Thiol-disulfide oxidoreductase which is required in disulfide reduction during c-type cytochrome synthesis. May accept reducing equivalents from CcdA, leading to breakage of disulfide bonds in apocytochrome c; following this reduction heme can be covalently attached. Protein modification; cytochrome c assembly. The thioredoxin-like motif is exposed on the outside of the membrane. Belongs to the thioredoxin family. ResA subfamily.