Correlation of reaction-center chlorophyll (P-700) oxidation and bound iron-sulfur protein photoreduction in chloroplast photosystem I at low temperatures.

The extent of P-700 photooxidation at 18 degrees K has been followed in three different chloroplast preparations (unfractionated chloroplasts and two preparations enriched in Photosystem I). More than 90% of P-700+ formation in all preparations was eliminated by the addition of sodium dithionite at pH 10. Photoreduction of a bound chloroplast iron-sulfur protein was also decreased by at least 90% under similar conditions. Electron paramagnetic resonance spectra of the chloroplast preparations in the presence of dithionite showed chemical reduction of bound iron-sulfur protein under conditions where primary photochemistry is eliminated. These results indicate that P-700 photooxidation is concomitant with photoreduction of a bound iron-sulfur protein and that this iron-sulfur protein functions as the primary electron acceptor of Photosystem I.