PROJECT SUMMARY / ABSTRACT Since the discovery of penicillin in the 1920s, bioactive peptide natural products have been used as antibiotic, antiviral, immunosuppressive, and anti-cancer agents. Many of these bioactive peptides harbor backbone α-N-methylations and/or macrocyclizations, since these tailorings significantly improve peptide pharmacokinetics. As seen in the blockbuster immunosuppressant cyclosporin A, α-N-methylated peptides have increased structural rigidity, proteolytic resistance, and membrane permeability. Additionally, cyclic α-N- methylated peptides are able to bind large, flat surfaces with high affinity, making them attractive targets for disrupting protein-protein interactions. Despite these advantages, inefficient synthetic and in vitro processes hinder the production, screening, and optimization of α-N-methylated peptides. In addition, natural sources of amide backbone-methylated peptides were thought to be completely limited to inflexible nonribosomal peptide biosynthetic pathways. The goal of this work is to tease out the mechanistic and structural constraints of α-N- methylating biocatalysts within our newly discovered ribosomally encoded peptide natural product family called the borosins. Our first objective identifies a potent bioactive model system encoded in a basidiomycete fungus to tease out the rules and limitations of borosin α-N-methylation, N-to-C macrocyclization, and bioactivity. Our second objective focuses on structurally distinct borosin family members, where we will perform detailed kinetic measurements to tease out the mechanism for chemically challenging α-N-methylation. We also outline our longer-term visions to develop methods for studying peptide natural products in basidiomycete fungi as well as our efforts to tease out the potential biological roles of borosin metabolites. This research will create a diverse toolbox of flexible and efficient catalysts for the biological production, screening, and optimization of genetically templated bioactive α-N-methylated peptides.