Zn2+ Increases Siderophore Production in Azotobacter vinelandii

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When Azotobacter vinelandii was grown in the presence of low levels of iron, the addition of 20 or 40 μM ZnSO4 caused earlier production of the catechol siderophores and a dramatic increase in the amount of azotobactin. The level of cellular iron was not significantly lowered in Zn2+ -grown cells, which suggested that Zn2+ was not causing more severe, or earlier, iron limitation. Also, Zn2+ did not appear to affect production of the high-molecular-weight outer membrane iron-repressible proteins that presumably function as ferrisiderophore receptors. Spectrophotometric examination of ion binding to the siderophores revealed that while the siderophores appeared to bind Zn2+, only in the case of azotochelin was iron unable to completely overcome any Zn2+ -induced changes in the absorption spectra. This appeared to rule out direct competition of Zn2+ with iron for binding to the siderophores. 55Fe uptake was depressed both in Zn2+ -grown cells and in Zn2+ -free cells to which Zn2+ was added during the uptake assay, except with azotobactin, with which the level of 55Fe uptake by Zn2+ -grown cells was close to control levels. These results suggested two possible sites where Zn2+ could be acting, one involving the biosynthesis of siderophores and possibly the genetic regulation of the iron assimilation system and the other involving an internal point common to iron assimilation by both high- and low-affinity iron uptake.

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