Peptide natural products arise from two distinct pathway types: nonribosomal peptide synthetases (NRPSs) and ribosomally synthesized and posttranslationally modified peptides (RiPPs) [1]. NRPSs activate and couple individual amino acids, requiring large, modular enzyme complexes with tethered intermediates for peptide assembly. In contrast, RiPPs are generated by the maturation of small gene-encoded precursor proteins by a streamlined series of posttranslational modification (PTM) enzymes (Figure 1) [2]. These enzymes are largely tolerant to amino acid substitutions in the targeted C-terminal core region of the precursor proteins, relying rather on protein–protein contacts in the N-terminal leader or C-terminal follower to bring the enzymes in proximity for catalysis [3]. The mature cores are proteolytically released from the leader (and follower, if present), sometimes concomitant with export outside the cell. Modifications may also occur as postcleavage tailoring events, such as acylation of the liberated N-terminal amine [4]. Unlike NRPSs, which can couple nonproteinogenic residues, RiPPs are limited to standard α-amino acids polymerized by the ribosome. However, a broad and growing diversity of PTM enzymes have been characterized, including many that install features in RiPPs once thought to be exclusive to NRPS products — such as d-amino acid and β-amino acid configurations, various cyclic structures, backbone N-methylations, and lipidations, among others [2].

Natural nonribosomal peptides (NRPs) from fungal and bacterial producers and their related derivatives are clinically used as anti-infectives. Examples include the penicillins, gramicidins, polymyxins, bacitracins, glycopeptides, and, most recently, daptomycin as antibacterials and echinocandins as antifungals. They comprise nonproteinogenic residues, contain a macrocyclized region, and some are conjugated to nonpeptide parts such as fatty acids. These features are particularly important for the biological function of peptides, improving their target-binding properties by providing conformational constraints and increasing resistance to chemical or proteolytic degradation. It has historically been difficult to engineer nonribosomal megasynthetases, although strides have been made in recent years to recombine their modular parts to generate new-to-nature chimeras [1]. The growing burden of antimicrobial resistance to these nonribosomal drugs and other classes of antibiotics reinforces the need for new molecules, drug targets, and treatment regimens [5]. RiPPs and their engineered variants are a promising resource for the efficient identification of novel bioactive peptides.

Here, I report recent trends in the discovery of new PTMs and lead structures from RiPP pathways, highlighting how promiscuous modifications can be exploited for biotechnology applications with an emphasis on recent literature reports.