The central protein of the sulfur-oxidizing enzyme system of Paracoccus pantotrophus, SoxYZ, formed complexes with subunits associated and covalently bound. In denaturing SDS-polyacrylamide gel electrophoresis (PAGE) SoxY migrated at 12 and SoxZ at 16 kDa. SDS-PAGE of homogeneous SoxYZ without reductant separated dimeric complexes of 25, 29, and 32 kDa identified by the N-terminal amino acid sequences as SoxY-Y, SoxY-Z, and SoxZ-Z, and subunit cleavage by reduction suggested their linkage via protein disulfide bonds. SoxYZ was reversibly redox active between -0.25 and 0.2 V, as monitored by a combined electrochemical and FTIR spectroscopic approach. The dimanganese SoxB protein (58.611 Da) converted the covalently linked heterodimer SoxY-Z to SoxYZ with associated subunits which in turn aggregated to the heterotetramer Sox(YZ)(2). This reaction depended on time and the SoxB concentration, and demonstrated the interaction of these two Sox proteins. (C) 2003 Elsevier Inc. All rights reserved.