Frankobactin Metallophores Produced by Nitrogen-Fixing Frankia Actinobacteria Function in Toxic Metal Sequestration

A series of new metallophores, referred to as frankobactins, were extracted from cultures of the symbiotic and nitrogen-fixing actinobacterium Frankia sp. CH37. Structure elucidation revealed a 2-hydroxyphenyl-substituted oxazoline core and a chain composed of five proteinogenic and nonproteinogenic amino acids, suggesting nonribosomal peptide synthesis as the biosynthetic origin. By whole-genome sequencing, bioinformatic analysis, and comparison with other Frankia strains, the genetic locus responsible for the biosynthesis was detected. Spectrophotometric titration of frankobactin with Fe(III) and Cu(II) and mass spectrometry established the 1:1 (metal:frankobactin) coordination. Uptake experiments suggested that frankobactin A₁ (1) did not serve to recruit iron, but to detoxify Cu(II). As frankobactin A₁ prevents the cellular entry of Cu(II), it could play a crucial role in the symbiosis of Frankia sp. and its host in the reclamation of copper-contaminated soil.