Increase of optical purtiy of L-lactic produced by engineered Escherichia coli by enhancing anaerobic expression of dld gene
In order to eliminate D-lactic acid in cheap fermentation raw materials and improve the optical purity of L-lactic acid in fermentation final product,the expression plasmids of D-lactate dehydrogenase gene(dld)with different anaerobic inducible promoters(pflBp6,pnirB,pflBp6-pnirB)were constructed.They were transferred into the Escherichia coli HBUT-L to enhance its ability to eliminate exogenous D-lactic acid while producing L-lactic acid by fermentation.The optimal plasmid expression conditions and strains were screened by single factor experiments of liquid loading volume and rotation speed,and the fermentation was verified in inorganic salt and corn pulp media.The results showed that the D-lactate dehydrogenase ac-tivity of engineered strain HBUT-L4 containing expression plasmid with promoter pflBp6-pnirB was the highest(81.1 U/g)fermentation for 18 h un-der the conditions of liquid loading volume 200 ml/250 ml and rotation speed 150 r/min.When fermentation with inorganic salt and corn pulp media under these conditions,compared to original strain HBUT-L,the D-lactic acid consumption rate of engineered strain HBUT-L4 increased by 250％and 217％,respectively,and the optical purity of L-lactic acid increased from 96.32％,98.48％to 99.95％,99.98％,respectively.In the process of L-lactic acid fermentation by engineered E.coli,the expression plasmid with anaerobic inducible promoter pflBp6-pnirB could increase the D-lactate dehydrogenase activity,and strengthen the ability of strain to eliminate exogenous D-lactic acid,and improve the optical purity of L-lactic acid,which had important industrial application value.
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