Iso-Propanol Production By Engineered Cyanobacteria, Synechococcus Elongatus pcc 7942
Cyanobacteria converting carbon dioxide into organic compounds using solar energy is an attractive host for bio production. The introduction of synthetic metabolic pathway composed of multiple genes derived from other organisms enables some cyanobacteria to produce valuable chemicals and fuels. In this study, we used Synechococcus elongatus PCC 7942 as host strain and firstly accomplished iso-propanol production from carbon dioxide by engineered strain.
A synthetic metabolic pathway composed of acetyl-CoA acetyl transferase (encoded by thl), acetoacetyl-CoA transferase (encoded by atoAD), acetoacetate decarboxylase (encoded by adc), and secondary alcohol dehydrogenase (encoded by sadh) was introduced into S. elongatus PCC 7942. The extrinsic acetate, precursor of acetyl-CoA, enabled the constructed strain to produce iso-propanol, indicating that the introduced pathway is functional in S. elongatus PCC 7942. Although no iso-propanol was produced under light condition, iso-propanol and acetate were detected under dark and anaerobic conditions. Therefore, two-phase incubation was applied for iso-propanol production. At first, the constructed strain was grown under light condition (growth phase). Next, the grown culture was transferred to dark and anaerobic conditions (production phase). The culture at early stationary phase in growth phase was optimal for iso-propanol production with acetate accumulation. The shifting dark and anaerobic to light condition during production phase dramatically increased iso-propanol production by conversion to iso-propanol from acetate. After optimization of the conditions, 2.42 mM (146 mg/l) of iso-propanol was produced.
The transition to dark and anaerobic conditions is complicate process and not preferable for practical application. It was expected that the iso-propanol production under light condition is accomplished by self-production of acetate. The introduction of phosphate acetyltransferase (encoded by pta) catalyzing reversible conversion between acetyl-CoA and acetyl phosphate led S. elongatus PCC 7942 to produce acetate after stationary phase. To prevent produced iso-propanol evaporation, the strain introduced both a synthetic metabolic pathway and pta was incubated in tight-capped flask with addition of NaHCO3. Under these conditions, the constructed strain successfully produced 0.55 mM (33.1 mg/l) of iso-propanol. The enforcement of metabolic flux to acetyl-CoA would be necessary for more improvement of productivity.