title: Hydrogen formation and its regulation in Ruminococcus albus: involvement of an electron-bifurcating [FeFe]-hydrogenase, of a non-electron-bifurcating [FeFe]-hydrogenase, and of a putative hydrogen-sensing [FeFe]-hydrogenase.
abstract: Ruminococcus albus 7 has played a key role in the development of the concept of interspecies hydrogen transfer. The rumen bacterium ferments glucose to 1.3 acetate, 0.7 ethanol, 2 CO2, and 2.6 H2 when growing in batch culture and to 2 acetate, 2 CO2, and 4 H2 when growing in continuous culture in syntrophic association with H2-consuming microorganisms that keep the H2 partial pressure low. The organism uses NAD(+) and ferredoxin for glucose oxidation to acetyl coenzyme A (acetyl-CoA) and CO2, NADH for the reduction of acetyl-CoA to ethanol, and NADH and reduced ferredoxin for the reduction of protons to H2. Of all the enzymes involved, only the enzyme catalyzing the formation of H2 from NADH remained unknown. Here, we report that R. albus 7 grown in batch culture on glucose contained, besides a ferredoxin-dependent [FeFe]-hydrogenase (HydA2), a ferredoxin- and NAD-dependent electron-bifurcating [FeFe]-hydrogenase (HydABC) that couples the endergonic formation of H2 from NADH to the exergonic formation of H2 from reduced ferredoxin. Interestingly, hydA2 is adjacent to the hydS gene, which is predicted to encode an [FeFe]-hydrogenase with a C-terminal PAS domain. We showed that hydS and hydA2 are part of a larger transcriptional unit also harboring putative genes for a bifunctional acetaldehyde/ethanol dehydrogenase (Aad), serine/threonine protein kinase, serine/threonine protein phosphatase, and a redox-sensing transcriptional repressor. Since HydA2 and Aad are required only when R. albus grows at high H2 partial pressures, HydS could be a H2-sensing [FeFe]-hydrogenase involved in the regulation of their biosynthesis.
authors.affiliation: Max Planck Institute for Terrestrial Microbiology, Marburg, Germany.
authors.forename: Yanning
authors.initials: Y
authors.lastname: Zheng
authors.affiliation: Max Planck Institute for Terrestrial Microbiology, Marburg, Germany.
authors.forename: Jörg
authors.initials: J
authors.lastname: Kahnt
authors.affiliation: Department of Animal Sciences, Institute for Genomic Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA.
authors.forename: In Hyuk
authors.initials: IH
authors.lastname: Kwon
authors.affiliation: Department of Animal Sciences, Institute for Genomic Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA.
authors.forename: Roderick I
authors.initials: RI
authors.lastname: Mackie
authors.affiliation: Max Planck Institute for Terrestrial Microbiology, Marburg, Germany thauer@mpi-marburg.mpg.de.
authors.forename: Rudolf K
authors.initials: RK
authors.lastname: Thauer
mesh_terms: D000105:Acetyl Coenzyme A / Q000378:metabolism; D000445:Aldehyde Oxidoreductases / Q000235:genetics / Q000378:metabolism; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D004579:Electron Transport; D005285:Fermentation; D005560:Formate Dehydrogenases / Q000235:genetics / Q000378:metabolism*; D015964:Gene Expression Regulation, Bacterial / Q000502:physiology*; D015971:Gene Expression Regulation, Enzymologic / Q000502:physiology; D005947:Glucose / Q000378:metabolism; D006859:Hydrogen / Q000378:metabolism*; D006864:Hydrogenase / Q000378:metabolism*; D007501:Iron / Q000378:metabolism; D009243:NAD; D009249:NADP / Q000378:metabolism; D050743:Pyruvate Synthase / Q000235:genetics / Q000378:metabolism; D045857:Ruminococcus / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D009243:NAD; D009249:NADP; D000105:Acetyl Coenzyme A; D006859:Hydrogen; D007501:Iron; D006864:Hydrogenase; D005560:Formate Dehydrogenases; D000445:Aldehyde Oxidoreductases; D050743:Pyruvate Synthase; D005947:Glucose