Trends in Biotechnology
Volume 19, Issue 3, 1 March 2001, Pages 109-114
Journal home page for Trends in Biotechnology

Review
Redox-operated genetic switches: the SoxR and OxyR transcription factors

https://doi.org/10.1016/S0167-7799(00)01542-0Get rights and content

Abstract

Two redox-responsive transcription regulators have been well defined in Escherichia coli and serve as paradigms of redox-operated genetic switches. SoxR contains iron–sulfur centers that activate the protein when they are one-electron oxidized, or nitrosylated by nitric oxide. OxyR contains a pair of redox-active cysteine residues that activate the protein when they are oxidized to form a disulfide bond.

Section snippets

The SoxR and SoxS proteins: partners in redox-sensing and global transcription activation

The exposure of E. coli to sublethal doses of superoxide-generating agents, such as paraquat and menadione, renders the cells resistant to higher doses of these agents 4. The inducible resistance to superoxide-generating agents depends on the integrity of the soxRS locus [5], [6]. The soxR and soxS genes encode two separate transcription activators that participate in a two-step activation cascade that ultimately modulates more than 16 other genes (Fig. 1; see 2 for a more detailed review). The

SoxR: a transcription activator with iron–sulfur clusters

SoxR is a 17-kDa polypeptide that belongs to the MerR family of transcriptional activators [9], [10]. The primary structure of SoxR predicts a helix-turn-helix motif in the amino end, and this region of proteins in the MerR family appears to confer sequence-specific DNA binding. SoxR forms a dimer in solution, with each monomer containing a [2Fe–2S] cluster [11], [12] (Fig. 2). The ligands for metal binding in SoxR are the only four cysteine residues in the polypeptide, in a cx2cxcx5c sequence

Sensing free radical stress: distinct activation pathways for SoxR by superoxide stress or NO

The [2Fe–2S] clusters in SoxR can undergo reversible one-electron oxidation and reduction. Because the reduced [2Fe–2S] clusters are paramagnetic, these changes can be followed quantitatively by electron paramagnetic resonance (EPR) spectroscopy [11], [12]. Although the normal in vivo level of SoxR (50–100 molecules per cell) is too low to detect by EPR, the signal for reduced SoxR can be detected in intact E. coli when SoxR is expressed at ∼1% of total cellular protein, under which conditions

OxyR: hydrogen peroxide sensor and transcription activator

In parallel with the effects of superoxide-generating agents, low levels of hydrogen peroxide (H2O2) trigger peroxide resistance in E. coli 27. Much of this inducible resistance depends on the oxyR gene, which governs a set of genes that constitute the oxyR regulon 28. The OxyR protein belongs to the LysR family of transcription factors 29, and it can also respond to nitrosothiols 30. The OxyR-activated genes include dps (a DNA- and iron-binding protein), gorA (GSH reductase), grxA

Redox sensing by OxyR: activation by intramolecular disulfide bond formation

The OxyR polypeptide has a mass of 34 kD, and forms tetramers in solution (Fig. 3). In vivo transcriptional activation by OxyR in response to H2O2 depends on two cysteine residues (Cys199 and Cys208). Mass spectrometry analysis and proteolytic studies showed that, upon treatment of purified OxyR with H2O2, Cys199 and Cys208 form a disulfide bond 39. Because oxidized OxyR does not seem to form covalently crosslinked multimers 35, the Cys199–Cys208 disulfide is intramolecular. In vitro, the

Biotechnology applications of SoxR and OxyR?

An obvious application of the soxRS and oxyR systems is for toxicology testing to detect compounds that cause cellular oxidative stress. Indeed, companies now use these systems to test their own products or provide systems to test the products of other companies. Note that this type of application can be somewhat generic, in that a broad array of structurally distinct compounds can generate the in vivo signals to activate SoxR. This generality can be useful in providing a ‘first-pass’ sorting

Acknowledgments

Work in the authors’ laboratory has been supported by the National Cancer Institute of the US National Institutes of Health.

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