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Sulfur oxidation of Paracoccus pantotrophus: the sulfur-binding protein SoxYZ is the target of the periplasmic thiol–disulfide oxidoreductase SoxS

Lehrstuhl für Technische Mikrobiologie, Fakultät Bio- und Chemieingenieurwesen, Technische Universität Dortmund, D-44221 Dortmund, Germany

Correspondence
Cornelius G. Friedrichcornelius.
friedrich{at}udo.edu

The periplasmic thiol–disulfide oxidoreductase SoxS is essential for chemotrophic growth of Paracoccus pantotrophus with thiosulfate. To trap its periplasmic partner, the cysteine residues of the CysXaaXaaCys motif of SoxS (11 kDa) were changed to alanine by site-directed mutagenesis. The disrupted soxS gene of the homogenote mutant G S was complemented with plasmids carrying the mutated soxS[C13A] or soxS[C16A] gene. Strain G S(pRD179.6[C16A]^S) displayed a marginal thiosulfate-oxidizing activity, suggesting that Cys13^S binds the target protein. Evidence is presented that SoxS specifically binds SoxY. (i) Immunoblot analysis using non-reducing SDS gel electrophoresis and anti-SoxS and anti-SoxYZ antibodies identified the respective antigens of strain G S(pRD179.6[C16A]^S) at the 25 kDa position, suggesting an adduct of about 14 kDa, close to the value expected for SoxY migration. (ii) A mutant unable to produce SoxYZ, such as strain G X(pRD187.7[C16A]^S), did not form a SoxS(C16A) adduct, while addition of homogeneous SoxYZ resulted in the 25 kDa adduct. (iii) The SoxY and SoxZ subunits were distinguished by site-directed mutagenesis of the cysteine residue in SoxZ. SoxYZ(C53S) formed the 25 kDa adduct with SoxS(C16A). These results demonstrate that the target of SoxS is the sulfur-binding protein SoxY of the SoxYZ complex. As SoxYZ is reversibly inactivated, SoxS may activate SoxYZ as a crucial function for chemotrophy of P. pantotrophus.