Metabolic Engineering of for Efficient Biosynthesis of Lacto--tetraose Using a Novel β-1,3-Galactosyltransferase from .

Human milk oligosaccharides (HMOs) attract considerable interest in recent years because of their particular role in infant health. Lacto--tetraose (LNT), one of the most abundant HMOs, has been commercially added in the infant formula as a functional fortifier. In this study, a novel LNT-producing β-1,3-galactosyltransferase (β-1,3-GalT) from was screened from 14 putative candidates, and a highly LNT-producing metabolically engineered strain was constructed based on a previously constructed lacto--triose II (LNT II)-producing strain, by strengthening UDP-galactose synthesis and introduction of β-1,3-GalT. The engineered strain produced 3.11 and 25.49 g/L LNT in shake-flask and fed-batch cultivation, with the molar conversion ratio of LNT II to LNT of 88.15 and 85.09%, respectively. The productivity and specific yield of LNT in fed-batch cultivation were measured to be 0.61 g/L·h and 0.76 g/g dry cell weight, respectively. To the best of our knowledge, it is the highest LNT yield ever reported.