Siderophore alcaligin E production by Alcaligenes eutrophus CH34: features of the ale-operon and application perspectives of alcaligin E for leaching of heavy metals

Abstract

In virtually all microorganisms, iron plays an irreplaceable role as cofactor for a variety of functional proteins and enzymes. Due to the formation of insoluble ferric hydroxide complexes under aerobic conditions and at neutral pH, the bioavailability of iron is severely restricted. Therefore, microorganisms have evolved specialised high affinity transport systems in order to obtain sufficient amounts of this essential element. Most bacteria have the ability to produce and excrete siderophores, small compounds exhibiting very high affinity for ferric iron [21]. A cognate-specific transport system mediates the uptake of the ferric-siderophore complex into the cell [13, 22]. In general, the biosynthesis of the siderophore and associated transport machinery is initiated under conditions of iron limitation. In E. coli the Fur protein acts as a transcriptional repressor of iron regulated genes: when complexed to divalent ferrous ions, Fur binds to a specific DNA-sequence (Fur-box) located in the promoter regions of iron-responsive genes and inhibits transcription [9]. All bacteria studied to date possess a fur-like gene. In addition to the regulation by Fur, some bacteria possess additional regulatory elements. For example, in Vibrio anguillarum positive regulation as well as antisense RNA have been implicated in the production and transport of its siderophore, anguibactin [25, 311. In several Pseudomonas species positive regulators involved in siderophore-mediated iron acquisition have been described [30]. In P aeruginosa 7NSK2, a site-specific recombinase encoded by a gene called sss (stress induced siderophore synthesis) was found to be involved in the regulation of pyoverdin synthesis by Zn²⁺ [15].