Mutational analysis of photosystem I polypeptides in Synechocystis sp. PCC 6803: Subunit requirements for reduction of NADP⁺ mediated by ferredoxin and flavodoxin

The subunit requirements for NADP⁺ reduction by photosystem I were assessed in mutants of Synechocystis sp. PCC 6803 created by targeted inactivation of the psaD, psaE, psaF, and psaL genes. The PsaE-less, PsaF-PsaJ-less, and PsaL-less mutants showed normal photo-autotrophic growth, while the growth of PsaD-less mutants was slower without glucose. In isolated wild-type membranes, the rate of flavodoxin reduction and flavodoxin-mediated NADP⁺ reduction were 800 and 480 μmol/mg of chlorophyll/h, respectively. The rate of ferredoxin-mediated NADP⁺ photoreduction was 460 μmol/ mg of chlorophyll/h. There was no diminution in NADP⁺ photoreduction in membranes isolated from the PsaF-less and PsaL-less mutants. The rates of ferredoxin-mediated NADP⁺ photoreduction in membranes of the PsaE-less mutants were 25 μmol/mg of chlorophyll/h. However, the rate of flavodoxin reduction was 380 μmol/mg of chlorophyll/h, and that of flavodoxin-mediated NADP⁺ photoreduction was 170 μmol/mg of chlorophyll/h. PsaD-less membranes showed <20% of the wild-type rates of flavodoxin-mediated NADP⁺ photoreduction, but were completely deficient in ferredoxin-mediated NADP⁺ photoreduction. Therefore, the roles of PsaE and PsaD are more crucial for "docking" of ferredoxin than of flavodoxin. Proteolysis studies showed that while PsaD was susceptible to rapid in vitro degradation by thermolysin, the number and sizes of protease-resistant fragments were not affected by the absence of PsaE. Protease accessibility studies further indicated that the C-terminal domain of PsaD is surface-exposed on the n-side. These results suggest that PsaE and the C-terminal domain of PsaD generate the docking site for the electron acceptors of photosystem I.