[thioredoxin]-dithiol + NADP(+) = [thioredoxin]-disulfide + H(+) + NADPH Binds 1 FAD per subunit. Homodimer. 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. The active site is a redox-active disulfide bond. Belongs to the class-II pyridine nucleotide-disulfide oxidoreductase family.