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Lipogenesis and redox balance in nitrogen-fixing pea bacteroids

Terpolilli, J.J.ORCID: 0000-0003-4306-3346, Masakapalli, S.K., Karunakaran, R., Webb, I.U.C., Green, R., Watmough, N.J., Kruger, N.J., Ratcliffe, R.G., Poole, P.S. and Zhulin, I.B. (2016) Lipogenesis and redox balance in nitrogen-fixing pea bacteroids. Journal of Bacteriology, 198 (20). pp. 2864-2875.

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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 [C-13] 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-beta-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.

Item Type: Journal Article
Murdoch Affiliation(s): Centre for Rhizobium Studies
Publisher: American Society for Microbiology
Copyright: © 2016, American Society for Microbiology.
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