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Natural products have provided many molecules to treat and prevent illnesses in humans, animals and plants. While only a small fraction of the existing microbial diversity has been explored for bioactive metabolites, tens of thousands of molecules have been reported in the literature over the past 80 years. Thus, the main challenge in microbial metabolite screening is to avoid the re-discovery of known metabolites in a cost-effective manner. In this perspective, we report and discuss different approaches used in our laboratory over the past few years, ranging from bioactivity-based screening to looking for metabolic rarity in different datasets to deeply investigating a single Streptomyces strain. Our results show that it is possible to find novel chemistry through a limited screening effort, provided that appropriate selection criteria are in place.

Microbial natural products impress by their bioactivity, structural diversity and ingenious biosynthesis. While screening the rare actinobacterial genus Planomonospora, cyclopeptides 1A and 1B were discovered, featuring an unusual Tyr-His biaryl-bridging across a tripeptide scaffold, with the sequences N-acetyl-Tyr-Tyr-His (1A) and N-acetyl-Tyr-Phe-His (1B). Genome analysis of the 1A producing strain pointed to-wards a ribosomal synthesis of 1A, from a pentapeptide precursor encoded by the tiny 18-nucleotide gene bycA, to our knowledge the smallest gene ever reported. Further, biaryl instalment is performed by the closely linked gene bycB, encoding a cytochrome P450 monooxygenase. Biosynthesis of 1A was confirmed by heterologous production in Streptomyces, yielding the mature product. Bioinformatic analysis of related cytochrome P450 monooxygenases indicated that they constitute a widespread family of pathways, associated to 5-aa coding sequences in approximately 200 (actino)bacterial genomes, all with potential for a biaryl linkage between amino acids 1 and 3. We propose the name biarylicins for this newly discovered family of RiPPs.