Thioredoxin-like protein TlpA from Bradyrhizobium japonicum is a reductant for the copper metallochaperone ScoI

► Protein-disulfide reductases are important in the oxidizing environment of the bacterial periplasm for keeping target proteins with vicinal cysteine residues in the reduced, dithiol state. ► The Bradyrhizobium japonicum TlpA protein is reported here to reduce the oxidized form of ScoI, a widespread copper-binding chaperone. ► By reducing ScoI, the TlpA protein prepares this target protein for the binding of copper, thus ensuring that the metal can subsequently become inserted into heme-copper cytochrome oxidases.