Active photosynthesis in cyanobacterial mutants with directed modifications in the ligands for two iron-sulfur clusters in the PsaC protein of photosystem I

The PsaC protein of the Photosystem I (PSI) complex in thylakoid membranes coordinates two [4Fe-4S] clusters, F(A) and F(B). Although it is known that PsaC participates in electron transfer to ferredoxin, the pathway of electrons through this protein is unknown. To elucidate the roles of F(A) and F(B), we created two site-directed mutant strains of the cyanobacterium Anabaena variabilis ATCC 29413. In one mutant, cysteine 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 (C50D). Low-temperature electron paramagnetic resonance studies demonstrated that the C50D mutant has a normal F(B) center and a modified F(A) center. In contrast, the C13D strain has normal F(A), but failed to reveal any signal from F(B). Room-temperature optical studies showed that C13D has only one functional electron acceptor in PsaC, whereas two such 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 half that of C50D or wild type. These data show that (i) F(B) is not essential for the assembly of the PsaC protein in PSI and (ii) F(B) is not absolutely required for electron transfer from the PSI reaction center to ferredoxin.