Improving l-serine formation by Escherichia coli by reduced uptake of produced l-serine
Chenyang Wang, Junjun Wu, Shi Binchao, Jiping Shi, Zhijun Zhao
Abstract
Abstract Background Microbial de novo production of l -serine, which is widely used in a range of cosmetic and pharmaceutical products, has attracted increasing attention due to its environmentally friendly characteristics. Previous pioneering work mainly focused on l -serine anabolism; however, in this study, it was found that l -serine could be reimported through the l -serine uptake system, thus hampering l -serine production. Result To address this challenge, engineering via deletion of four genes, namely, sdaC , cycA , sstT and tdcC , which have been reported to be involved in l -serine uptake in Escherichia coli , was first carried out in the l -serine producer E. coli ES. Additionally, the effects of these genes on l -serine uptake activity and l -serine production were investigated. The data revealed an abnormal phenomenon regarding serine uptake activity. The serine uptake activity of the Δ sdaC mutant was 0.798 nmol min −1 (mg dry weight) −1 after 30 min, decreasing by 23.34% compared to that of the control strain. However, the serine uptake activity of the single sstT , cycA and tdcC mutants increased by 34.29%, 78.29% and 48.03%, respectively, compared to that of the control strain. This finding may be the result of the increased level of sdaC expression in these mutants. In addition, multigene-deletion strains were constructed based on an sdaC knockout mutant. The Δ sdaC Δ sstT Δ tdcC mutant strain exhibited 0.253 nmol min −1 (mg dry weight) −1 l -serine uptake activity and the highest production titer of 445 mg/L in shake flask fermentation, which was more than three-fold the 129 mg/L production observed for the parent. Furthermore, the Δ sdaC Δ sstT Δ tdcC mutant accumulated 34.8 g/L l -serine with a yield of 32% from glucose in a 5-L fermenter after 36 h. Conclusion The results indicated that reuptake of l -serine impairs its production and that an engineered cell with reduced uptake can address this problem and improve the production of l -serine in E. coli .