Improvement of l-Leucine Production in by Altering the Redox Flux.

The production of l-leucine was improved by the disruption of encoding transcriptional regulator and overexpression of the key genes () of the l-leucine biosynthesis pathway in XQ-9. In order to improve l-leucine production, we rationally engineered to enhance l-leucine production, by improving the redox flux. On the basis of this, we manipulated the redox state of the cells by mutating the coenzyme-binding domains of acetohydroxyacid isomeroreductase encoded by , inserting NAD-specific leucine dehydrogenase, encoded by from , and glutamate dehydrogenase encoded by from , instead of endogenous branched-chain amino acid transaminase and glutamate dehydrogenase, respectively. The yield of l-leucine reached 22.62 ± 0.17 g·L by strain ΔLtbR-acetohydroxyacid isomeroreductase (AHAIR)/ABNCE, and the concentrations of the by-products (l-valine and l-alanine) increased, compared to the strain ΔLtbR/ABNCE. Strain ΔLtbR-AHAIRLeuDH/ABNCLDH accumulated 22.87±0.31 g·L l-leucine, but showed a drastically low l-valine accumulation (from 8.06 ± 0.35 g·L to 2.72 ± 0.11 g·L), in comparison to strain ΔLtbR-AHAIR/ABNCE, which indicated that LeuDH has much specificity for l-leucine synthesis but not for l-valine synthesis. Subsequently, the resultant strain ΔLtbR-AHAIRLeuDHRocG/ABNCLDH accumulated 23.31 ± 0.24 g·L l-leucine with a glucose conversion efficiency of 0.191 g·g.