Introduction of a Synthetic CO2fixing Photorespiratory Bypass into a Cyanobacterium

Background: Photorespiration limits carbon fixation. Results: Heterologous expression and functional activity of six enzymes from the 3-hydroxypropionate bi-cycle are demonstrated in cyanobacteria. Conclusion: A synthetic CO2-fixing photorespiratory bypass can be introduced into cyanobacteria. Significance: The results lay the foundation for expressing an alternative CO2 fixation pathway in cyanobacteria, algae, and plants. Global photosynthetic productivity is limited by the enzymatic assimilation of CO2 into organic carbon compounds. Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), the carboxylating enzyme of the Calvin-Benson cycle, poorly discriminates between CO2 and O-2, leading to photorespiration and the loss of fixed carbon and nitrogen. With the advent of synthetic biology, it is now feasible to design, synthesize, and introduce biochemical pathways in vivo. We engineered a synthetic photorespiratory bypass based on the 3-hydroxypropionate bi-cycle into the model cyanobacterium, Synechococcus elongatus sp. PCC 7942. The heterologously expressed cycle is designed to function as both a photorespiratory bypass and an additional CO2-fixing pathway, supplementing the Calvin-Benson cycle. We demonstrate the function of all six introduced enzymes and identify bottlenecks to be targeted in subsequent bioengineering. These results have implications for efforts to improve photosynthesis and for the green production of high value products of biotechnological interest.