Transient EPR Studies of In Vivo Uptake of Substituted Anthraquinones by Photosystem I in Phylloquinone Biosynthetic Pathway Mutants of Synechocystis sp. PCC 6803

It is shown that for mutant strains of Synechocystis sp. PCC 6803, which are unable to synthesize phylloquinone, supplementing the growth medium with various substituted anthraquinones (AQs) allows the mutants to grow under high light conditions. Chromatographic analysis of PS I particles isolated from cells grown in this way show varying levels of incorporation of AQ. Here we present room temperature spin polarized transient EPR data for such particles and show that they are remarkably different from both the wild type and the mutants grown without AQ. At room temperature, the spectra reveal that the AQ incorporated into PS I is active in electron transport and the kinetic traces show that electron transfer to the iron sulfur clusters occurs with lifetimes that depend on which AQ has been incorporated. The spin polarization patterns also provide evidence that the rate of electron transfer from A0 to AQ is slow and that significant singlet-triplet mixing occurs in the primary radical pair.