L-tyrosine production by deregulated strains of Escherichia coli

The excretion of the aromatic amino acid (L)-tyrosine was achieved by manipulating three gene targets in the wild-type Escherichia coli K12: The feedback-inhibition-resistant (fbr) derivatives of aroG and tyrA were expressed on a low-copy-number vector, and the TyrR-mediated regulation of the aromatic amino acid biosynthesis was eliminated by deleting the tyrR gene. The generation of this (L)-tyrosine producer, strain T1, was based only on the deregulation of the aromatic amino acid biosynthesis pathway, but no structural genes in the genome were affected. A second tyrosine over- producing strain, E. coli T2, was generated considering the possible limitation of precursor substrates. To enhance the availability of the two precursor substrates phosphoenolpyruvate and erythrose- 4-phosphate, the ppsA and the tktA genes were over-expressed in the strain T1 background, increasing L- tyrosine production by 80% in 50ml batch cultures. Fed-batch fermentations revealed that (L)-tyrosine production was tightly correlated with cell growth, exhibiting the maximum productivity at the end of the exponential growth phase. The final L-tyrosine concentrations were 3.8 g/l for E. coli T1 and 9.7 g/l for E. coli T2 with a yield of (L)-tyrosine per glucose of 0.037 g/g (T1) and 0.102 g/g (T2), respectively.