title: Lipogenesis and Redox Balance in Nitrogen-Fixing Pea Bacteroids.
abstract: Within legume root nodules, rhizobia differentiate into bacteroids that oxidize host-derived dicarboxylic acids, which is assumed to occur via the tricarboxylic acid (TCA) cycle to generate NAD(P)H for reduction of N2 Metabolic flux analysis of laboratory-grown Rhizobium leguminosarum showed that the flux from [(13)C]succinate was consistent with respiration of an obligate aerobe growing on a TCA cycle intermediate as the sole carbon source. However, the instability of fragile pea bacteroids prevented their steady-state labeling under N2-fixing conditions. Therefore, comparative metabolomic profiling was used to compare free-living R. leguminosarum with pea bacteroids. While the TCA cycle was shown to be essential for maximal rates of N2 fixation, levels of pyruvate (5.5-fold reduced), acetyl coenzyme A (acetyl-CoA; 50-fold reduced), free coenzyme A (33-fold reduced), and citrate (4.5-fold reduced) were much lower in bacteroids. Instead of completely oxidizing acetyl-CoA, pea bacteroids channel it into both lipid and the lipid-like polymer poly-β-hydroxybutyrate (PHB), the latter via a type III PHB synthase that is active only in bacteroids. Lipogenesis may be a fundamental requirement of the redox poise of electron donation to N2 in all legume nodules. Direct reduction by NAD(P)H of the likely electron donors for nitrogenase, such as ferredoxin, is inconsistent with their redox potentials. Instead, bacteroids must balance the production of NAD(P)H from oxidation of acetyl-CoA in the TCA cycle with its storage in PHB and lipids.
  
  
  
  Biological nitrogen fixation by symbiotic bacteria (rhizobia) in legume root nodules is an energy-expensive process. Within legume root nodules, rhizobia differentiate into bacteroids that oxidize host-derived dicarboxylic acids, which is assumed to occur via the TCA cycle to generate NAD(P)H for reduction of N2 However, direct reduction of the likely electron donors for nitrogenase, such as ferredoxin, is inconsistent with their redox potentials. Instead, bacteroids must balance oxidation of plant-derived dicarboxylates in the TCA cycle with lipid synthesis. Pea bacteroids channel acetyl-CoA into both lipid and the lipid-like polymer poly-β-hydroxybutyrate, the latter via a type II PHB synthase. Lipogenesis is likely to be a fundamental requirement of the redox poise of electron donation to N2 in all legume nodules.
authors: Centre for Rhizobium Studies, Murdoch University, Perth, Australia Department of Molecular Microbiology, John Innes Centre, Norwich, United Kingdom.
authors: Jason J
authors: JJ
authors: Terpolilli
authors: Department of Plant Sciences, University of Oxford, Oxford, United Kingdom.
authors: Shyam K
authors: SK
authors: Masakapalli
authors: Department of Molecular Microbiology, John Innes Centre, Norwich, United Kingdom.
authors: Ramakrishnan
authors: R
authors: Karunakaran
authors: Department of Molecular Microbiology, John Innes Centre, Norwich, United Kingdom Department of Plant Sciences, University of Oxford, Oxford, United Kingdom.
authors: Isabel U C
authors: IU
authors: Webb
authors: Department of Molecular Microbiology, John Innes Centre, Norwich, United Kingdom.
authors: Rob
authors: R
authors: Green
authors: Centre for Molecular Structure and Biochemistry, University of East Anglia, Norwich, United Kingdom.
authors: Nicholas J
authors: NJ
authors: Watmough
authors: Department of Plant Sciences, University of Oxford, Oxford, United Kingdom.
authors: Nicholas J
authors: NJ
authors: Kruger
authors: Department of Plant Sciences, University of Oxford, Oxford, United Kingdom.
authors: R George
authors: RG
authors: Ratcliffe
authors: Department of Molecular Microbiology, John Innes Centre, Norwich, United Kingdom Department of Plant Sciences, University of Oxford, Oxford, United Kingdom philip.poole@plants.ox.ac.uk.
authors: Philip S
authors: PS
authors: Poole
mesh_terms: D000105:Acetyl Coenzyme A / Q000378:metabolism; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D002244:Carbon / Q000378:metabolism; D002952:Citric Acid Cycle; D006885:Hydroxybutyrates / Q000378:metabolism; D050155:Lipogenesis; D009584:Nitrogen / Q000378:metabolism*; D010084:Oxidation-Reduction; D018532:Pisum sativum / Q000382:microbiology* / Q000502:physiology; D011091:Polyesters / Q000378:metabolism; D019289:Pyruvic Acid / Q000378:metabolism; D016961:Rhizobium leguminosarum / Q000235:genetics / Q000378:metabolism*; D013559:Symbiosis; D000094122:Polyhydroxybutyrates
chemical_list: D000105:Acetyl Coenzyme A; D001426:Bacterial Proteins; D002244:Carbon; D006885:Hydroxybutyrates; D009584:Nitrogen; D011091:Polyesters; D019289:Pyruvic Acid; C003182:poly-beta-hydroxybutyrate; D000094122:Polyhydroxybutyrates