Reconstitution of Septacidin Biosynthesis in Escherichia coli by Redirecting an ADP-Heptose Precursor from Primary Metabolism

Septacidins (SEPs) represent a group of nucleoside antibiotics featuring an N-6-glycosylated l-heptosamine-adenine core (SEP-327), which is challenging for chemical synthesis. The SEP analogues with diverse bioactivities are usually obtained by the long fermentation process of Streptomyces. After delineating the whole biosynthetic pathway of SEPs, we embarked on the reconstitution of SEP and its key intermediate biosynthesis in the fast-growing Escherichia coli cells. We sequentially constructed a set of engineered E. coli strains that can synthesize the N-6-glycosylated d-heptose-adenine intermediate (SEP-328), SEP-327, and two SEP congeners with linear fatty acyl groups (SEP-E594 and SEP-E622). For all these constructions, the engineered SEP pathways were shortened significantly by redirecting a key precursor, ADP-l-glycero-beta-d-manno-heptose (ADP-heptose), from lipopolysaccharide biosynthesis. The titer of SEP-327 was increased to 470.07 +/- 12.81 mg/L at the flask level by improving the solubility of a key oxidase (SepI), supplementing precursors, and optimizing the culture conditions. During fed-batch fermentation, the titer of SEP-327 could reach 2.53 g/L in a 5 L bioreactor using glycerol, a major byproduct of biodiesel industry, as the carbon source, which will significantly facilitate the preparation of SEP-327 for semisynthetic drug development efforts. In addition, we also obtained SEP-E594 and SEP-E622 with a linear fatty acyl terminus that exhibits antifungal activities comparable to those of their congeners with a branched fatty acyl group.