Cyanobacteria have promising potential to generate valuable compounds from light energy and atmospheric carbon dioxide (CO2). Many strains have been engineered to express pathways to generate a variety of products from pharmaceuticals to biofuels. Even with cyanobacteria’s high photosynthetic efficiencies most strains remain economically inviable. Many strains are grown in bioreactors where ample light and CO2 are provided, however, photosynthetic conversion of carbon and light to useful compounds is limited. I propose photosynthesis is feedback limited, or sink limited, and the mechanisms of feedback inhibition can be alleviated in a Synechococcus elongatus PCC7942 strain engineered to export sucrose. By inducing expression of sucrose permease, in various salt conditions, the amount of sucrose can be tuned, which allowed us to measure a transient increase in the operating efficiency of PSII proportional to the rate of sucrose export. Concomitantly, an oxidized electron transport chain and lowered PSI acceptor side limitation were observed. These two photosynthetic parameters could be consequences of increased carbon fixation since the ratio of RbcL to PSII increases as well as greater accumulation of RbcS-GFP following sucrose export. These data suggest that sucrose export leads to greater carbon fixation and as a consequence increase light use efficiency within PSII. Furthermore, using this knowledge, we created strains that could partition carbon from growth and division to bioproduction and observed greater total and specific productivities for sucrose production. The ideas herein aid in the understanding of bioproduction strain inefficiencies and novel avenues for future genetic engineering for increased bioproduction.
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