The location of the mobile electron carrier ferredoxin in vascular plant photosystem I

In this study, we present the location of the ferredoxin-binding site in ph otosystem I from spinach. Image analysis of negatively stained two-dimensio nal crystals indicates that the addition of ferredoxin and chemical cross-l inkers do not significantly alter the unit cell, parameters (for untreated photosystem I, a = 26.4 nm, b = 27.6 nm, and gamma = 90 degrees, space grou p p22(1)2(1) and for ferredoxin cross-linked photosystem I, a = 26.2 nm, b = 27.2 nm, and gamma = 90 degrees, space group p22(1)2(1)), Fourier differe nce analysis reveals that ferredoxin is bound on top of the stromal ridge p rincipally interacting with the extrinsic subunits PsaC and PsaE, This loca tion would be accessible to the stroma, thereby promoting efficient electro n transfer away from photosystem I. This observation is significantly diffe rent from that of the ferredoxin binding site proposed for cyanobacteria, A model for the binding of ferredoxin in vascular plants is proposed and is discussed-relative to observations in cyanobacteria.