PSAC SUBUNIT OF PHOTOSYSTEM-I IS ORIENTED WITH IRON-SULFUR CLUSTER F-B AS THE IMMEDIATE ELECTRON-DONOR TO FERREDOXIN AND FLAVODOXIN

The PsaC subunit of photosystem t (PS I) binds two [4Fe-4S] clusters, F-A and F-B, functioning as electron carriers between F-X and soluble ferredoxin, To resolve the issue whether F-A, or F-B is proximal to F- X, we used single-turnover flashes to promote step-by-step electron tr ansfer between electron carriers in control (both F-A and F-B present) and HgCl2-treated (F-B-less) PS I complexes from Synechococcus so. PC C 6301 and analyzed the kinetics of P700(+) reduction by monitoring th e absorbance changes at 832 nm in the presence of a fast electron dono r (phenazine methosulfate (PMS)). In control PS I complexes exogenousl y added ferredoxin, or flavodoxin could be photoreduced on each flash, thus allowing P700(+) to be reduced from PMS. In F-B-less complexes, both in the presence and in the absence of ferredoxin or flavodoxin, P 700(+) was reduced from PMS only on the first flash and was reduced fr om F-X(-) on the following flashes, indicating lack of electron transf er to ferredoxin or flavodoxin. In the F-B-less complexes, a normal le vel of P700 photooxidation was detected accompanied by a high yield of charge recombination between P700(+) and F-A(-) in the presence of a slow donor, 2,6-dichlorophenol-indophenol. This recombination remained the only pathway of F-A(-) reoxidation in the presence of added ferre doxin, consistent with the lack of forward electron transfer, F-A(-) c ould be reoxidized by methyl viologen in F-B-less PS I complexes, alth ough at a concentration two orders of magnitude higher than is require d in wild-type PS I complexes, thus implying the presence of a diffusi on barrier. The inhibition of electron transfer to ferredoxin and flav odoxin was completely reversed after reconstituting the F-B cluster. U sing rate versus distance estimates for electron transfer rates from F -X to ferredoxin for two possible orientations of PsaC, we conclude th at the kinetic data are best compatible with PsaC being oriented with F-A as the cluster proximal to F-X and F-B as the distal cluster that donates electrons to ferredoxin.