Zhang, X., O. Baars, and F. M. M. Morel. “
Genetic, structural, and functional diversity of low- and high-affinity siderophores in strains of nitrogen fixing Azotobacter chroococcum. doi:10.1039/c8mt00236c.”
Metallomics (2018).
DOI: 10.1039/C8MT00236CAbstractTo increase iron (Fe) bioavailability in surface soils, microbes secrete siderophores, chelators with widely
varying Fe affinities. Strains of the soil bacterium Azotobacter chroococcum (AC), plant-growth
promoting rhizobacteria used as agricultural inoculants, require high Fe concentrations for aerobic
respiration and nitrogen fixation. Recently, A. chroococcum str. NCIMB 8003 was shown to synthesize
three siderophore classes: (1) vibrioferrin, a low-affinity a-hydroxy carboxylate (pFe = 18.4),
(2) amphibactins, high-affinity tris-hydroxamates, and (3) crochelin A, a high-affinity siderophore with
mixed Fe-chelating groups (pFe = 23.9). The relevance and specific functions of these siderophores
in AC strains remain unclear. We analyzed the genome and siderophores of a second AC strain,
A. chroococcum str. B3, and found that it also produces vibrioferrin and amphibactins, but not crochelin
A. Genome comparisons indicate that vibrioferrin production is a vertically inherited, conserved strategy
for Fe uptake in A. chroococcum and other species of Azotobacter. Amphibactin and crochelin
biosynthesis reflects a more complex evolutionary history, shaped by vertical gene transfer, gene gain
and loss through recombination at a genomic hotspot. We found conserved patterns of low vs. highaffinity
siderophore production across strains: the low-affinity vibrioferrin was produced by mildly Fe
limited cultures. As cells became more severely Fe starved, vibrioferrin production decreased in favor of
high-affinity amphibactins (str. B3, NCIMB 8003) and crochelin A (str. NCIMB 8003). Our results show
the evolution of low and high-affinity siderophore families and conserved patterns for their production
in response to Fe bioavailability in a common soil diazotroph.
Baars, Oliver, François M. M. Morel, and Xinning Zhang. “
The purple non-sulfur bacterium Rhodopseudomonas palustris produces novel petrobactin related siderophores under aerobic and anaerobic conditions.”
Environmental Microbiology 20, no. 5 (2018): 1667-1676.
DOI:10.1111/1462-2920.14078AbstractMany bacteria produce siderophores to bind and take up Fe(III), an essential trace metal with extremely low solubility in oxygenated environments at circumneutral pH. The purple non‐sulfur bacterium Rhodopseudomonas palustris str. CGA009 is a metabolically versatile model organism with high iron requirements that is able to grow under aerobic and anaerobic conditions. Siderophore biosynthesis has been predicted by genomic analysis, however, siderophore structures were not identified. Here, we elucidate the structure of two novel siderophores from R. palustris: rhodopetrobactin A and B. Rhodopetrobactins are structural analogues of the known siderophore petrobactin in which the Fe chelating moieties are conserved, including two 3,4‐dihydroxybenzoate and a citrate substructure. In the place of two spermidine linker groups in petrobactin, rhodopetrobactins contain two 4,4′‐diaminodibutylamine groups of which one or both are acetylated at the central amine. We analyse siderophore production under different growth modes and show that rhodopetrobactins are produced in response to Fe limitation under aerobic as well as under anaerobic conditions. Evaluation of the chemical characteristics of rhodopetrobactins indicates that they are well suited to support Fe acquisition under variable oxygen and light conditions.