ACTIVE PHOTOSYNTHESIS IN CYANOBACTERIAL MUTANTS WITH DIRECTED MODIFICATIONS IN THE LIGANDS FOR 2 IRON-SULFUR CLUSTERS IN THE PSAC PROTEIN OF PHOTOSYSTEM-I

The PsaC protein of the Photosystem I (PSI) complex in thylakoid membr anes coordinates two [4Fe-4S] clusters, F-A and F-B. Although it is kn own that PsaC participates in electron transfer to ferredoxin, the pat hway of electrons through this protein is unknown. To elucidate the ro les of F-A and F-B, we created two site-directed mutant strains of the cyanobacterium Anabaena variabilis ATCC 29413. In one mutant, cystein e 13, a ligand for F-B was replaced by an aspartic acid (C13D); in the other mutant, cysteine 50, a ligand for F-A was modified similarly (C 50D). Low-temperature electron paramagnetic resonance studies demonstr ated that the C50D mutant has a normal F-B center and a modified F-A c enter. In contrast, the C13D strain has normal F-A, but failed to reve al any signal from F-B. Room-temperature optical studies showed that C 13D has only one functional electron acceptor in PsaC, whereas two suc h accepters are functional in the C50D and wild-type strains. Although both mutants grow under photoautotrophic conditions, the rate of PSI- mediated electron transfer in C13D under low light levels is about hal f that of C50D or wild type. These data show that (i) F-B is not essen tial for the assembly of the PsaC protein in PSI and (ii) F-B is not a bsolutely required for electron transfer from the PSI reaction center to ferredoxin.