Ferredoxin thioredoxin reductase is a [4FE-4S] protein which plays an important role in the ferredoxin/thioredoxin regulatory chain. It converts an electron signal (photoreduced ferredoxin) to a thiol signal (reduced thioredoxin), regulating enzymes by reduction of specific disulphide groups. It catalyses the light-dependent activation of several photosynthetis enzymes. Ferredoxin thioredoxin reductase is a heterodimer of subunit a and subunit b. Subunit a is the variable subunit, and b is the catalytic chain. This family is the alpha chain.