The rice seedling blight fungus Rhizopus microsporus harbors endosymbiotic bacteria (Burkholderia rhizoxinica) that produce the virulence factor rhizoxin and control host development. Genome mining indicated a massive inventory of cryptic nonribosomal peptide synthetase (NRPS) genes, which have not yet been linked to any natural products. The discovery and full characterization of a novel cyclopeptide from endofungal bacteria is reported. In silico analysis of an orphan, symbiont-specific NRPS predicted the structure of a nonribosomal peptide, which was targeted by LC-MS/MS profiling of wild-type and engineered null mutants. NMR spectroscopy and chemical derivatization elucidated the structure of the bacterial cyclopeptide. Phylogenetic analyses revealed the relationship of starter C domains for rare N-acetyl-capped peptides. Heptarhizin is produced under symbiotic conditions in geographically constrained strains from the Pacific clade; this indicates a potential ecological role of the peptide.
Keywords: cyclopeptides; enzymes; genomics; natural products; symbiotic bacteria.
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