Nitration of a ribosomal pentapeptide generates a noncanonical precursor for nonribosomal peptide synthesis
Peptide natural products possess a fascinating array of complex structures and diverse functions. Central to this is a repertoire of modified amino acid building blocks, which stem from fundamentally different biosynthesis pathways for peptides of nonribosomal and ribosomal origins. Given these origins, integration of nonribosomal and ribosomal pathways have previously been thought unlikely. Now, we demonstrate that ribosomal biosynthesis generates a key noncanonical 3-nitrotyrosine building block for the nonribosomal synthesis of rufomycin. In this pathway, a biarylitide-type ribosomal peptide is nitrated by a modified cytochrome P450 crosslinking enzyme, with the nitrated residue liberated by the actions of a dedicated protease found within the rufomycin gene cluster before being incorporated into rufomycin by the rufomycin nonribosomal peptide synthetase. This resolves the enigmatic origins of 3-nitrotyrosine within rufomycin biosynthesis and demonstrates unexpected integration of ribosomal peptide synthesis as a mechanism for the generation of noncanonical building blocks within nonribosomal synthesis pathways.