Structural and Stereochemical Elucidation of Cyanochelin C, a Siderophore Associated with Novel Class of Cyanobacterial Acyl Hydrolases

Iron is a key micronutrient that constrains microbial growth and productivity in many aquatic and terrestrial environments due to its limited bioavailability. Microorganisms evolved sophisticated acquisition strategies, including the production of siderophores, high-affinity iron-chelating molecules that facilitate iron solubilisation and uptake. Cyanobacteria, photosynthetic prokaryotes and major contributors to global primary production, also depend on iron as a cofactor to their core metabolic enzymes. However, very few cyanobacterial siderophores were described so far, and cyanobacteria remain an underxplored source of possibly novel siderophores. Here we report a novel cyanobacterial siderophore, cyanochelin C, that employs two {beta}-hydroxyaspartate residues for iron chelation. We provide extensive nuclear magnetic resonance (NMR) and mass spectrometry (MS) evidence on the molecular structure and identify the corresponding biosynthetic gene cluster (BGC). Bioinformatic analysis of the BGC further revealed the presence of an acylase CcsQ clustering with a broader cyanobacteria-specific family of acylases associated with predicted siderophore-encoding BGCs. Discovery of cyanochelin C and its deacylation by CcsQ expands the known structural diversity of cyanobacterial siderophores and improves the understanding of important enzymatic reactions.