Cyanochelins, an overlooked class of widely distributed cyanobacterial siderophores, discovered by silent gene cluster awakening.

Cyanobacteria require iron for growth and often inhabit iron-limited habitats, yet only a few siderophores are known to be produced by them. We report that cyanobacterial genomes frequently encode PKS/NRPS biosynthetic pathways for synthesis of lipopeptides featuring β -hydroxyaspartate ( β -OH-Asp), a residue known to be involved in iron chelation. Iron starvation triggered the synthesis of β -OH-Asp lipopeptides in the cyanobacteria Rivularia sp. PCC 7116, Leptolyngbya sp. NIES-3755 and Rubidibacter lacunae KORDI 51-2. The induced compounds were confirmed to bind iron by mass spectrometry and were capable of Fe 3+ to Fe 2+ photoreduction accompanied by their cleavage, when exposed to sunlight. The siderophore from Rivularia , named cyanochelin A, was structurally characterized by MS and NMR and contains a hydrophobic tail bound to phenolate and oxazole moieties followed by five amino acids including two modified aspartate residues for iron chelation. Phylogenomic analysis revealed twenty-six additional cyanochelin-like gene clusters across a broad range of cyanobacterial lineages. Our data suggests that cyanochelins and related compounds are widespread, β -OH-Asp-featuring cyanobacterial siderophores produced by phylogenetically distant species upon iron starvation. Production of photolabile siderophores by phototrophic cyanobacteria raises questions to what extent the compounds facilitate iron monopolization by the producer or provide Fe 2+ for the whole microbial community via photoreduction. Significance: All living organisms depend on iron as an essential cofactor for indispensable enzymes. However, the sources of bioavailable iron are often limited. To face this problem, microorganisms synthesize low molecular weight metabolites capable of iron scavenging - the siderophores. Although cyanobacteria inhabit the majority of the Earth's ecosystems, their repertoire of known siderophores is remarkably poor. Their genomes are known to harbour a rich variety of gene clusters with unknown function. Here we report the awakening of a widely distributed class of silent gene clusters by iron starvation, yielding cyanochelins, β -hydroxy aspartate lipopeptides involved in iron acquisition. Our results expand the limited arsenal of known cyanobacterial siderophores and propose products with ecological function to a number of previously silent gene clusters.