Reconstitution of iron-sulfur center F_B results in complete restoration of NADP⁺ photoreduction in Hg-treated Photosystem I complexes from Synechococcus sp. PCC 6301

The F_B iron-sulfur cluster is destroyed preferentially by treating Photosystem I complexes with HgCl₂(Kojima Y, Niinomi Y, Tsuboi S, Hiyama T and Sakurai H (1987) Bot Mag 100: 243-53). When F_B is 95% depleted but F_Ais quantitatively retained in cyanobacterial PS I complexes, the reduction potential of F_A remains highly electronegative (E_m=-530 mV, n=1), the EPR spectral and spin relaxation properties of F_A and F_Xremain unchanged, but NADP⁺ photoreduction rates decline from 552 to 72 μmol mg Chl^-1 h^-1.When F_B is reconstituted with FeCl₃, Na₂S and β-mercaptoethanol, NADP⁺photoreduction rates recover to 528 μmol mg Chl^-1 h^-1. The correlation between the presence of F_Band NADP⁺ photoreduction provides direct experimental evidence that this iron-sulfur cluster is required for electron throughput from cytochrome c₆ to flavodoxin or ferredoxin in Photosystem I.