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Phosphoketolase pathway contributes to carbon metabolism in cyanobacteria

Nature Plants volume 2, Article number: 15187 (2016) Cite this article

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Abstract

Central carbon metabolism in cyanobacteria comprises the Calvin–Benson–Bassham (CBB) cycle, glycolysis, the pentose phosphate (PP) pathway and the tricarboxylic acid (TCA) cycle. Redundancy in this complex metabolic network renders the rational engineering of cyanobacterial metabolism for the generation of biomass, biofuels and chemicals a challenge. Here we report the presence of a functional phosphoketolase pathway, which splits xylulose-5-phosphate (or fructose-6-phosphate) to acetate precursor acetyl phosphate, in an engineered strain of the model cyanobacterium Synechocystis (ΔglgC/xylAB), in which glycogen synthesis is blocked, and xylose catabolism enabled through the introduction of xylose isomerase and xylulokinase. We show that this mutant strain is able to metabolise xylose to acetate on nitrogen starvation. To see whether acetate production in the mutant is linked to the activity of phosphoketolase, we disrupted a putative phosphoketolase gene (slr0453) in the ΔglgC/xylAB strain, and monitored metabolic flux using 13C labelling; acetate and 2-oxoglutarate production was reduced in the light. A metabolic flux analysis, based on isotopic data, suggests that the phosphoketolase pathway metabolises over 30% of the carbon consumed by ΔglgC/xylAB during photomixotrophic growth on xylose and CO2. Disruption of the putative phosphoketolase gene in wild-type Synechocystis also led to a deficiency in acetate production in the dark, indicative of a contribution of the phosphoketolase pathway to heterotrophic metabolism. We suggest that the phosphoketolase pathway, previously uncharacterized in photosynthetic organisms, confers flexibility in energy and carbon metabolism in cyanobacteria, and could be exploited to increase the efficiency of cyanobacterial carbon metabolism and photosynthetic productivity.

In this study, we examined the metabolic network of the model cyanobacterium Synechocystis sp. PCC 6803 (hereafter Synechocystis) in which the central carbon metabolism was engineered to enable simultaneous organic acids excretion14 and xylose catabolism2. Experimental and computational analyses of double and triple knockout/knock-in strains grown under various nutrient conditions uncovered a phosphoketolase pathway, previously uncharacterized in photosynthetic organisms. We systematically examined its function in organic acids (acetate, pyruvate and 2-oxoglutarate) excretion and estimated its contribution to the central carbon metabolism by isotope tracer experiments and metabolic modelling. Our results indicate that the phosphoketolase pathway plays a major role in the central carbon metabolism in this model cyanobacterium.

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Figure 1: The engineered central carbon metabolism in the cyanobacterium Synechocystis.
Figure 2: Identification of acetate in cell-free ΔglgC/xylAB culture medium supplemented with xylose.
Figure 3: Production of organic acids by ΔglgC/xylAB/Δslr0453 (red columns) and ΔglgC/xylAB (white columns).
Figure 4: Acetate productivity in the presence or absence of Slr0453.
Figure 5: Simulation of fractional 13C-labelling of serine from [U-13C]- and [1-13C]xylose tracer experiments.

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Acknowledgements

This work was supported by National Renewable Energy Laboratory Director's Postdoc Fellowship (to W.X.), and by the US Department of Energy (DOE), Office of Science, Basic Energy Science (to M.G., M.C., J.Y.). The latter funded in part the conception and execution of the work as well as preparation of the manuscript. It was also supported in part by the DOE Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office (to P.C.M.), BioEnergy Technologies Office (to E.G.), Science Undergraduate Laboratory Internship program (to S.R.), and a Dragon-Gate grant (to T.C.L.) from Ministry of Science and Technology in Taiwan. The authors acknowledge M. Seibert from NREL and A. Grossman from the Carnegie Institution for Science for helpful discussion. The software Metran was developed and kindly provided by Maciek R. Antoniewicz from the University of Delaware. The US Government retains and the publisher, by accepting the article for publication, acknowledges that the US Government retains a nonexclusive, paid up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for US Government purposes.

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  1. National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, 80401, Colorado, USA

    Wei Xiong, Tai-Chi Lee, Sarah Rommelfanger, Erica Gjersing, Melissa Cano, Pin-Ching Maness, Maria Ghirardi & Jianping Yu

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Contributions

W.X., P.C.M., M.G. and J.Y. planned the project; W.X., T.C.L., E.G., M.C. and S.R. performed the experiments; W.X., E.G. and J.Y. analysed the data and drafted the manuscript; all authors edited the manuscript.

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Correspondence to Jianping Yu.

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Xiong, W., Lee, TC., Rommelfanger, S. et al. Phosphoketolase pathway contributes to carbon metabolism in cyanobacteria. Nature Plants 2, 15187 (2016). https://doi.org/10.1038/nplants.2015.187

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