CO₂ reduction and organic compounds production by photosynthetic bacteria with surface displayed carbonic anhydrase and inducible expression of phosphoenolpyruvate carboxylase

In Rhodobacter sphaeroides, carbonic anhydrase (CA; EC 4.2.1.1) is a zinc-containing metalloenzyme that catalyzes the reversible hydration of CO₂ to HCO₃⁻ while phosphoenolpyruvate carboxylase (PEPC; 4.1.1.31), an enzyme involved in the carbon metabolism that catalyzed the fixation of CO₂ to PEP, is a key factor for biological fixation of CO₂ and enhances the production of organic compounds. In this study, the recombinant R. sphaeroides with highly-expressed CA was developed based on a surface displayed system of CA (pJY-OmpCA) on the outer membrane of R. sphaeroides using outer membrane protein (Omp) in R. sphaeroides, Finally, two more different recombinant R. sphaeroides were developed, which transformed with a two-vector system harboring cytosolic expressed CA (pJY-OmpCA-CA)or PEPC (pJY-OMPCA-PEPC) in R. sphaeroides with surface displayed CA on the outer membrane. In case of recombinant R. sphaeroides with the pJY-OmpCA-PEPC, it has shown the highest CO₂ reduction efficiency and the production of several organic compounds (carotenoids, polyhydroxybutyrate, malic acid, succinic acid). It means that the surface displayed CA on the R. sphaeroides would accelerate the CO₂-bicabonate conversion on the bacterial outer membrane. Moreover, inducible over-expression of PEPC with surface-displayed CA was successfully used to facilitate a rapider CO₂ reduction and quicker production of organic compounds.