The periplasmic thioredoxin SoxS plays a key role in activation in vivo of chemotrophic sulfur oxidation of Paracoccus pantotrophus.

The significance of the soxS gene product on chemotrophic sulfur oxidation of Paracoccus pantotrophus was investigated. The thioredoxin SoxS was purified, and the N-terminal amino acid sequence identified SoxS as the soxS gene product. The wild-type formed thiosulfate-oxidizing activity and Sox proteins during mixotrophic growth with succinate plus thiosulfate, while there was no activity, and only traces of Sox proteins, under heterotrophic conditions. The homogenote mutant strain GBOmegaS is unable to express the soxSR genes, of which soxR encodes a transcriptional regulator. Strain GBOmegaS cultivated mixotrophically showed about 22 % of the specific thiosulfate-dependent O(2) uptake rate of the wild-type, and when cultivated heterotrophically it produced 35 % activity. However, under both mixotrophic and heterotrophic conditions, strain GBOmegaS formed Sox proteins essential for sulfur oxidation in vitro at the same high level as the wild-type produced them during mixotrophic growth. Genetic complementation of strain GBOmegaS with soxS restored the activity upon mixotrophic and heterotrophic growth. Chemical complementation by reductants such as L-cysteine, DTT and tris(2-carboxyethyl)phosphine also restored the activity of strain GBOmegaS in the presence of chloramphenicol, which is an inhibitor of de novo protein synthesis. The data demonstrate that SoxS plays a key role in activation of the Sox enzyme system, and this suggests that SoxS is part of a novel type of redox control in P. pantotrophus.