Syringotoxin Biosynthetic Gene Cluster Required for Antiamoebal Defense in Pseudomonas syringae

Syringotoxins are chlorinated cyclic lipopeptides from Pseudomonas syringae that were structurally characterized more than three decades ago. The corresponding biosynthetic gene cluster required for their production, however, has not been experimentally assigned. Based on targeted genetics, metabolite profiling, and structure analysis, we identify a divergent syringomycin-type nonribosomal peptide synthetase gene cluster in P. syringae DSM 1242 and show that psyt is the syringotoxin biosynthetic gene cluster. Deletion of psyt abolishes syringotoxin production and renders the mutant susceptible to grazing by the amoebal predator Dictyostelium discoideum. Besides nanomolar amoebicidal activity, syringotoxin B shows strong antifungal activity, particularly against Candida pathogens. Comparative genomic and structural analyses place psyt within the syringomycin-type lipopeptide family, while the activity loss after macrolactone opening links syringotoxin bioactivity to its cyclic lipopeptide architecture. Together, this work assigns the biosynthetic origin of syringotoxin and establishes this cyclic lipopeptide as a defense factor required for full resistance of P. syringae to amoebal predation.