Production Of Xylitol From CO₂ By Genetically Engineered Cyanobacteria

Xylitol is a good sweetener,and it has been widely used in food,medicine and chemical industry.However,plants and human body tend to accumulate only low quantities of xylitol,and extraction and purification are often complicated.Chemical synthesis is technically challenging and uneconomical.Recent advances in synthetic biology and metabolic engineering have enabled the production of xylitol via microbial metabolism of carbohydrates.However,economical and sustainable biosynthesis of natural products remains a tremendous challenge.This thesis focuses on xylitol production through several new methods,including constructing the photosynthetic system,extending pentose phosphate pathway?PPP?.Natural pathways are the result of evolution over a long period and seem to be more efficient and stable.More importantly,natural synthetic pathways are often connected with basal metabolism.Cyanobacteria are unicellular photosynthetic microorganisms that have several advantages for synthetising compounds.However,the production of xylitol directly from CO₂ has remained largely unexplored.Here,by extending a natural pathway of PPP to construct a new approach to produce xylitol in S.elongatus PCC7942.The genes?xdh and pho13?encoding the xylitol dehydrogenase and phosphatase were introduced into S.elongatus PCC7942 to construct a photosynthetic platform.With this platform,xylitol can not be generated directly from CO₂ and solar energy.Then,we introduced the gene?xpdh?encoding the xylitol-phosphate dehydrogenase into S.elongatus PCC7942 to construct recombinant bacteria PX.Engineered strain PX can produce 25.5 mg/L xylitol from CO₂ and solar energy in sixteen days.To optimize the synthesis pathway of xylitol,xpdh and the gene?phoA?encoding phosphatase were introduced into S.elongatus PCC7942 to construct recombinant bacteria PXP.Engineered strain PXP can produce 16.94 mg/L xylitol under the condition of continuous ventilation.Thus,we have confirmed that xpdh is really suitable for xylitol biosynthesis and alkaline phosphatase from S.elongatus PCC7942 in the production of xylitol plays a better role than phoA.The novel synthetic strategy makes the environmentally friendly,low-cost and sustainable production of xylitol a reality.