Kinetics of the Oxidation of the [2Fe-2S] Cluster in SoxR by Redox-Active Compounds as Studied by Pulse Radiolysis.

SoxR containing a [2Fe-2S] cluster required for its transcription activity functions as a bacterial stress-response sensor that is activated through oxidation by redox-active compounds (RACs). SoxR from Escherichia coli (EcSoxR) is activated by nearly all RACs nonspecifically. In contrast, nonenteric SoxRs such as Pseudomonas aeruginosa (PaSoxR), and Streptomyces coelicolor (ScSoxR) activate their target genes in response to RAC including endogenously produced metabolites. To investigate the determinants of SoxR's activity, the endogenous or various synthetic RACs-mediated oxidation of the [2Fe-2S] cluster of EcSoxR, PaSoxR, and ScSoxR were measured by pulse radiolysis. Radiolytically generated hydrated electrons (e_aq^-) very rapidly reduced the oxidized form of the [2Fe-2S] cluster of SoxR. In the presence of RAC, a subsequent increase in absorption in the visible region corresponding to reoxidation of the [2Fe-2S] cluster was observed on a time scale of milliseconds. Both EcSoxR and PaSoxR reacted very rapidly (2.0 × 10⁸ to 2.0 × 10⁹ M^-1 s^-1) with various RACs, including viologen, phenazines, and quinones. No differences in kinetic behaviors were evident between EcSoxR and PaSoxR, whereas ScSoxR reacted with a limited range of RACs.