Rational Metabolic Engineering of Escherichia Coli for High-yield L-serine Production

Abstract Background: L-serine is widely used in the food, cosmetic and pharmaceutical industries, and direct fermentation of L-serine from glucose is an attractive technique. However, L-serine producers have historically been developed via classical random mutagenesis due to the complicated metabolic network and regulatory mechanism of L-serine production, leading to un-optimal productivity and yield of L-serine and thus limiting its large-scale industrial production. Result: In this study, a high-yield and high-productivity Escherichia coli strain was constructed by a defined genetic modification methodology for L-serine production. First, L-serine-mediated feedback inhibition was removed and L-serine biosynthetic pathway genes (serAfr, serC and serB) associated with phosphoglycerate kinase (pgk) were overexpressed. Secondly, L-serine conversion pathway was further examined by introducing a glyA mutation (K229G) and deleting other degrading enzymes based on deletion of initial sdaA. Finally, the L-serine transport system was rationally engineered to reduce the uptake and accelerate the export of L-serine. The optimally engineered strain produced 35 g/L L-serine with a productivity of 0.98 g/L/h and yield of 0.42 g/g glucose in a 5-L fermenter, the highest productivity and yield of L-serine from glucose reported to date. Transcriptome and intermediate metabolite were analyzed to further understand the regulatory mechanism of L-serine production. Conclusion: These results demonstrated that combined metabolic and bioprocess engineering strategies can improve L-serine productivity and yield, thus providing basic principles for rationally designing of high-yield production strains and paving the way for towards a simple and economical process for industrial L-serine production.