Rational Tuning of Superoxide Sensitivity in SoxR, the [2Fe-2S] Transcription Factor: Implications of...

In Escherichia coli, the [2Fe-2S] transcriptional factor, SoxR, functions as a sensor of oxidative stress. The transcriptional activity in SoxR is regulated by the reversible oxidation and reduction of [2Fe-2S] clusters. We previously proposed that superoxide (O₂^•-) has a direct role as a signal for E. coli SoxR and that the sensitivity of the E. coli SoxR response to O₂^•- is 10-fold higher than that of Pseudomonas aeruginosa SoxR. The difference between the two homologues reflects interspecies differences in the regulatory role of O₂^•- activation. To investigate the determinants of SoxR's sensitivity to O₂^•-, we substituted several amino acids that are not conserved among enteric bacteria SoxR homologues and investigated the interaction of SoxR with O₂^•- using pulse radiolysis. The substitution of E. coli SoxR Lys residues 89 and 92 with Ala residues (K89AK92A), located close to [2Fe-2S] clusters, dramatically affected this protein's reaction with O₂^•-. The second-order rate constant of the reaction was 3.3 × 10⁷ M^-1 s^-1, which was 10 times smaller than that of wild-type SoxR. Conversely, the corresponding substitution of Ala90 with Lys in P. aeruginosa SoxR increased the rate approximately 10-fold. In contrast, introductions of the Arg127Ser128Asp129 → Leu127Gln128Ala129 substitution into E. coli SoxR, and the corresponding substitution (Leu125Gln126Ala127 → Arg125Ser126Asp127) in P. aeruginosa SoxR, did not affect the reaction rates. In addition, the Lys mutation in E. coli SoxR (K89AK92A) showed a defect in vivo transcriptional activity by measuring β-galactosidase expression in response to paraquat. Our findings clearly support the idea Lys is critical to the response to O₂^•- and further transcriptional activity of SoxR.