Improving L-serine formation by Escherichia coli by reduced uptake of produced L-serine

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.

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 ΔsdaC 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 ΔsdaCΔsstTΔtdcC mutant strain exhibited 0.253 nmol min^-1 (mg dry weight) ^-1L-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 ΔsdaCΔsstTΔtdcC mutant accumulated 34.8 g/L L-serine with a yield of 32% from glucose in a 5-L fermenter after 36 h.

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.