Removal of PsaF Alters Forward Electron Transfer in Photosystem I: Evidence for Fast Reoxidation of Q_K-A in Subunit Deletion Mutants of Synechococcus sp. PCC 7002

Recent studies of point mutations in photosystem I have suggested that the two kinetic phases of phylloquinone reoxidation represent electron transfer in the two branches of cofactors. This interpretation implies that changes in the relative amplitudes of the two kinetic phases represent a change in the extent of electron transfer in the two branches. Using time-resolved electron paramagnetic resonance (EPR), this issue is investigated in subunit deletion mutants of Synechococcus sp. PCC 7002. The spin-polarized EPR signals of P ₇₀₀⁺A₁^- and P₇₀₀ ⁺FeS^-, both at room temperature and in frozen solution, are altered by deletion of PsaF and/or PsaE, and the differences from the wild type are much more pronounced in PS I complexes isolated from the mutants using Triton X-100 rather than n-dodecyl β-D-maltopyranoside. The changes in the transient EPR data for the mutant complexes are consistent with a significant fraction of reaction centers showing (i) faster electron transfer from A ₁^- to Fx, (ii) slower forward electron transfer from A₀^- to A₁, and (iii) slightly altered quinone hyperfine couplings, possibly as a result of a change in the hydrogen bonding. The fraction of fast electron transfer and its dependence on the isolation procedure are estimated approximately from simulations of the room temperature EPR data, The results are discussed in terms of possible models for the electron transfer. It is suggested that the detergent-induced fraction of fast electron transfer is most likely due to alteration of the environment of the quinone in the PsaA branch of cofactors and is not the result of a change in the directionality of electron transfer.