Design of a recombinant Escherichia coli for producing l-phenylalanine from glycerol

Abstract

A recombinant Escherichia coli was engineered to produce the commercially important amino acid l-phenylalanine (l-Phe) using glycerol as the carbon source. Compared to the conventionally used glucose and sucrose, glycerol is a less expensive carbon source. As phenylalanine dehydrogenase (PheDH) activity is involved in the last step of l-Phe synthesis in E. coli, a phenylalanine dehydrogenase gene (phedh) from the thermotolerant Bacillus lentus was cloned into pRSFDuet-1 (pPheDH) and expressed in E. coli BL21(DE3). The resulting clone had a limited ability to produce l-Phe from glycerol, possibly because of a poor glycerol uptake by the cell, or an inability to excrete l-Phe, or both. Therefore, yddG gene encoding an aromatic amino acid exporter and glpF gene encoding a glycerol transport facilitator were coexpressed with the phedh in a reengineered E. coli. In a glycerol medium, the maximum l-Phe production rates of the clones pPY (phedh and yddG genes) and pPYF (phedh, yddG and glpF genes) were 1.4- and 1.8-fold higher than the maximum production rate of the pPheDH clone. The better producing pPYF clone was further evaluated in a 5 l stirred-tank fermenter (37 °C, an aeration rate of 1 vvm, an agitation speed of 400 rpm). In the fermenter, the maximum concentration of l-Phe (366 mg/l) was achieved in a much shorter period compared to in the shake flasks. In the latter, the highest titer of l-Phe was only 76 % of the maximum value attained in the fermenter.