THE ACCESSORY ELECTRON-DONOR TYROSINE-D OF PHOTOSYSTEM-II IS SLOWLY REDUCED IN THE DARK DURING LOW-TEMPERATURE STORAGE OF ISOLATED THYLAKOIDS

Storage of thylakoids for several months at 203 K in the dark changes the flash pattern of oxygen evolution by gradually shifting the first oxygen maximum from the third flash, where it is usually observed, to the fourth flash. This effect is accompanied with the increase of a fast phase (t 1 2 = 1.5 s) in the decay of the S2 and S3 states of the water-oxidizing complex in Photosystem II. In parallel to the changes in the oxygen flash pattern the EPR signal from the stable tyrosine-D+ radical (Signal IIslow) completely disappears with a half-time of approx. 13 weeks. The normal oxygen yield sequence, showing the first maximum at the third flash, as well as the original amplitude of Signal IIslow can be restored by a single flash or by continuous illumination at room temperature. These data show that tyrosine-D+ is reduced by an endogenous redox component in Photosystem II during dark storage of thylakoids at 203 K. In parallel with the reduction of tyrosine-D+ we observed the oxidation of high potential cytochrome b-559, and it is proposed that at low temperature an electron can be transferred from high-potential cytochrome b-559 to tyrosine-D+ in a slow reaction in most of the centers. © 1990.