Frankia produces a hydroxamate siderophore under iron limitation

Abstract

Frankia strain 52065, a nitrogen‐fixing symbiont of the actinorhizal plant, Ceanothus americanus, was grown under both iron‐replete and iron‐limited culture conditions. Cells were monitored over time for both growth and siderophore production. Culture media was tested for the presence of siderophores using a combination of chemical and Chromatographic techniques. Both iron‐replete and iron‐limited cultures tested negative for catechol structures using the Arnow assay. The Csaky test for hydroxamate structures showed an increase in activity with increasing age of culture, reaching a maximal production of 87 nmolar N per ml supernatant in iron‐limited cultures of Frankia on day 65. Iron‐replete cultures were consistently negative for the presence of hydroxamates throughout their growth cycle. An HPLC assay for the detection of iron‐55 labelled siderophores indicated a single iron‐binding compound eluting at 12.3 minutes. This peak was first detected in iron‐limited culture supernatant during the early exponential stage of cell growth (circa day 14) and increased in size through late exponential to stationary phase (circa day 50). This increase was blocked by the addition of iron to iron‐limited cultures. Iron‐replete cultures did not show any iron‐binding activity over this same period of time. Based on this preliminary evidence we conclude that Frankia strain 52065 produces a hydroxamate siderophore in response to iron limitation in culture.