Ferredoxin has been chemically cross-linked to thylakoids by using N-ethyl-3-(3-dimethylaminopropyl)-carbodiimide. The membranes thus treated became able to photoreduce cytochrome c and to catalyze the NADPH-cytochrome c reductase reaction without adding exogenous ferredoxin. Preincubation of thylakoids with an antibody against ferredoxin-NADP+ reductase before carbodiimide treatment or removal of the reductase by mild trypsin treatment after the cross-linking reaction did not alter the cytochrome c photoreduction activity of the treated membranes. Two independent binding sites of ferredoxin to thylakoids are thus inferred: one site is shown to be the membrane-bound reductase, the second is suggested to be at the level of the photosystem I complex.
