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. Microorganisms 2020, 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.

Automated summary

In this study, the biosynthetic gene cluster of albucidin was identified in Streptomyces, and it was confirmed that alsA and alsB genes are necessary and sufficient for the biosynthesis of albucidin. Moreover, the two-step construction of albucidin 4'-phosphate from 2'-dAMP was shown to be catalyzed by the enzymes AlsB and AlsA. Isotope labeling studies provided insights into the initial steps of the catalytic reaction.