Reduces sulfate for Cys biosynthesis. Substrate preference is adenosine-5'-phosphosulfate (APS) >> 3'-phosphoadenosine-5'-phosphosulfate (PAPS). Uses glutathione or DTT as source of protons. AMP + glutathione disulfide + 2 H(+) + sulfite = adenosine 5'-phosphosulfate + 2 glutathione Binds 1 [4Fe-4S] cluster. Stimulated by sodium sulfate > ammonium sulfate. A number of isoforms are produced. According to EST sequences. Leaves, roots and stem. By sulfate starvation. The C-terminal domain may function as glutaredoxin and mediates the interaction of the enzyme with glutathione (GSH). Active in GSH-dependent reduction of hydroxyethyldisulfide, cystine, dehydroascorbate, insulin disulfides and ribonucleotide reductase (By similarity). Belongs to the APS reductase family.