title: Phosphoketolase pathway contributes to carbon metabolism in cyanobacteria.
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 (13)C 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.
authors.affiliation: National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, USA.
authors.forename: Wei
authors.initials: W
authors.lastname: Xiong
authors.affiliation: National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, USA.
authors.forename: Tai-Chi
authors.initials: TC
authors.lastname: Lee
authors.affiliation: National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, USA.
authors.forename: Sarah
authors.initials: S
authors.lastname: Rommelfanger
authors.affiliation: National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, USA.
authors.forename: Erica
authors.initials: E
authors.lastname: Gjersing
authors.affiliation: National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, USA.
authors.forename: Melissa
authors.initials: M
authors.lastname: Cano
authors.affiliation: National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, USA.
authors.forename: Pin-Ching
authors.initials: PC
authors.lastname: Maness
authors.affiliation: National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, USA.
authors.forename: Maria
authors.initials: M
authors.lastname: Ghirardi
authors.affiliation: National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, USA.
authors.forename: Jianping
authors.initials: J
authors.lastname: Yu
mesh_terms: D000085:Acetates / Q000378:metabolism; D000446:Aldehyde-Lyases / Q000378:metabolism*; D019747:Aldose-Ketose Isomerases / Q000235:genetics / Q000378:metabolism; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D002244:Carbon / Q000378:metabolism*; D005816:Genetic Complementation Test; D052836:Heterotrophic Processes; D007656:Ketoglutaric Acids / Q000378:metabolism; D009584:Nitrogen / Q000378:metabolism; D010428:Pentosephosphates / Q000378:metabolism; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000235:genetics / Q000378:metabolism; D046939:Synechocystis / Q000235:genetics / Q000378:metabolism*; D014994:Xylose / Q000378:metabolism
chemical_list: D000085:Acetates; D001426:Bacterial Proteins; D007656:Ketoglutaric Acids; D010428:Pentosephosphates; C031625:xylulose-5-phosphate; D002244:Carbon; D014994:Xylose; D017853:Phosphotransferases (Alcohol Group Acceptor); C032700:xylulokinase; D000446:Aldehyde-Lyases; C022367:phosphoketolase; D019747:Aldose-Ketose Isomerases; C023298:xylose isomerase; D009584:Nitrogen