N-Carbamoylation of 2,4-diaminobutyrate reroutes the outcome in padanamide biosynthesis

Padanamides are linear tetrapeptides notable for the absence of proteinogenic amino acids in their structures. In particular, two unusual heterocycles, (S)-3-amino-2-oxopyrrolidine-1-carboxamide (S-Aopc) and (S)-3-aminopiperidine-2,6-dione (S-Apd), are found at the C-termini of padanamides A and B, respectively. Here we show that padanamides are synthesized by highly dissociated hybrid nonribosomal peptide synthetase/polyketide synthase machinery. We furthermore demonstrate that carbamoyltransferase gene padQ is critical to the formation of padanamide A but dispensable for biosynthesis of padanamide B. Biochemical investigations show that PadQ carbamoylates the rare biosynthetic precursor L-2,4-diaminobutyrate, generating L-2-amino-4-ureidobutyrate, the presumed precursor to the C-terminal residue of padanamide A.