Biosynthesis of Sialyllacto-<i>N</i>-tetraose c in Engineered <i>Escherichia coli</i>

Human milk oligosaccharides (HMOs) have attracted considerable interest for their vital role in supporting infant health. Among these, sialyllacto-<i>N</i>-tetraose c (LST c), a pentasaccharide with the structure Neu5Ac(α2,6)Gal(β1,4)GlcNAc(β1,3)Gal(β1,4)Glc, stands out due to its critical importance in the development and application of complex HMOs. In this study, we employed multivariate modular metabolic engineering (MMME) to screen for efficient sialyltransferases and balance metabolic fluxes, successfully constructing strains capable of LST c biosynthesis. Additionally, by blocking competing pathway genes, enhancing the supply of UDP-GlcNAc and UDP-Gal precursors, and establishing a CTP cofactor regeneration system, we developed a high-yielding <i>Escherichia coli</i> strain, W15. This strain achieved an LST c titer of 220.9 mg/L in shake flask cultures. In a 3-L fed-batch fermentation, the LST c concentration reached 922.2 mg/L, with a productivity of 10.25 mg/L/h and a specific yield of 38.70 mg/g DCW. This research provides an effective strategy for producing LST c in microbial cell factories.