SoxR, a [2Fe-2S] transcription factor, is active only in its oxidized form.

Abstract

SoxR protein is known to function both as a sensor and as a transcriptional activator for a superoxide response regulon in Escherichia coli. The activity of SoxR was tested by its ability to enable the transcription of its target gene, soxS, in vitro. The activity of the oxidized form was lost when its [2Fe-2S] clusters were reduced by dithionite under anaerobic conditions, and it was rapidly restored by autooxidation. This result is consistent with the hypothesis that induction of the regulon is effected by the univalent oxidation of the Fe-S centers of SoxR. In vivo, this oxidation may be caused by an alteration of the redox balance of electron chain intermediates that normally maintains SoxR in an inactive, reduced state. Oxidized SoxR was about twice as effective as reduced SoxR in protecting the soxS operator from endonucleolytic cleavage. However, this difference could not account for a greater than 50-fold difference in their activities and therefore could not support a model in which oxidation activates SoxR by enabling it to bind to DNA. NADPH, ferredoxin, flavodoxin, or ferredoxin (flavodoxin):NADP+ reductase could not reduce SoxR directly in vitro at a measurable rate. The midpoint potential for SoxR was measured at -283 mV.

Regulation of the soxRS Oxidative Stress Regulon

“…The first evidence in this regard was that the SoxR apoprotein was transcriptionally inactive in vitro (9). Subsequently, it was found that whereas SoxR ox is active in vitro (6,10), it can be inactivated by anaerobic reduction with dithionite and then rapidly restored by autooxidation (3). In this work, we obtained parallel findings in vivo.…”

“…Many deletion or substitution mutations of the C terminus result in a regulon-constitutive phenotype, provided that they spare the four cysteines (3,13,22). In strains carrying these mutations, SoxR is in an activated state even in the absence of an inducer of the regulon.…”

“…The model for the oxidative activation of SoxR implies that constitutively activated mutant proteins might be constitutively oxidized. It has been suggested, for example, that the mutations might alter a recognition site for a SoxR reductase (3,22).…”

“…[1][2][3], mediates an oxidative stress response that protects the cell against the superoxide anion radical (O 2 . ), nitric oxide, and redox cycling reagents, such as paraquat (methyl viologen), plumbagin, and PMS.…”

Preliminary structural studies on the multi‐ligand‐binding domain of the transcription activator, BmrR, from bacillus subtilis

“…The amino terminal region of BmrR contains a putative helixturn-helix motif and shows substantial sequence homology to the amino terminal DNA-binding domains of a family of bacterial regulatory proteins, which include SoxR from Escherichia coli (Gaudu & Weiss, 1996;Hidalgo et al, 1997), the mercury resistance operon regulator, MerR, of several bacterial species (Summers, 1992;Ansari et al, 1995;Utschig et al, 1995) and TipAL from Streptomyces Zividans (Holmes et al, 1993). Whereas the carboxy terminal activation domains of the latter two proteins bind a single specific ligand, BmrR is different in its high-affinity binding of the structurally dissimilar molecules, rhodamine (& = 1.4 p M ) and tetraphenylphosphonium (Kd = 100 p M ) (Ahmed et al, 1994).…”