The AusAB non-ribosomal peptide synthetase of <i>Staphylococcus aureus</i> preferentially generates phevalin in host-mimicking media

Peptide and protein synthesis are fundamental processes in nature which are largely mediated by the ribosomal machinery. An alternative pathway for peptide synthesis is non-ribosomal mRNA template-independent synthesis, performed by so-called NRPSs. NRPSs are multi-enzyme complexes which serve the simultaneous role of template and biosynthetic machinery. They are mostly found in bacteria and fungi and are responsible for the biosynthesis of many pharmacologically significant products, including antibiotics, anticancer compounds, or immunosuppressants. The human pathogen <i>S. aureus</i> possesses one such NRPS, AusA, which synthesizes three cyclic dipeptides termed "aureusimines" using the aromatic amino acids phenylalanine and tyrosine and the branched-chain amino acids leucine and valine. Although the biological role of aureusimines remains unknown, AusA appears to play a role in the interaction of <i>S. aureus</i> with the host. In addition, owing to its minimal canonical NRPS structure and autonomous function (i.e., most NRPS pathways require the assembly of several NRPS proteins), AusA represents an excellent model system for studying such molecular assembly lines. Our observation is, to our knowledge, the first report of an NRPS showing preferential incorporation of aromatic amino acids, despite their similar availability.