Abstract
BackgroundMicrobial 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.ResultTo 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.ConclusionThe 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.
Highlights
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
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
Investigating whether l‐serine could be reimported by E. coli To determine whether l-serine was reimported, E. coli ES was inoculated into Luria-Bertani (LB) medium with
Summary
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. The direct fermentation of cheaper carbon sources to obtain l-serine has become a promising production method because this method is environmentally friendly and allows easy extraction [3]. Numerous exciting studies have demonstrated the successful microbial production of l-serine. Production of 36 g/L l-serine was achieved by controlling SHMT activity with a folate supply in a 60-h fed-batch fermentation process [5]. Zhu et al [6] obtained a C. glutamicum strain engineered to minimize the byproducts l-alanine and l-valine by deleting alaT, avtA and ilvN and achieved l-serine production of 42.6 g/L in a 96-h fedbatch fermentation process.
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