Increased Ethylene Production by Overexpressing Phosphoenolpyruvate Carboxylase in the Cyanobacterium Synechocystis PCC 6803

Background: Cyanobacteria can be metabolically engineered to convert CO₂ to fuels and chemicals such as ethylene. A major challenge in such efforts is to optimize carbon fixation and partition towards target molecules. Results: The efe gene encoding an ethylene-forming enzyme was introduced into a strain of the cyanobacterium Synechocystis PCC 6803 with increased phosphoenolpyruvate carboxylase (PEPc) levels. The resulting engineered strain (CD-P) showed significantly increased ethylene production (10.5 ± 3.1 μg mL^-1 OD^-1 day^-1) compared to the control strain (6.4 ± 1.4 μg mL^-1 OD^-1 day^-1). Interestingly, extra copies of the native pepc or the heterologous expression of PEPc from the cyanobacterium Synechococcus PCC 7002 (Synechococcus) in the CD-P, increased ethylene production (19.2 ± 1.3 and 18.3 ± 3.3 μg mL^-1 OD^-1 day^-1, respectively) when the cells were treated with the acetyl-CoA carboxylase inhibitor, cycloxydim. A heterologous expression of phosphoenolpyruvate synthase (PPSA) from Synechococcus in the CD-P also increased ethylene production (16.77 ± 4.48 μg mL^-1 OD^-1 day^-1) showing differences in the regulation of the native and the PPSA from Synechococcus in Synechocystis. Conclusions: This work demonstrates that genetic rewiring of cyanobacterial central carbon metabolism can enhance carbon supply to the TCA cycle and thereby further increase ethylene production.