The genetic origin of evolidine, the first cyclopeptide discovered in plants, and related orbitides

Cyclic peptides are reported to have antibacterial, antifungal and other bioactivities. Orbitides are a class of cyclic peptide that are small, head-to-tail cyclized, composed of proteinogenic amino acids, and lack disulfide bonds; they are also known in several genera of the plant family Rutaceae. Melicope xanthoxyloides is the Australian rain forest tree of the Rutaceae family in which evolidine - the first plant cyclic peptide - was discovered. Evolidine (cyclo-SFLPVNL) has subsequently been all but forgotten in the academic literature, so to redress this we used tandem mass spectrometry and de novo transcriptomics to rediscover evolidine and decipher its biosynthetic origin from a short precursor just 48 residues in length. We also identify another six M. xanthoxyloides orbitides using the same techniques. These peptides have atypically diverse C-termini consisting of residues not recognized by either of the known proteases plants use to macrocyclize peptides, suggesting new cyclizing enzymes a wait discovery. We examined the structure of two of the novel orbitides by NMR, finding one had a definable structure, whereas the other did not. Mining RNA-seq and whole genome sequencingdata from other species of the Rutaceae familyrevealed a large and diverse family of peptides is encoded by similar sequences across the family and demonstrates how powerful de novo transcriptomics can be at accelerating the discovery of new peptide families.