Component of the thioredoxin-thioredoxin reductase system. Participates in various redox reactions through the reversible oxidation of its active center dithiol to a disulfide and catalyzes dithiol-disulfide exchange reactions. Stimulates complex formation between sigma factor SigR and its cognate anti-sigma factor RsrA probably by reducing RsrA. Expressed from 2 promoters, 1 of which (trxBp1) is under control of SigR, and further transiently induced (about 50-fold) by the thiol-oxidizing agent diamide. Part of the trxB-trxA operon. Belongs to the thioredoxin family.