Structure and Biosynthesis of Mangotoxin Produced by the Plant Pathogen Pseudomonas syringae

Pathogenic bacteria interfere with plant metabolism through specialized metabolites and mechanisms. Mangotoxin was identified in 2003 from Pseudomonas syringae pv. syringae, which causes devastating diseases globally in various crops, including mango. Mangotoxin was found to inhibit arginine biosynthesis in tomato and Escherichia coli; however, its chemical structure remained unknown. We determined the structure of mangotoxin from the native bacterial producer P. syringae to be an alanine-citrulline dipeptide that contains an internal alkyne on citrulline. We showed that mangotoxin also inhibits the growth of a multidrug-resistant human pathogen Klebsiella pneumoniae strain. In vitro characterization of mangotoxin biosynthesis revealed that alkyne formation is catalyzed by a heme oxygenase-like domain-containing oxidase with a redox partner on a tripeptide intermediate. We discovered that the mangotoxin biosynthetic enzymes can couple various canonical and noncanonical amino acids into tripeptides and install two- or four-electron oxidations on several tripeptides, providing a foundation for bioengineering of the pathway. Our work answered a two-decade-old question on the structure of an elusive toxin and will facilitate the elucidation of the molecular mechanism used by mangotoxin to inhibit amino acid metabolism in plants and bacteria.