At the genome level, Synechococcus elongatus UTEX 2973 (Synechococcus 2973) is nearly identical to the model cyanobacterium Synechococcus elongatus PCC 7942 (Synechococcus 7942) with only 55 single nucleotide differences separating the two strains. Despite the high similarity between the two strains, Synechococcus 2973 grows three times faster, accumulates significantly more glycogen, is tolerant to extremely high light intensities, and displays higher photosynthetic rates. The high homology between the two strains provides a unique opportunity to examine the factors that lead to increased photosynthetic rates. We compared the photophysiology of the two strains and determined the differences in Synechococcus 2973 that lead to increased photosynthetic rates and the concomitant increase in biomass production. In this study, we identified inefficiencies in the electron transport chain of Synechococcus 7942 that have been alleviated in Synechococcus 2973. Photosystem II (PSII) capacity is the same in both strains. However, Synechococcus 2973 exhibits a 1.6-fold increase in PSI content, a 1.5-fold increase in cytochrome b6f content, and a 2.4-fold increase in plastocyanin content on a per cell basis. The increased content of electron carriers allows a higher flux of electrons through the photosynthetic electron transport chain, while the increased PSI content provides more oxidizing power to maintain upstream carriers ready to accept electrons. These changes serve to increase the photosynthetic efficiency of Synechococcus 2973, the fastest growing cyanobacterium known.

cyanobacteria are attractive microbial hosts for production of chemicals using light and co 2. 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 −1 of sucrose with a highest productivity of 1.9 g L −1 day −1 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 −1 day −1 without the need of salt induction. the engineered Synechococcus 2973 in this study demonstrated the highest productivity of sucrose in cyanobacteria. Microbial production of fuels and commodity chemicals provides alternative solutions to reduce the reliance on fossil fuel. However, the requirement of sugar feedstock is one of the challenges for sustainable bioproduction. Cyanobacteria are photosynthetic prokaryotes that use light, CO 2 , and trace amounts of minerals for growth. Compared to terrestrial plants, cyanobacteria have higher efficiencies to utilize solar energy 1. In recent years, many synthetic biology tools have been developed for cyanobacteria 2. These tools have enabled metabolic engineering of cyanobacteria to produce various chemicals, including fuels 3 , petrochemicals 4 , sugars 5 , fragrances 6 , and biopolymers 7. Although cyanobacteria demonstrate the potential of converting CO 2 into desired products, most of the reported titers and productivities are still too low for commercial applications 8,9. A more efficient photosynthetic chassis is needed to improve CO 2 utilization and carbon partitioning toward products. Sucrose is an important feedstock in food industry and bioethanol production. Cyanobacteria synthesize sucrose as a compatible solute to tolerate high salt environments. By synthesizing sucrose, the osmotic pressure can be maintained to avoid desiccation in salt stress conditions. Studies of various cyanobacterial strains showed that more than 60 strains accumulate sucrose under high salt conditions 10. In cyanobacterial cells, sucrose is synthesized from uridine diphosphate glucose (UDP-Glu) and fructose 6-phosphate (F6P) by sucrose-phosphate synthase (SPS) and sucrose-phosphate phosphatase (SPP) (Fig. 1A). CscB is a sucrose/H + symporter which b...

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Terpenoids are a large and diverse group of natural products with commercial applications. Microbial production of terpenes is considered as a feasible approach for the stable supply of these complex hydrocarbons. Cyanobacteria, photosynthetic prokaryotes, are attractive hosts for sustainable bioproduction, because these autotrophs require only light and CO 2 for growth. Despite cyanobacteria having been engineered to produce a variety of compounds, their productivities of terpenes are generally low. Further research is needed to determine the bottleneck reactions for enhancing terpene production in cyanobacteria. In this study, we engineered the fast-growing cyanobacterium Synechococcus elongatus UTEX 2973 to produce a commercially-used terpenoid, limonene. We identified a beneficial mutation in the gene encoding geranylgeranyl pyrophosphate synthase crtE , leading to a 2.5-fold increase in limonene production. The engineered strain produced 16.4 ​mg ​L −1 of limonene at a rate of 8.2 ​mg ​L −1 day −1 , which is 8-fold higher than limonene productivities previously reported in other cyanobacterial species. Furthermore, we employed a combinatorial metabolic engineering approach to optimize genes involved in the upstream pathway of limonene biosynthesis. By modulating the expression of genes encoding the enzymes in the MEP pathway and the geranyl pyrophosphate synthase, we showed that optimization of the expression level is critical to enhance limonene production in cyanobacteria.

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