Constructing a synthetic constitutive metabolic pathway in Escherichia coli for (R, R)-2,3-butanediol production

Many microorganisms could naturally produce (R, R)-2,3-butanediol ((R, R)-2,3-BD), which has unique applications due to its special chiral group and spatial configuration. But the low enantio-purity of the product hindered the development of large-scale production. In this work, a synthetic constitutive metabolic pathway for enantiomerically pure (R, R)-2,3-BD biosynthesis was constructed in Escherichia coli with vector pUC6S, which does not contain any lac sequences. The expression of this artificial constructed gene cluster was optimized by using two different strength of promoters (AlperPLTet01 (P₀₁) and AlperBB (P_BB)). The strength of P₀₁ is twice stronger than P_BB. The fermentation results suggested that the yield of (R, R)-2,3-BD was higher when using the stronger promoter. Compared with the wild type, the recombinant strain E. coli YJ2 produced a small amount of acetic acid and showed higher glucose consumption rate and higher cell density, which indicated a protection against acetic acid inhibition. In order to further increase the (R, R)-2,3-BD production by reducing the accumulation of its precursor acetoin, the synthetic operon was reconstructed by adding the strong promoter P₀₁ in front of the gene ydjL coding for the enzyme of (R, R)-2,3-BD dehydrogenase which catalyzes the conversion of acetoin to (R, R)-2,3-BD. The engineered strain E. coli YJ3 showed a 20 % decrease in acetoin production compared with that of E. coli YJ2. After optimization the fermentation conditions, 30.5 g/L of (R, R)-2,3-BD and 3.2 g/L of acetoin were produced from 80 g/L of glucose within 18 h, with an enantio-purity over 99 %.