Strains of synechocystis sp. PCC 6803 with altered PsaC. I. Mutations incorporated in the cysteine ligands of the two [4Fe-4S] clusters F(A) and F(B) of photosystem I

Two [4Fe-4S] clusters, F(A) and F(B), function as terminal electron carriers in Photosystem I (PS I), a thylakoid membrane-bound protein-pigment complex. To probe the function of these two clusters in photosynthetic electron transport, site-directed mutants were created in the transformable cyanobacterium Synechocystis sp. PCC 6803. Cysteine ligands in positions 14 or 51 to F(B) and F(A), respectively, were replaced with aspartate, serine, or alanine, and the effect on the genetic, physiological, and biochemical characteristics of PSI complexes from the mutant strains were studied. All mutant strains were unable to grow photoautotrophically, and compared with wild type, mixotrophic growth was inhibited under normal light intensity. The mutant cells supported lower rates of whole-chain photosynthetic electron transport. Thylakoids isolated from the aspartate and serine mutants have lower levels of PSI subunits PsaC, PsaD, and PsaE and lower rates of PS I- mediated substrate photoreduction compared with the wild type. The alanine and double aspartate mutants have no detectable levels PsaC, PsaD, and PsaE. Electron transfer rates, measured by cytochrome c₆-mediated NADP⁺ photoreduction, were lower in purified PSI complexes from the aspartate and serine mutants. By measuring the P700⁺ kinetics after a single turnover flash, a large percentage of the backreaction in the aspartate and serine mutants was found to be derived from A₁ and F(X), indicating an inefficiency at the F(X) -· F(A)/F(B) electron transfer step. The alanine and double aspartate mutants failed to show any backreaction from [F(A)/F(B)]^-. These results indicate that the various mutations of the cysteine 14 and 51 ligands to F(B) and F(A) affect biogenesis and electron transfer differently depending on the type of substitution, and that the effects of mutations on biogenesis and function can be biochemically separated and analyzed.