Catalytic subunit of the ferredoxin-thioredoxin reductase (FTR), which catalyzes the two-electron reduction of thioredoxins by the electrons provided by reduced ferredoxin. [thioredoxin]-disulfide + 2 H(+) + 2 reduced [2Fe-2S]-[ferredoxin] = [thioredoxin]-dithiol + 2 oxidized [2Fe-2S]-[ferredoxin] Binds 1 [4Fe-4S] cluster. Heterodimer of subunit A (variable subunit) and subunit B (catalytic subunit). Heterodimeric FTR forms a complex with ferredoxin and thioredoxin (By similarity). Belongs to the ferredoxin thioredoxin reductase beta subunit family.