Fungal arginine-containing cyclodipeptide synthases are finally revealed.

Biosynthetic pathways without any identifiable core enzymes may encode unknown (biosynthetic route)-unknown (molecular structure) natural products. However, bioinformatics-guided mining for such unknown-unknown metabolites is challenging. Recently, an unknown-unknown biosynthetic route has been deciphered in fungi. It was found that a class of enzymes previously annotated as hypothetical proteins catalyze the biosynthesis of arginine-containing cyclodipeptides (CDPs). This advances the understanding of the biosynthesis of CDPs and highlights the vast potential of unknown-unknown natural products encoded by microbial genomes.