Growth and siderophore formation in six iron-limited strains of Frankia

Iron is an essential component in the enzyme nitrogenase and is therefore necessary for nitrogen fixation. To examine how nitrogen-fixing Frankia species obtain their required iron, six different isolates were grown under nitrogen-limiting conditions with and without added iron. These cultures were tested for their ability to produce high affinity iron chelators (termed siderophores) using a combination of Chromatographie and chemical assays. Frankia sp. strain 52065 produced a novel hydroxamate-containing peptide siderophore (trivial name: frankobactin) under iron-limiting conditions. No siderophore was produced under iron-replete conditions. Three Frankia strains (HFPCcI3, HFPArI3 and ArI5) showed no siderophore activity using five different assays under both iron-replete and iron-limiting conditions. Two strains (HFPCpIl and AvsI3) tested positive using one assay (CAS) but the other four assays gave negative results. It is unknown if this CAS-positive compound functions as a siderophore in these two species. Siderophore-mediated iron uptake does occur in selected Frankia isolates (e.g. strain 52065) but is not uniformly distributed among all strains.