Ribosomal pentapeptide nitration for non-ribosomal peptide antibiotic precursor biosynthesis

Peptide natural products possess a fascinating array of complex structures and diverse biological activities. Central to this is a repertoire of structurally modified amino acid building blocks, which stem from fundamentally different biosynthetic pathways for peptides of non-ribosomal and ribosomal origins. Given these origins, the integration of non-ribosomal and ribosomal peptide biosynthesis has previously been thought implausible. Now, we report how nature has synergized ribosomal and non-ribosomal peptide pathways in the biosynthesis of the rufomycins, exceptionally potent antitubercular antibiotics. In this pathway, a biarylitide-type ribosomal pentapeptide precursor is nitrated by a modified cytochrome P450 biaryl-crosslinking enzyme. The nitrated residue, key for antibiotic activity, is liberated by a dedicated protease before activation and peptide incorporation by the non-ribosomal rufomycin synthetase assembly line. This resolves the enigmatic origins of 3-nitrotyrosine within rufomycin biosynthesis and unveils a novel function for ribosomally synthesized peptides as templates for biosynthesis of modified non-ribosomal peptide building blocks.