EPR signals of oxidized plastocyanin in intact algae

(1) An electron paramagnetic resonance (EPR) signal was observed at g = 2.05 in the low temperature spectra of intact cells of green, red and blue-green algae and of spinach chloroplasts. The g-value and the shape of the signal were similar to that of purified, soluble plastocyanin. (2) The amount of the copper protein, determined from the EPR signal height, was estimated to be nearly the same in all the studied organisms on the basis of the concentration of chlorophyll. Furthermore, it was found that the amount of the copper protein, determined from the EPR signal height in spinach chloroplasts corresponds with that of plastocyanin as determined chemically by Katoh, S., Suga, I., Shiratori, I. and Takamiya, A. (1961) Arch. Biochem. Biophys. 94, 136-141. (3) Experiments with far-red and red illumination show that the site of the copper protein in vivo is in the electron transport pathway between Photosystems 1 and 2. Plastocyanin is not oxidized by illumination at 77 degrees K, indicating that no electron transfer occurs between the primary electron donor of Photosystem 1, P700, and plastocyanin at that temperature. Furthermore, the experiments suggest that in the intact cells of the studied algae, plastocyanin is not only reduced by Photosystem 2 but also by cyclic electron transport around Photosystem 1.