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, Prbc225, PcpcB300, PcpcBm1, PrbcLm17, and PrbcLm15. 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 Prbc225‐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 PcpcB300‐mtlD module. It is by far the highest obtained mannitol yield from the engineered cyanobacteria.