Crochelins: Siderophores with an Unprecedented Iron‐Chelating Moiety from the Nitrogen‐Fixing Bacterium Azotobacter chroococcum

Abstract

Microbes use siderophores to access essential iron resources in the environment. Over 500 siderophores are known, but they utilize a small set of common moieties to bind iron. Azotobacter chroococcum expresses iron‐rich nitrogenases, with which it reduces N2. Though an important agricultural inoculant, the structures of its iron‐binding molecules remain unknown. Here, the “chelome” of A. chroococcum is examined using small molecule discovery and bioinformatics. The bacterium produces vibrioferrin and amphibactins as well as a novel family of siderophores, the crochelins. Detailed characterization shows that the most abundant member, crochelin A, binds iron in a hexadentate fashion using a new iron‐chelating γ‐amino acid. Insights into the biosynthesis of crochelins and the mechanism by which iron may be removed upon import of the holo‐siderophore are presented. This work expands the repertoire of iron‐chelating moieties in microbial siderophores.