific activity and optimal pH, both preferentially catalyzed H₂ evolution rather than H₂ uptake. H₂ evolution of at least the short-form enzyme was still active even under high hydrogen partial pressure. H₂ evolution activity was detected in the hydrogenosomal fraction of P. grassii; however, the vigorous H₂ uptake activity of its endosymbiotic bacteria compensated the strong H₂ evolution activity of the host protists. The results suggest that termite-gut symbionts are a rich reservoir of novel Fe-hydrogenases whose property is adapted to the gut environment, and that the potential of H₂ production in termite guts had been largely underestimated.