Genetic engineering of Escherichia coli to enhance production of L-tryptophan.

Reducing the accumulation of acetate in Escherichia coli cultures can decrease carbon efflux as by-products and reduce acetate toxicity, and therefore enable high cell density cultivation. The concentration of intracellular amino acids can be decreased by genetic modifications of the corresponding amino acid transport systems. This can increase the levels of amino acids in the fermentation broth by decreasing the feedback inhibition on the corresponding biosynthetic pathways. Here, the effects of genetic manipulation of phosphate acetyltransferase (pta), high affinity tryptophan transporter (mtr) and aromatic amino acid exporter (yddG) on L-tryptophan production were investigated. The pta mutants accumulated less acetate and showed higher capacity for producing L-tryptophan as compared with the parental strain. The strains lacking mtr, or overexpressed yddG, or with the both mtr knockout and yddG overexpression, accumulated lower concentrations of intracellular tryptophan but higher production of extracellular L-tryptophan. In the L-tryptohan fed-batch fermentation of an E. coli derived from TRTH0709/pMEL03 having deletion of pta-mtr and overexpression of yddG in a 30-L fermentor, the maximum concentration of L-tryptophan (48.68 g/L) was obtained, which represented a 15.96 % increase as compared with the parental strain. Acetate accumulated to a concentration of 0.95 g/L. The intracellular concentration of L-tryptophan was low, and the glucose conversion rate reached a high level of 21.87 %, which was increased by 15.53 % as compared with the parent strain.