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Volume 271, Number 52, Issue of December 27, 1996 pp. 33173-33175
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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COMMUNICATION:
The Redox State of the [2Fe-2S] Clusters in SoxR Protein Regulates Its Activity as a Transcription Factor

(Received for publication, September 16, 1996, and in revised form, October 28, 1996)

From the Department of Molecular and Cellular Toxicology, Harvard School of Public Health, Boston, Massachusetts 02115

SoxR protein is a redox-responsive transcription factor that governs a regulon of oxidative stress and antibiotic resistance genes in Escherichia coli. Purified SoxR contains oxidized [2Fe-2S] clusters and stimulates in vitro transcription of its target gene soxS up to 100-fold. SoxR transcriptional activity, but not DNA binding, is completely dependent on the [2Fe-2S] clusters; apo-SoxR prepared in vitro binds the soxS promoter with unchanged affinity but does not have transcription activity. Thus, modulation of the SoxR [2Fe-2S] clusters was proposed to control the protein's function in transcription. Here, we provide evidence that SoxR with reduced [2Fe-2S] clusters is inactive. Redox titration of purified SoxR revealed a midpoint potential of 285 ± 10 mV (pH 7.6). In vitro transcription assays showed that SoxR was inactivated when the [2Fe-2S] cluster was reduced (380 mV), and full activity was restored upon reoxidation (+100 mV). The results suggest that one-electron oxidation and reduction of the [2Fe-2S] cluster regulate SoxR transcriptional activity.

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