The cysteine-proximal aspartates in the F-X-binding niche of photosystem I- Effect of alanine and lysine replacements on photoautotrophic growth, electron transfer rates, single-turnover flash efficiency, and EPR spectral properties

The F-X electron acceptor in Photosystem I (PS I) is a highly electronegati ve (E-m = -705 mV) interpolypeptide [4Fe-4S] cluster ligated by cysteines 5 56 and 565 on PsaB and cysteines 574 and 583 on PsaA in Synechocystis sp, P CC 6803. An aspartic acid is adjacent to each of these cysteines on PsaB an d adjacent to the proline-proximal cysteine on PsaA We investigated the eff ect of D566(saB) and D557(PsaB) on electron transfer through F-X by changin g each aspartate to the neutral alanine or to the positively charged lysine either singly (D566A(PsaB), D557A(Psab), D566K(PsaB), and D557K(PsaB)) or in pairs (D557A(PsaB)/D566A(PsaB) and D557K(PsaB)/D566A(PsaB)). All mutants except for D557K(PsaB)/D566A(PsaB) grew photoautotrophically, but the grow th of D557K(PsaB) and D557A(PsaB)/D566A(PsaB) was impaired under low light. The doubling time was increased, and the chlorophyll content per cell was lower in D557K(PsaB) and D557A(PsaB)/D566A(PsaB) relative to the wild type and the other mutants. Nevertheless, the rates of NADP(+) photoreduction in PS I complexes from all mutants were no less than 75% of that of the wild type. The kinetics of back-reaction of the electron accepters on a single-t urnover flash showed efficient electron transfer to the terminal accepters F-A and F-B in PS I complexes from all mutants. The EPR spectrum of F-X was identical to that in the wild type in all but the single and double D566A( PsaB) mutants, where the high-field resonance was shifted downfield. We con clude that the impaired growth of some of the mutants is related to a reduc ed accumulation of PS I rather than to photosynthetic efficiency. The chemi cal nature and the charge of the amino acids adjacent to the cysteine ligan ds on PsaB do not appear to be significant factors in the efficiency of ele ctron transfer through F-X.