The importance of Cytochrome P450-catalyzed modifications of natural products produced by non-ribosomal peptide synthetase machineries is most apparent during glycopeptide antibiotic biosynthesis: specifically, the formation of essential amino acid side chains crosslinks in the peptide backbone of these clinically relevant antibiotics. These cyclization reactions take place whilst the peptide substrate remains bound to the non-ribosomal peptide synthetase in a process mediated by a conserved domain of previously unknown function-the X-domain. This review addresses recent advances in understanding P450 recruitment to non-ribosomal peptide synthetase-bound substrates and highlights the importance of both carrier proteins and the X-domain in different P450-catalyzed reactions.

Feglymycin, a peptide antibiotic produced by Streptomyces sp. DSM 11171, consists mostly of nonproteinogenic phenylglycine-type amino acids. It possesses antibacterial activity against methicillin-resistant Staphylococcus aureus strains and antiviral activity against HIV. Inhibition of the early steps of bacterial peptidoglycan synthesis indicated a mode of action different from those of other peptide antibiotics. Here we describe the identification and assignment of the feglymycin (feg) biosynthesis gene cluster, which codes for a 13-module nonribosomal peptide synthetase (NRPS) system. Inactivation of an NRPS gene and supplementation of a hydroxymandelate oxidase mutant with the amino acid l-Hpg proved the identity of the feg cluster. Feeding of Hpg-related unnatural amino acids was not successful. This characterization of the feg cluster is an important step to understanding the biosynthesis of this potent antibacterial peptide.