Assignment of a kinetic component to electron transfer between iron–sulfur clusters FX and FA/B of Photosystem I

We studied the kinetics of reoxidation of the phylloquinones in Chlamydomonas reinhardtii Photosystem I using site-directed mutations in the PhQA-binding site and of the residues serving as the axial ligand to ec3A and ec3B chlorophylls. In wild type PS I, these kinetics are biphasic, and mutations in the binding region of PhQA induced a specific slowing down of the slow component. This slowing allowed detection of a previously unobserved 180-ns phase having spectral characteristics that differ from electron transfer between phylloquinones and FX. The new kinetic phase thus reflects a different reaction that we ascribe to oxidation of FX− by the FA/B FeS clusters. These absorption changes partly account for the differences between the spectra associated with the two kinetic components assigned to phylloquinone reoxidation. In the mutant in which the axial ligand to ec3A (PsaA–Met688) was targeted, about 25% of charge separations ended in P700+A0− charge recombination; no such recombination was detected in the B-side symmetric mutant. Despite significant changes in the amplitude of the components ascribed to phylloquinone reoxidation in the two mutants, the overall nanosecond absorption changes were similar to the wild type. This suggests that these absorption changes are similar for the two different phylloquinones and that part of the differences between the decay-associated spectra of the two components reflect a contribution from different electron acceptors, i.e. from an inter-FeS cluster electron transfer.