Siderophore formation in iron-limited cultures of Frankia sp. strain 52065 and Frankia sp. strain CeSI5

Frankia sp. strain 52065 (DDB 03010210) produces a high-affinity iron chelator or siderophore termed frankobactin to obtain iron needed for nitrogen fixation under iron-limiting conditions. Cultures of Frankia sp. strain 52065 and Frankia sp. strain CeSI5 (UFG 026605) were grown under iron-limiting and iron-replete conditions and examined for siderophore formation throughout the growth cycle using the HPLC ⁵⁵Fe-binding assay and the Csaky chemical assay. Both cultures produced frankobactin under iron-limiting, but not iron-replete, conditions. This is the first positive report of hydroxamate siderophore formation in a Frankia isolate other than Frankia sp. strain 52065. A detailed analysis of siderophore formation throughout the culture cycle shows the presence of a second, strong iron-binding compound in both Frankia sp. strain 52065 and Frankia strain CeSI5. Chemical characterization by mass spectroscopy indicates that this second siderophore, named frankobactin A, is likely to be the open oxazoline ring conformer of frankobactin. Solution concentrations of frankobactin and frankobactin A increased during the rapid growth phase of Frankia in culture, reaching a maximum concentration of 20-25 μM, then decreased once the cultures entered stationary phase. Uptake studies using ⁵⁵Fe-labeled frankobactin indicated this siderophore forms part of an inducible, high-affinity iron-uptake mechanism.