Ferric uptake regulators (Fur) from Vibrio cholerae and Helicobacter pylori bind a [2Fe–2S] cluster in response to elevation of intracellular free iron content

Abstract

Intracellular iron homeostasis in bacteria is primarily regulated by ferric uptake regulator (Fur). Since its discovery, Fur has been assumed to bind ferrous iron and regulate expression of target genes. However, the iron-bound Fur has never been isolated from any bacteria. In previous studies, we have shown that Escherichia coli Fur and Haemophilus influenzae Fur bind a [2Fe–2S] cluster via the conserved Cys-93 and Cys-96 when expressed in the E. coli mutant cells in which intracellular free iron content is elevated. Here we report that Fur homologs from Vibrio cholerae and Helicobacter pylori which contain Cys-93 and Cys-96 can also bind a [2Fe–2S] cluster. On the other hand, Fur homolog from Magnetospirillum gryphiswaldense MSR-1 which has no cysteine residues fails to bind any [2Fe–2S] clusters. Interestingly, different Fur proteins with the conserved Cys-93 and Cys-96 have distinct binding activities for the [2Fe–2S] cluster, with H. influenzae Fur having the highest, followed by E. coli Fur, V. cholera Fur, and H. pylori Fur. Binding of the [2Fe–2S] cluster in the Fur proteins is significantly decreased when expressed in wild-type E. coli cells, indicating that binding of the [2Fe–2S] clusters in Fur proteins is regulated by the levels of intracellular free iron content. Finally, unlike the [2Fe–2S] clusters in E. coli ferredoxin, the [2Fe–2S] clusters in the Fur proteins are not stable and quickly release ferrous iron when the clusters are reduced, suggesting that Fur may undergo reversible binding of the [2Fe–2S] cluster in response to intracellular free iron content in bacteria.