Two Radical SAM Enzymes Are Necessary and Sufficient for the In Vitro Production of the Oxetane Nucleoside Antiviral Agent Albucidin.

Oxetanocin A and albucidin are two oxetane natural products. While the biosynthesis of oxetanocin A has been described, less is known about albucidin. In this work, the albucidin biosynthetic gene cluster is identified in Streptomyces. Heterologous expression in a nonproducing strain demonstrates that the genes alsA and alsB are necessary and sufficient for albucidin biosynthesis confirming a previous study (Myronovskyi et al. Microorganisms2020, 8, 237). A two‐step construction of albucidin 4′‐phosphate from 2′‐deoxyadenosine monophosphate (2′‐dAMP) is shown to be catalyzed in vitro by the cobalamin dependent radical S‐adenosyl‐l‐methionine (SAM) enzyme AlsB, which catalyzes a ring contraction, and the radical SAM enzyme AlsA, which catalyzes elimination of a one‐carbon fragment. Isotope labelling studies show that AlsB catalysis begins with stereospecific H‐atom transfer of the C2′‐pro‐R hydrogen from 2′‐dAMP to 5′‐deoxyadenosine, and that the eliminated one‐carbon fragment originates from C3′ of 2′‐dAMP. [ABSTRACT FROM AUTHOR]