D-sedoheptulose 1,7-bisphosphate + H2O = D-sedoheptulose 7-phosphate + phosphate Binds 3 Mg(2+) ions per subunit. In continuous light, by inorganic phosphate, sed7P, glycerate and ribulose 1,5-bisphosphate. Carbohydrate biosynthesis; Calvin cycle. Homodimer. Expressed in all tissues examined except root. Highest levels found in leaves and flowers. Light activation through pH changes, Mg(2+) levels and also by light-modulated reduction of essential disulfide groups via the ferredoxin-thioredoxin f system. In etiolated seedlings, induction occurs only after 8 hours of illumination. Belongs to the FBPase class 1 family.