BACTERIAL ferredoxin was isolated from C. pasteurianum by Mortenson1 and characterized as a nonhaem iron protein. The term ferredoxin was subsequently broadened to cover other iron-sulphur proteins including those involved in the photosynthetic process in chlroplasts. All ferredoxins have similar electron paramagnetic resonance (EPR) spectra and very low E0 values in the neighbourhood of −0.4 V.