Fur protein

The Fur protein regulates iron uptake systems in many Gram-negative bacteria. The striking phenotype of the first fur mutants isolated was the overexpression of the outer membrane receptors for siderophore iron transport. In addition, excretion of siderophores under iron-rich growth conditions was observed in these mutants, indicating that the biosynthesis of siderophores is also regulated by Fur. 

The Fur protein from E. coli was isolated in one step due to its high affinity for metal-chelate columns loaded with zinc. In DNase footprinting experiments, the Fur protein was shown to bind DNA in the promoter region of several iron-regulated genes. The consensus sequence, called the Fur box, is GATAATGATAATCATT ATC. In vitro binding is dependent on the divalent cations Co2+ Mn2+ /s Cd2+ Cu2+ at 150 iM, while Fe2+ seemed to be less active at this concentration, probably due to oxidation to Fe3+ (De Lorenzo et al., 1987). The unspecificity for divalent metals observed in vitro shows that the cells have to select the ions transported carefully and have to balance their active concentrations. In addition, it is a caveat for the experimenter to test a hypothesis on metal-ion specificity not only in vitro, but also in vivo. 

In the Gram-positive bacterium Bacillus subtilis (DNA with low G + C content), three Fur-like proteins have been characterized (Bsat et al., 1998). One, called Fur, regulates mainly iron uptake and siderophore biosynthesis. A second one, called PerR, regulates peroxide stress response genes and acts with manganese as corepressor. A third one, Zur, regulates genes for zinc uptake. The Zur protein found in E. coli shows only 25 % identity to the B. subtilis Zur, while the two Fur proteins have 32 % identical amino acids. 

Once the siderophore-iron complexes are inside the bacteria, the iron is released and utilized for vital cell functions. The iron-free hydroxamate siderophores are commonly re-excreted to bring in an additional iron load (Enterobactin is at least partially degraded by a cytoplasmic esterase This cycle is repeated until specific intracellular ferric uptake regulation proteins (Fur proteins) bind iron, and signal that the intracellular iron level is satisfactory, at -which point ne-w siderophore and siderophore-receptor biosynthesis are halted and the iron-uptake process stops. This intricate feedback mechanism allows a meticulous control over iron(III) uptake and accumulation against an unfavorable concentration gradient so as to maintain the intracellular iron(III) level within the required narrow window. Several excellent reviews concerning siderophore-iron transport mechanisms have been recently published i.3,i6, is,40,45,60-62 ... 

Fungicides, 18 177, 244 fur protein, 36 414-415 Fuscoredoxin, 47 485 in sulfur-reducing bacteria, 47 378-382 Fused-ring nitrides... 

Bccau.se they arc ionic in solution, proteins bind with cations and anions depending on the pH of the environment. Sometimes, complex salts arc formed, and precipitation lakes place (e.g.. irichloroacclic acid is a precipitating agent fur proteins and is used fur dcprolcinizing. solutions). 

The x-ray structure of Fur from Pseudomonas aeruginosa, solved recently [64], is the first structure of a Fur protein. In this organism the protein has only one sulf-hydryl, and it lacks the domain with the high-affinity zinc site of the E. coli homolog. Still, it was found to have two metal sites, one (O4/N2) octahedral site for iron and one tetrahedral (O2/N2) site occupied by zinc. All the metal-binding amino acids are conserved in the P. aeruginosa and E. coli homologs, so an unidentified... 

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