A complex thiolate switch regulates the Bacillus subtilis organic peroxide sensor OhrR.

Oxidation of protein thiolates is central to numerous redox-regulated processes. Bacillus subtilis OhrR is an organic peroxide sensor that represses expression of an inducible peroxiredoxin, OhrA. Here, we present evidence that oxidation of the sole cysteine residue in OhrR leads to a sulfenic acid-containing intermediate that retains DNA-binding activity: further reaction to generate either a mixed disulfide (S-thiolation) or a protein sulfenamide (sulfenyl-amide) derivative is essential for derepression. Protein S-thiolation protects OhrR from overoxidation and provides for a facile regeneration of active OhrR by thiol-disulfide exchange reactions. The sulfenamide can also be reduced by thiol-disulfide exchange reactions, although this process is much slower than for mixed disulfides. Recovery of oxidized OhrR from B. subtilis identifies three distinct S-thiolated species, including mixed disulfides with a novel 398-Da thiol, cysteine, and CoASH. Evidence for in vivo formation of the sulfenamide derivative is also presented.