Pyrroline Ring Assembly via N‑Prenylation and Oxidative Carbocyclization during Biosynthesis of Aeruginosin Derivatives

Aeruginosins are linear peptide natural products isolated from cyanobacteria and contain various arginine derivatives at their termini. 1-Amino-2-(<i>N</i>-amidino-3-Δ3-pyrrolinyl)ethane (Aeap) is a structurally unique arginine derivative, as it has an unusual pyrroline ring with two additional carbon atoms of unknown biosynthetic origin. Here, we demonstrate that Aer3, a member of a newly identified subfamily of prenyltransferases, catalyzes selective isopentenylation of the internal N atom of agmatine. Rieske oxygenase AerC then catalyzes both carbocyclization and C–C bond cleavage to construct the pyrroline ring in Aeap. This pyrroline ring formation in Aeap biosynthesis, involving two novel enzymes, represents a unique route for heterocycle formation in nature.