Metabolic engineering of Synechococcus elongatus for photoautotrophic production of mannitol

Abstract

With multiple applications in food, pharmaceutical, and chemical industries as antioxidant or nonmetabolizable sweetener; the bioproduction of d-mannitol is gaining global attention, especially with photosynthetic organisms as hosts. Considering the sustainability prospects, the current work encompasses metabolic engineering of a widely used cyanobacterial strain, Synechococcus elongatus PCC 7942, and two newly isolated fast-growing cyanobacterial strains; S. elongatus PCC 11801 and S. elongatus PCC 11802, for mannitol production. We engineered these strains with a two-step pathway by cloning genes for mannitol-1-phosphate dehydrogenase (mtlD) and mannitol-1-phosphatase (mlp), where the mtlD expression was under the control of different promoters from PCC 7942, namely, P_rbc225 , P_cpcB300 , P_cpcBm1 , P_rbcLm17 , and P_rbcLm15 . The strains were tested under the "switch conditions," where the growth conditions were switched after the first 3 days, thereby resulting in differential promoter activity. Among the engineered strains of PCC 11801 and PCC 11802, the strains possessing P_rbc225 -mtlD module produced relatively high mannitol titers of 401 ± 18 mg/L and 537 ± 18 mg/L, respectively. The highest mannitol titer of 701 ± 15 mg/L (productivity 60 mg/L.d, yield 895 µM/OD₇₃₀ ) was exhibited by the engineered strain of PCC 7942 expressing P_cpcB300 -mtlD module. It is by far the highest obtained mannitol yield from the engineered cyanobacteria.

Keywords: CO2 to mannitol; Synechococcus elongatus; cyanobacteria; switch conditions.