Production of L-valine by system-wide metabolic engineering strategies

Abstract

We have previously reported the development of a metabolically engineered Escherichia coli strain capable of producing high-level L-valine based on in silico flux response analysis. Here we report further strain improvement by using comparative transcriptome analysis. To examine the transcriptional response of the engineered E. coli strain to L-valine production, a comparative transcriptome analysis between an L-valine producing Escherichia coli strain of VAMF (pKBRilvIHmutCED, pTrc184ygaZHlrp) and the control strain of E. coli WL (pKBR, pTrc184) was per-formed. The genes encoding L-valine biosynthesis enzyme were found to be up-regulated in the L-valine producing strain, whereas the genes involved in glycolysis and TCA cycle were all down-regulated. The genes involved in acetic acid assimila-tion were found to be up-regulated. Notably, the ilvIH genes encoding acetohydroxy acid synthase (AHAS) isoenzyme III were found to be down-regulated to 0.79- and 0.82-fold, respectively. As this enzyme is involved in the committed step towards L-valine formation, these genes were selected and overexpressed, resulting in 77.31 % increase in L-valine production. Furthermore, the yigFG genes, which were found to be significantly up-regulated by 12.14- and 10.68-fold, respectively, were newly identified as an L-valine exporter. Overexpression of these genes further increased L-valine production by 19.22%. The final engineered E. coli strain was able to produce 1.6 g/L L-valine in flask cultivation, which is 98.04 % higher than that obtained with the parental strain.

With previously engineered L-valine producing strain, Lrp regulatory modes in presence of various concentration of L-leucine was determined. The approach of employing transcriptome analysis in further strain improvement can also be applied as a good strategy for the enhanced production of other bioproducts.