Oxidation of protein thiolates is central to numerous redoxregulated 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.protein oxidation ͉ redox regulation ͉ S-thiolation ͉ sulfenamide

The manganese transport regulator (MntR) represses the expression of genes involved in manganese uptake in Bacillus subtilis. It selectively responds to Mn2+ and Cd2+ over other divalent metal cations including Fe2+, Co2+ and Zn2+. Previous work has shown that MntR forms binuclear complexes with Mn2+ or Cd2+ at two binding sites, labeled A and C, that are separated by 4.4 Å. Zinc activates MntR poorly and binds only to the A site, forming a mononuclear complex. The difference in metal binding stoichiometry suggested a mechanism for selectivity in MntR. Larger metal cations are strongly activating because they can form the binuclear complex, while smaller metal ions cannot bind with the geometry needed to fully occupy both metal-binding sites. To investigate this hypothesis, structures of MntR in complex with two other non-cognate metal ions, Fe2+ and Co2+, have been solved. Each metal forms a mononuclear complex with MntR with the metal ion bound in the A site, supporting the conclusions drawn from the Zn2+ complex. Additionally, we investigated two site-specific mutants of MntR, E11K and H77A, that contain substitutions to metal binding residues in the A site. While metal binding in each mutant is significantly altered relative to wild-type MntR, both mutants retain activity and selectivity for Mn2+ in vitro and in vivo. That observation, coupled with previous studies, suggests that the A and C sites both contribute to the selectivity of MntR.

SummaryThe Bacillus subtilis OhrR protein functions as a transcriptional repressor of the inducible peroxidase, OhrA. Derepression is mediated by the organicperoxide selective oxidation of an active site cysteine (C15). In the presence of cumene hydroperoxide (CHP), oxidation of OhrR leads to a sulphenic acid intermediate which reacts to form either a mixeddisulphide or a protein sulphenamide. These inactive forms of OhrR can be reactivated by thiol-disulphide exchange reactions allowing restoration of repression. Here, we demonstrate that linoleic acid hydroperoxide (LHP) is a potent oxidant for OhrR and even low levels lead to overoxidation of OhrR to cysteine sulphinic (and sulphonic) acid derivatives. Kinetic competition experiments indicate that further oxidation of the initial OhrR sulphenate product occurs at least 100-fold more rapidly with LHP than with CHP. Thus, depending on the oxidant, OhrR can be either reversibly oxidized or can instead function as a sacrificial regulator.

OhrR proteins can be divided into two groups based on their inactivation mechanism: 1-Cys (represented by Bacillus subtilis OhrR) and 2-Cys (represented by Xanthomonas campestris OhrR). A conserved cysteine residue near the amino terminus is present in both groups of proteins and is initially oxidized to the sulfenic acid. The B. subtilis 1-Cys OhrR protein is subsequently inactivated by formation of a mixed-disulfide bond with low-molecular-weight thiols or by cysteine overoxidation to sulfinic and sulfonic acids. In contrast, the X. campestris 2-Cys OhrR is inactivated when the initially oxidized cysteine sulfenate forms an intersubunit disulfide bond with a second Cys residue from the other subunit of the protein dimer. Here, we demonstrate that the 1-Cys B. subtilis OhrR can be converted into a 2-Cys OhrR by introducing another cysteine residue in either position 120 or position 124. Like the X. campestris OhrR protein, these mutants (G120C and Q124C) are inactivated by intermolecular disulfide bond formation. Analysis of oxidized 2-Cys variants both in vivo and in vitro indicates that intersubunit disulfide bond formation can occur simultaneously at both active sites in the protein dimer. Rapid formation of intersubunit disulfide bonds protects OhrR against irreversible overoxidation in the presence of strong oxidants much more efficiently than do the endogenous low-molecular-weight thiols.

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