EstB-mediated hydrolysis of the siderophore triacetylfusarinine C optimizes iron uptake of Aspergillus fumigatus.

Aspergillus fumigatus excretes the fusarinine-type siderophore desferri-triacetylfusarinine C (DF-TafC) to mobilize iron. DF-TafC is a cyclic peptide consisting of three N(5)-cis-anhydromevalonyl-N(5)-hydroxy-N(2)-acetyl-l-ornithine residues linked by ester bonds; these linkages are in contrast to peptide linkages found for ferrichrome-type siderophores. Subsequent to the binding of iron and uptake, triacetylfusarinine C (TafC) is hydrolyzed, the cleavage products are excreted, and the iron is transferred to the metabolism or to the intracellular siderophore desferri-ferricrocin (DF-FC) for iron storage. Here we report the identification and characterization of the TafC esterase EstB, the first eukaryotic siderophore-degrading enzyme to be characterized at the molecular level. The encoding gene, estB, was found to be located in an iron-regulated gene cluster, indicating a role in iron metabolism. Deletion of estB in A. fumigatus eliminated TafC esterase activity of cellular extracts and caused increased intracellular accumulation of TafC and TafC hydrolysis products in vivo. Escherichia coli-expressed EstB displayed specific TafC esterase activity but did not hydrolyze fusarinine C, which has the same core structure as TafC but lacks three N(2)-acetyl residues. Localization of EstB via enhanced green fluorescent protein tagging suggested that TafC hydrolysis takes place in the cytoplasm. EstB abrogation reduced the intracellular transfer rate of iron from TafC to DF-FC and delayed iron sensing. Furthermore, EstB deficiency caused a decreased radial growth rate under iron-depleted but not iron-replete conditions. Taken together, these data suggest that EstB-mediated TafC hydrolysis optimizes but is not essential for TafC-mediated iron uptake in A. fumigatus.