Spx mediates oxidative stress regulation of the methionine sulfoxide reductases operon in Bacillus subtilis

We characterized the methionine sulfoxide reductase genes msrA and msrB in Bacillus subtilis, forming an operon transcribed from a single sigma A-dependent promoter. The msrAB operon was specifically induced by oxidative stress caused by paraquat (PQ) but not by H2O2. Spx, a global oxidative stress regulator in B. subtilis, is primarily responsible for this PQ-specific induction of msrAB expression. In support of this finding, an spx deletion mutant is extremely sensitive to PQ, and increased expression of msrA was identified in a clpX mutant in which Spx accumulated. However, the Spx effect was also visible under conditions where the protein did not accumulate (PQ treatment), suggesting a specific molecular effect at the level of the Spx protein. Indeed, the CXXC motif of Spx was found essential for its function in the PQ-specific induction of msrAB expression. PQ caused a modification of Spx requiring at least one of the cysteines of the CXXC motif of Spx. The PQ modified form of Spx showed a dynamic change in vivo.The Spx mediated PQ-specific regulation pathway of the msrAB operon in B. subtilis is reported. Our results suggest that PQ induced the expression of msrAB partially through an oxidation on Spx via modification of its CXXC motif.Among the 20 protein-building amino acids, methionine has a special status. It is the first amino acid in all nascent peptides, and often retained in the mature polypeptides. Methionine residues in proteins are also involved in catalytic centers [1-5] and binding of metals, copper in particular [6,7]. However, its major role in the cell is sometimes attributed to the reactivity of its sulfur atom. Methionine is highly sensitive to reactive oxygen species (ROS) that modify it covalently, yielding methionine sulfoxide in two enantiomeric forms: R- and S-. Remarkably, this reaction is reversible and two non-homologous methionine sulfoxide reductases MsrA and MsrB can restore intact methionines from the S- and R- forms, respectively