Unraveling the specific regulation of the shikimate pathway for tyrosine accumulation in Bacillus licheniformis.

L-Tyrosine serves as a common precursor for multiple valuable secondary metabolites. Synthesis of this aromatic amino acid in Bacillus licheniformis occurs via the shikimate pathway, but the underlying mechanisms involving metabolic regulation remain unclear. In this work, improved L-tyrosine accumulation was achieved in B. licheniformis via co-overexpression of aroG and tyrA from Escherichia coli to yield strain 45A12, and the L-tyrosine titer increased to 1005 mg/L with controlled glucose feeding. Quantitative RT-PCR results indicated that aroA, encoding DAHP synthase, and aroK, encoding shikimate kinase, were feedback-repressed by the end product L-tyrosine in the modified strain. Therefore, the native aroK was first expressed with multiple copies to yield strain 45A13, which could accumulate 1201 mg/L L-tyrosine. Compared with strain 45A12, the expression of aroB and aroF in strain 45A13 was upregulated by 21% and 27%, respectively, which may also have resulted in the improvement of L-tyrosine production. Furthermore, supplementation with 5 g/L shikimate enhanced the L-tyrosine titers of 45A12 and 45A13 by 29.1% and 24.0%, respectively. However, the yield of L-tyrosine per unit of shikimate decreased from 0.365 to 0.198 mol/mol after aroK overexpression in strain 45A12, which suggested that the gene product was also involved in uncharacterized pathways. This study provides a good starting point for further modification to achieve industrial-scale production of L-tyrosine using B. licheniformis, a generally recognized as safe workhorse.