IscR acts as an activator in response to oxidative stress for the suf operon encoding Fe‐S assembly proteins

Abstract

SummaryIn Escherichia coli, Fe-S clusters are assembled by gene products encoded from the isc and suf operons. Both the iscRSUA and sufABCDSE operons are induced highly by oxidants, reflecting an increased need for providing and maintaining Fe-S clusters under oxidative stress conditions. Three cis-acting oxidant-responsive elements (ORE-I, II, III) in the upstream of the sufA promoter serve as the binding sites for OxyR, IHF and an uncharacterized factor respectively. Using DNA affinity fractionation, we isolated an ORE-III-binding factor that positively regulates the suf operon in response to various oxidants. MALDI-TOF mass analysis identified it with IscR, known to serve as a repressor of the iscRSUA gene expression under anaerobic condition as a [2Fe-2S]-bound form. The iscR null mutation abolished ORE-III-binding activity in cell extracts, and caused a significant decrease in the oxidant induction of sufA in vivo. OxyR and IscR contributed almost equally to activate the sufA operon in response to oxidants. Purified IscR that lacked Fe-S cluster bound to the ORE-III site and activated transcription from the sufA promoter in vitro. Mutations in Fe-S-binding sites of IscR enabled sufA activation in vivo and in vitro. These results support a model that IscR in its demetallated form directly activates sufA transcription, while it de-represses isc operon, under oxidative stress condition.

Inefficient translation of nsrR constrains behaviour of the NsrR regulon in Escherichia coli

“…Further, there is some response to NO in an nsrR mutant, although the magnitude is smaller than in nsrR + strains (Table 4). Fur and IscR binding sites overlap in the suf promoter region (Lee et al, 2008), inhibition of Fur by NO (D'Autréaux et al, 2002) may lead to activation of the suf promoter by apoIscR (Yeo et al, 2006). Our data, nevertheless, suggest that NsrR is a regulator of the sufA promoter and contributes to its upregulation by exposure to NO.…”

“…Isc is the major housekeeping activity, while the Suf system is important during oxidative stress or iron starvation (Outten et al, 2004). The suf operon is subject to complex regulation, with roles for OxyR, Fur, IscR and IHF in controlling activity of the suf promoter (Lee et al, 2003(Lee et al, , 2004(Lee et al, , 2008Yeo et al, 2006). There is evidence that the suf genes are upregulated by exposure to NO (Justino et al, 2005;Pullan et al, 2007), and of an NsrR binding site in the sufA promoter (Partridge et al, 2009).…”

The unique regulation of iron-sulfur cluster biogenesis in a Gram-positive bacterium

“…IscR is a [2Fe-2S] cluster-containing transcriptional regulator encoded by the first gene of the iscRSUA-hscBA-fdx operon that regulates both ISC and SUF systems in E. coli and other Gram-negative bacteria (10,11). Apart from the sequence-unrelated SufR found in cyanobacteria (31,32), no IscR homolog was described in Grampositive bacteria, with the possible exception of some species of the Clostridium genus for which functional data are still lacking (30).…”

“…In T. potens, the TherJR_1914 gene encodes a protein of the Rrf2 family of transcriptional regulators, sharing only 37% identity with E. coli IscR but with full conservation of the cysteine residues known to coordinate the [2Fe-2S] cluster (Cys 92, 98, 104 , E. coli numbering) (Fig. 1C) (10).…”

“…According to the currently accepted model for Fe/S cluster biogenesis, under conditions unfavorable for Fe/S cluster formation, the labile IscR cluster is lost, and IscR-mediated repression of the isc (ironsulfur cluster) operon is alleviated. At the same time, apo-IscR activates the suf (sulfur assimilation) operon to further compensate for damage or loss of Fe/S clusters (9,10). Once the demand for Fe/S biogenesis is met, higher levels of cluster-containing holo-IscR exist, causing an increased repression of the ISC pathway.…”