Recruitment of a Foreign Quinone into the A1 Site of Photosystem I CONSECUTIVE FORWARD ELECTRON TRANSFER FROM A0 TO A1 TO FX WITH ANTHRAQUINONE IN THE A1 SITE AS STUDIED BY TRANSIENT EPR*

In photosystem I (PS I), phylloquinone (PhQ) acts as a low potential electron acceptor during light-induced electron transfer (ET). The origin of the very low mid-point potential of the quinone is investigated by intro-ducing anthraquinone (AQ) into PS I in the presence and absence of the iron-sulfur clusters. Solvent extrac-tion and reincubation is used to obtain PS I particles containing AQ and the iron-sulfur clusters, whereas in-cubation of the menB rubA double mutant yields PS I with AQ in the PhQ site but no iron-sulfur clusters. Transient electron paramagnetic resonance spectros-copy is used to investigate the orientation of AQ in the binding site and the ET kinetics. The low temperature spectra suggest that the orientation of AQ in all samples is the same as that of PhQ in native PS I. In PS I con