Photoreduction of the secondary Photosystem I electron acceptor vitamin K1 in intact spinach chloroplasts and Cyanobacteria in vivo

AbstractThe light-induced EPR changes in the g=2 region of intact spinach chloroplasts and of marine cyanobacteria Synechococcus PCC 7002 at room temperature have been analysed. Illumination after dark adaptation results in the well known transient oxidation of P700 and its subsequent re-reduction during the first 100 ms of the illumination, as monitored by EPR Signal I. Here we show, that after a distinct lag phase, coinciding with the re-reduction of P700, a second EPR signal is generated. This signal could be identified as the reduction signal of phylloquinone electron acceptor A1 by its asymmetric spectrum (g=2.0049, ΔHptp=1.00 mT) and by its sensitivity to conditions which suppress limitation of electron transport at the acceptor side of PS I during illumination. Since steady state reduction of A1 has to date been found only by photoaccumulation in prereduced PS I particles at cryogenic temperatures, this is the first observation of the species under physiological conditions. A1 reduction is shown to saturate with light intensity at a level of 0.5 per P700. The observation of substantial levels of reduced A1 in vivo should provide new insights into the mechanisms of energy dissipation and light regulation of the reaction centre of PS I under acceptor limited conditions