Enhanced production of sucrose in the fast-growing cyanobacterium Synechococcus elongatus UTEX 2973.

Cyanobacteria are attractive microbial hosts for production of chemicals using light and CO. However, their low productivity of chemicals is a major challenge for commercial applications. This is mostly due to their relatively slow growth rate and carbon partitioning toward biomass rather than products. Many cyanobacterial strains synthesize sucrose as an osmoprotectant to cope with salt stress environments. In this study, we harnessed the photosynthetic machinery of the fast-growing cyanobacterium Synechococcus elongatus UTEX 2973 to produce sucrose under salt stress conditions and investigated if the high efficiency of photosynthesis can enhance the productivity of sucrose. By expressing the sucrose transporter CscB, Synechococcus 2973 produced 8 g L of sucrose with a highest productivity of 1.9 g L day under salt stress conditions. The salt stress activated the sucrose biosynthetic pathway mostly via upregulating the sps gene, which encodes the rate-limiting sucrose-phosphate synthase enzyme. To alleviate the demand on high concentrations of salt for sucrose production, we further overexpressed the sucrose synthesis genes in Synechococcus 2973. The engineered strain produced sucrose with a productivity of 1.1 g L day without the need of salt induction. The engineered Synechococcus 2973 in this study demonstrated the highest productivity of sucrose in cyanobacteria.