title: Different Functions of Phylogenetically Distinct Bacterial Complex I Isozymes.
abstract: NADH:quinone oxidoreductase (complex I) is a bioenergetic enzyme that transfers electrons from NADH to quinone, conserving the energy of this reaction by contributing to the proton motive force. While the importance of NADH oxidation to mitochondrial aerobic respiration is well documented, the contribution of complex I to bacterial electron transport chains has been tested in only a few species. Here, we analyze the function of two phylogenetically distinct complex I isozymes in Rhodobacter sphaeroides, an alphaproteobacterium that contains well-characterized electron transport chains. We found that R. sphaeroides complex I activity is important for aerobic respiration and required for anaerobic dimethyl sulfoxide (DMSO) respiration (in the absence of light), photoautotrophic growth, and photoheterotrophic growth (in the absence of an external electron acceptor). Our data also provide insight into the functions of the phylogenetically distinct R. sphaeroidescomplex I enzymes (complex IA and complex IE) in maintaining a cellular redox state during photoheterotrophic growth. We propose that the function of each isozyme during photoheterotrophic growth is either NADH synthesis (complex IA) or NADH oxidation (complex IE). The canonical alphaproteobacterial complex I isozyme (complex IA) was also shown to be important for routing electrons to nitrogenase-mediated H2 production, while the horizontally acquired enzyme (complex IE) was dispensable in this process. Unlike the singular role of complex I in mitochondria, we predict that the phylogenetically distinct complex I enzymes found across bacterial species have evolved to enhance the functions of their respective electron transport chains.
  
  
  OBJECTIVE
  Cells use a proton motive force (PMF), NADH, and ATP to support numerous processes. In mitochondria, complex I uses NADH oxidation to generate a PMF, which can drive ATP synthesis. This study analyzed the function of complex I in bacteria, which contain more-diverse and more-flexible electron transport chains than mitochondria. We tested complex I function in Rhodobacter sphaeroides, a bacterium predicted to encode two phylogenetically distinct complex I isozymes. R. sphaeroides cells lacking both isozymes had growth defects during all tested modes of growth, illustrating the important function of this enzyme under diverse conditions. We conclude that the two isozymes are not functionally redundant and predict that phylogenetically distinct complex I enzymes have evolved to support the diverse lifestyles of bacteria.
authors.affiliation: Department of Bacteriology, University of Wisconsin-Madison, Madison, Wisconsin, USA Microbiology Doctoral Training Program, University of Wisconsin-Madison, Madison, Wisconsin, USA Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, Wisconsin, USA.
authors.forename: Melanie A
authors.initials: MA
authors.lastname: Spero
authors.affiliation: Department of Bacteriology, University of Wisconsin-Madison, Madison, Wisconsin, USA.
authors.forename: Joshua R
authors.initials: JR
authors.lastname: Brickner
authors.affiliation: Department of Bacteriology, University of Wisconsin-Madison, Madison, Wisconsin, USA Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, Wisconsin, USA.
authors.forename: Jordan T
authors.initials: JT
authors.lastname: Mollet
authors.affiliation: Department of Bacteriology, University of Wisconsin-Madison, Madison, Wisconsin, USA Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, Wisconsin, USA Graduate Program in Cellular and Molecular Biology, University of Wisconsin-Madison, Madison, Wisconsin, USA.
authors.forename: Tippapha
authors.initials: T
authors.lastname: Pisithkul
authors.affiliation: Department of Bacteriology, University of Wisconsin-Madison, Madison, Wisconsin, USA Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, Wisconsin, USA.
authors.forename: Daniel
authors.initials: D
authors.lastname: Amador-Noguez
authors.affiliation: Department of Bacteriology, University of Wisconsin-Madison, Madison, Wisconsin, USA Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, Wisconsin, USA tdonohue@bact.wisc.edu.
authors.forename: Timothy J
authors.identifier: http://orcid.org/0000-0001-8738-2467
authors.initials: TJ
authors.lastname: Donohue
mesh_terms: D000693:Anaerobiosis; D015964:Gene Expression Regulation, Bacterial / Q000502:physiology*; D015971:Gene Expression Regulation, Enzymologic / Q000502:physiology*; D006859:Hydrogen / Q000378:metabolism; D011808:Quinone Reductases / Q000235:genetics / Q000378:metabolism*; D012242:Rhodobacter sphaeroides / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism
chemical_list: D006859:Hydrogen; C041831:NADH dehydrogenase (quinone); D011808:Quinone Reductases
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