Met. Eng., published online 14 June 2013; doi:10.1016/j.ymben.2013.06.001

Salvianic acid A is an efficient antioxidant and possibly has other biological roles, but limited availability and a lack of known biosynthetic enzymes of this polyphenolic acid has stymied a full understanding of its functions. Yao et al. noted that related aromatic compounds have been produced enzymatically in cells engineered for tyrosine production, but these studies focused primarily on tyrosine-derived compounds. The authors anticipated that rerouting of the tyrosine precursor, 4-hydroxyphenylpyruvate (4-HPP), in Escherichia coli would provide a more direct route to the desired product. The authors tested D-lactate dehydrogenase and two known activated mutants as catalysts for the chiral reduction of the ketone group and identified the flavoenzyme 4-hydroxyphenylacetate 3-hydroxylase hpaBC as likely to insert a 3-hydroxy group on the phenyl ring; expression of the pair of enzymes returned 259.27 mg l−1 salvianic acid A from a strain fed with 500 mg l−1 4-HPP. The authors separately adopted existing strategies to engineer tyrosine biosynthesis to optimize yield of the 4-HPP precursor, with the best success observed in a strain that overexpressed three enzymes that synthesize precursors very early in the pathway. Combination of the two strategies in fed-batch cultures led to an average titer of 7.1 g l−1, or 0.47 mol/mol glucose, with salvianic acid A toxicity limiting further accumulation. This artificial route provides a new source for a plant polyphenol.