ORCID Profile
0000-0001-8394-1575
Current Organisation
University of Oxford
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Publisher: American Association for the Advancement of Science (AAAS)
Date: 30-07-2021
Abstract: Catabolism of dicarboxylates at low oxygen drives N 2 fixation and promotes ammonia secretion in rhizobium-legume symbioses.
Publisher: American Society for Microbiology
Date: 15-10-2016
DOI: 10.1128/JB.00451-16
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 N 2 . 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 N 2 -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 N 2 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 N 2 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. IMPORTANCE 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 N 2 . 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 N 2 in all legume nodules.
Publisher: Elsevier BV
Date: 08-2012
Publisher: Springer Science and Business Media LLC
Date: 2002
Publisher: Oxford University Press (OUP)
Date: 25-10-2016
DOI: 10.1093/PCP/PCW174
Abstract: Oxygen deprivation leads to changes in mitochondrial morphology and impaired flow of reducing equivalents through the electron transport chain. The extent of these changes depends on the duration and severity of the treatment as well as on the species and cell type. Nitrate is known to ameliorate these effects in some instances, but it is possible that it is nitrite, rather than nitrate, that is the key to the mechanism. To test this, mitochondria were isolated from 21-day-old pea (Pisum sativum) roots and incubated for 90 min under normoxia or hypoxia in the presence or absence of 0.5 mM nitrite. The supply of nitrite under hypoxia led to nitric oxide production, improved mitochondrial integrity, improved energization of the inner mitochondrial membrane, increased ATP synthesis, decreased production of reactive oxygen species and decreased lipid peroxidation. It also resulted in higher levels and activities of complex I and the supercomplex I + III2. It is concluded that nitrite has an important role in maintaining mitochondrial function under hypoxia, and that it achieves this through the reduction of nitrite to nitric oxide by the mitochondrial electron transport chain.
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for R. George Ratcliffe.