RECONSTITUTION OF IRON-SULFUR CENTER FG RESULTS IN COMPLETE RESTORATION OF NADP(-TREATED PHOTOSYSTEM-I COMPLEXES FROM SYNECHOCOCCUS SP PCC-6301() PHOTOREDUCTION IN HG)

The Fg iron-sulfur cluster is destroyed preferentially by treating Pho tosystem 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 f rom 552 to 72 mu mol mg Chl(-1) h(-1). When F-B is reconstituted with FeCl3, Na2S and beta-mercaptoethanol, NADP(+) photoreduction rates rec over to 528 mu mol mg Chl(-1) h(-1). The correlation between the prese nce of F-B and NADP(+) photoreduction provides direct experimental evi dence that this iron-sulfur cluster is required for electron throughpu t from cytochrome c(6) to flavodoxin or ferredoxin in Photosystem I.