The iron-sulfur protein that transfers electrons from Photosystem I to NADP+ reductase is ferredoxin.
Photosystem I absorbs light and boosts the energy of incoming electrons. Those electrons pass through iron-sulfur centers within the photosystem before reaching soluble ferredoxin, a small iron-sulfur protein in the chloroplast stroma. Reduced ferredoxin then delivers electrons to ferredoxin–NADP+ reductase, also called FNR.
FNR uses two electrons from reduced ferredoxin, together with a proton, to reduce NADP+ to NADPH. NADPH supplies reducing power for carbon fixation and other biosynthetic reactions. Ferredoxin is therefore the mobile link between Photosystem I and the enzyme that produces NADPH.
A common mix-up is plastocyanin: it carries electrons to Photosystem I from the cytochrome b6f complex, whereas ferredoxin carries them away from Photosystem I. In cyclic electron flow, ferredoxin can instead help route electrons back into the photosynthetic electron-transport chain, contributing to ATP production without making NADPH.