Search

Siderophores are iron-chelating molecules that solubilize Fe 3+ for microbial utilization and facilitate colonization or infection of eukaryotes by liberating host iron for bacterial uptake. By fluorescently labeling membrane receptors and binding proteins, we created 20 sensors that detect, discriminate, and quantify apo- and ferric siderophores. The sensor proteins originated from TonB-dependent ligand-gated porins (LGPs) of Escherichia coli (Fiu, FepA, Cir, FhuA, IutA, BtuB), Klebsiella pneumoniae (IroN, FepA, FyuA), Acinetobacter baumannii (PiuA, FepA, PirA, BauA), Pseudomonas aeruginosa (FepA, FpvA), and Caulobacter crescentus (HutA) from a periplasmic E. coli binding protein (FepB) and from a human serum binding protein (siderocalin). They detected ferric catecholates (enterobactin, degraded enterobactin, glucosylated enterobactin, dihydroxybenzoate, dihydroxybenzoyl serine, cefidericol, MB-1), ferric hydroxamates (ferrichromes, aerobactin), mixed iron complexes (yersiniabactin, acinetobactin, pyoverdine), and porphyrins (hemin, vitamin B12). The sensors defined the specificities and corresponding affinities of the LGPs and binding proteins and monitored ferric siderophore and porphyrin transport by microbial pathogens. We also quantified, for the first time, broad recognition of diverse ferric complexes by some LGPs, as well as monospecificity for a single metal chelate by others. In addition to their primary ferric siderophore ligands, most LGPs bound the corresponding aposiderophore with ∼100-fold lower affinity. These sensors provide insights into ferric siderophore biosynthesis and uptake pathways in free-living, commensal, and pathogenic Gram-negative bacteria., Competing Interests: Conflict of interest D. A. S. is employed and compensated by Venatorx and may own stock or stock options in Venatorx as part of his remuneration for employment., (Copyright © 2022 The Authors. Published by Elsevier Inc. All rights reserved.)

Synthesis of the 86-kDa FatA outer membrane protein is repressed under iron-rich conditions. Complementation of transposition mutants derived from clones containing the pJM1 iron uptake region revealed the existence of an antisense RNA, RNA alpha. This RNA is only expressed under iron-rich conditions and acts as a negative regulator of FatA synthesis, with slight but discernible decrease in the steady-state level of fatA mRNA determined by RNase protection and by Northern blot analysis. Primer extension experiments revealed that the level of several possible fatA transcripts was reduced in the presence of RNA alpha. In addition, we found that fatA mRNA expression is slightly reduced in the presence of Escherichia coli Fur. We have identified and cloned a chromosomally encoded fur-like gene in Vibrio anguillarum.

Complementation of insertion mutants showed that the polypeptides FatD, FatC, FatB, and FatA are essential for the iron-transport process encoded by pJM1. Sequence analysis followed by homology studies indicated that transport of ferric anguibactin into Vibrio anguillarum 775 follows the same mechanism as reported for transport of Fe(3+)-hydroxamates, Fe(3+)-catecholates, ferric dicitrate, and vitamin B12 into Escherichia coli. Homology of FatA, part of the receptor complex, to seven E. coli receptor proteins involved in uptake of siderophores and vitamin B12 supports the idea of a common ancestral gene. A "TonB-Box" was found in FatA suggesting the existence of a TonB-like protein function in V. anguillarum. A high homology in the primary structure of FatB to FhuD, FecB, FepB, and BtuE suggests that FatB is the anguibactin-binding protein located in the periplasmic space. FatD and FatC are polytopic integral membrane proteins. According to their homologies to other proteins from other transport systems, they may be involved in the translocation of ferric anguibactin across the cytoplasmic membrane.

The iron-transport genes from the pJM1 plasmid of Vibrio anguillarum have been cloned and sequenced. Five open-reading frames have been identified, one of which encodes the outer membrane receptor for ferric anguibactin, OM2. This coding region corresponds to a protein of 726 amino acids with a Mr of 78,777. The protein has a hydrophobic signal sequence of 35 amino acids and a potential membrane-associated hydrophobic region at the carboxyl terminus. A 2.3-kilobase iron-regulated mRNA was transcribed from this region in vivo. The four other open-reading frames were shown to be involved in the regulation of OM2 expression and in iron transport by the use of insertion mutagenesis and complementation analysis. One of these open-reading frames, ORF3, encodes a 40-kDa polypeptide which, as deduced from the amino acid sequence and the hydropathy plot, is likely to be membrane-associated and together with OM2 may play a role in the transport of iron into the cell cytosol.