Lipogenesis and redox balance in nitrogen-fixing pea bacteroids

Terpolilli, J.J., 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.

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 [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:	Centre for Rhizobium Studies
Publisher:	American Society for Microbiology
Copyright:	© 2016, American Society for Microbiology.
URI:	http://researchrepository.murdoch.edu.au/id/eprint/34143

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