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

The Fg 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-A is 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-X remain unchanged, but NADP(+) photoreduction rates decline from 552 to 72 mu mol mg Chl(-1) h(-1). When F-B is reconstituted with FeCl3, Na2S and beta-mercaptoethanol, NADP(+) photoreduction rates recover to 528 mu mol mg Chl(-1) h(-1). The correlation between the presence of F-B and NADP(+) photoreduction provides direct experimental evidence that this iron-sulfur cluster is required for electron throughput from cytochrome c(6) to flavodoxin or ferredoxin in Photosystem I.