Septacidin

Septacidin is an adenine nucleoside antibiotic with antifungal and antitumor activities. During the efforts to construct a better septacidin producer, we obtained a high yield strain S. albus 1597 by putting the biosynthetic gene cluster (BGC) of septacidin under the control of the constitutive strong promoter ermE*. S. albus 1597 could produce new septacidin congeners SEP-538 and SEP-552 with shorter fatty acyl chains. Moreover, SEP-624 with an unprecedented hydroxylated fatty acyl chain was also isolated from this titre improved strain, enriching the diversity of septacidins. SEP-552 showed moderate inhibitory effects against Epidermophyton floccosum 57312 with MIC value 62.5 μM, while SEP-538 and SEP-624 only exhibited weak antifungal activities. The structure-activity relationship investigation revealed that the antifungal activity of septacidins is significantly influenced by the length of and the decoration on their fatty acyl chains.

Seven-carbon-chain-containing sugars exist in several groups of important bacterial natural products. Septacidin represents a group of l-heptopyranoses containing nucleoside antibiotics with antitumor, antifungal, and pain-relief activities. Hygromycin B, an aminoglycoside anthelmintic agent used in swine and poultry farming, represents a group of d-heptopyranoses-containing antibiotics. To date, very little is known about the biosynthesis of these compounds. Here we sequenced the genome of the septacidin producer and identified the septacidin gene cluster by heterologous expression. After determining the boundaries of the septacidin gene cluster, we studied septacidin biosynthesis by in vivo and in vitro experiments and discovered that SepB, SepL, and SepC can convert d-sedoheptulose-7-phosphate (S-7-P) to ADP-l-glycero-β-d-manno-heptose, exemplifying the involvement of ADP-sugar in microbial natural product biosynthesis. Interestingly, septacidin, a secondary metabolite from a gram-positive bacterium, shares the same ADP-heptose biosynthesis pathway with the gram-negative bacterium LPS. In addition, two acyltransferase-encoding genes sepD and sepH, were proposed to be involved in septacidin side-chain formation according to the intermediates accumulated in their mutants. In hygromycin B biosynthesis, an isomerase HygP can recognize S-7-P and convert it to ADP-d-glycero-β-d-altro-heptose together with GmhA and HldE, two enzymes from the Escherichia coli LPS heptose biosynthetic pathway, suggesting that the d-heptopyranose moiety of hygromycin B is also derived from S-7-P. Unlike the other S-7-P isomerases, HygP catalyzes consecutive isomerizations and controls the stereochemistry of both C2 and C3 positions.

Septacidin (1) represents a group of nucleoside antibiotics possessing a unique N6-glycosylated adenine core. They exhibit some fascinating bioactivities that are rare for other nucleoside antibiotics. Here we demonstrate that this unique structure in septacidin is formed by SepE and SepF. SepE is an unprecedented Fe(II)-dependent glycosyltransferase decorating the N6-position of AMP using ADP-l-glycero-β-d-manno-heptose (6) as a sugar donor. The Fe(II) may help SepE to bind AMP. SepF is an unusual glycosidase that detaches the N9-ribosyl-5-phosphate.