The study selected an L-isoleucine auxotrophic Corynebacterium glutamicum strain FA-1 to explore the transaminase that affects the synthesis of L-leucine and L-valine. The study first knocked out the ilvE gene, and found that after the ilvE gene was knocked out, the L-valine synthesis activity was lost, but there was still a weak L-leucine synthesis ability. It shows that there are other aminotransferases that catalyze the synthesis of L-leucine. Subsequently, the aspB gene was continued to be knocked out, and the ability to synthesize L-leucine was completely lost. On the basis of knocking out ilvE, overexpression of aspB gene restored the synthesis of L-leucine to a certain extent (Table 1). These results indicate that the protein encoded by aspB has L-leucine synthesis activity.
Furthermore, the study screened some endogenous and exogenous aminotransferase genes, and expressed these aminotransferase genes in ilvE and aspB co-knockout strains to explore whether these aminotransferases have branched-chain amino acid synthesis activity. Among them, TyrB shows specific L-leucine synthesis activity, while YbgE and CallvE show dual synthesis activities of L-leucine and L-valine. These results indicate that optimizing transaminase to switch substrate specificity has great potential to increase the yield of branched chain amino acids.
