Metabolic engineering of Escherichia coli for high-level astaxanthin production with high productivity

Astaxanthin is a reddish keto-carotenoid classified as a xanthophyll found in various microbes and marine organisms. As a powerful antioxidant having up to 100 times more potency than other carotenoids such as β-carotene, lutein, and lycopene, astaxanthin is a versatile compound utilized in animal feed, food pigment, health promotion and cosmetic industry. Here, we report development of metabolically engineered Escherichia coli capable of producing astaxanthin to a high concentration with high productivity. First, the heterologous crt genes (crtE, crtY, crtI, crtB, and crtZ) from Pantoea ananatis and the truncated BKT gene (trCrBKT) from Chlamydomonas reinhardtii were introduced to construct the astaxanthin biosynthetic pathway. Then, eight different fusion tags were examined by attaching them to the N- or C-terminus of the trCrBKT membrane protein to allow stable expression and to efficiently guide trCrBKT to the E. coli membrane. When the signal peptide of OmpF and TrxA were tagged to the N-terminus and C-terminus of trCrBKT, respectively, astaxanthin production reached 12.90 mg/L (equivalent to 3.84 mg/gDCW), which was 2.08-fold higher than that obtained without tagging. Upon optimization of culture conditions, this engineered strain WLGB-RPP harboring pAX15 produced 332.23 mg/L (5.38 mg/gDCW) of astaxanthin with the productivity of 3.79 mg/L/h by fed-batch fermentation. In order to further increase astaxanthin production, in silico flux variability scanning based on enforced objective flux (FVSEOF) was performed to identify gene overexpression targets. The engineered strain WLGB-RPP (pAX15, pTrc-ispDF) which simultaneously overexpressing the ispD and ispF genes identified by FVSEOF produced astaxanthin to a higher concentration of 377.10 mg/L (6.26 mg/gDCW) with a productivity of 9.20 mg/L/h upon induction with 1 mM IPTG. When cells were induced with 0.5 mM IPTG to reduce the metabolic burden, astaxanthin concentration further increased to 432.82 mg/L (7.12 mg/gDCW) with a productivity of 9.62 mg/L/h. To more stably maintain plasmid during the fed-batch fermentation of WLGB-RPP (pAX15, pTrc-ispDF), the post-segregational killing hok/sok system was introduced. This strain produced 385.04 mg/L (6.98 mg/gDCW) of astaxanthin with a productivity of 7.86 mg/L/h upon induction with 0.5 mM IPTG. The strategies reported here will be useful for the enhanced production of astaxanthin and related carotenoid products by engineered E. coli strains.