Functional Characterization of RseC in the SoxR Reducing System and Its Role in Oxidative Stress Response in<i> Escherichia coli</i>

The reducing system of SoxR consists of a putative electron transfer system encoded by the <i>rsxABCDGE</i> operon, RseC encoded from the unlinked <i>rpoE-rseABC</i> operon, and ApbE. RseC is composed of two transmembrane helices, with both the N-terminal and C-terminal domains located in the cytoplasm. The N-terminal domain has a four-cysteine motif, CX₅CX₂CX₅C, in the cytoplasm, with the latter three cysteines highly conserved in RseC homologs, allowing the SoxR reducer complex to function in <i>Escherichia coli</i>. These three cysteines can form an oxygen-sensitive Fe-S cluster when only the N-terminal domain is expressed in a truncated form. Without the C-terminal domain, RseC shows no significant difference in interaction with the SoxR reducer complex, but its ability to complement the function of an <i>rseC</i> mutant is greatly reduced. Additionally, the <i>rseC</i> mutant exhibits weak resistance to cumene hydrogen peroxide in the stationary phase and increased sensitivity to hydrogen peroxide in the exponential phase, independent of the SoxR regulon. This suggests that the full-length sequence of RseC is essential for its function and that it may have SoxR-independent additional roles.