Identification of the Early Steps in Herbicidin Biosynthesis Reveals an Atypical Mechanism of -Glycosylation.

Herbicidins are adenosine-derived nucleoside antibiotics with an unusual tricyclic core structure. Deletion of the genes responsible for formation of the tricyclic skeleton in sp. L-9-10 reveals the o importance of Her4, Her5, and Her6 in the early stages of herbicidin biosynthesis. o characterization of Her4 and Her5 demonstrates their involvement in an initial, two-stage C-C coupling reaction that results in net C5'-glycosylation of ADP/ATP by UDP/TDP-glucuronic acid. Biochemical analyses and intermediate trapping experiments imply a noncanonical mechanism of -glycosylation reminiscent of NAD-dependent -adenosylhomocysteine (SAH)-hydrolase catalysis. Structural characterization of the isolated metabolites suggests possible reactions catalyzed by Her6 and Her7. An overall herbicidin biosynthetic pathway is proposed based on these observations.