Kinetics of the photoreduction of cytochrome b-559 by photosystem II in chloroplasts.

The kinetics of the photoreduction of cytochrome b-559 and plastoquinone were measured using well-coupled spinach chloroplasts. High potential (i.e, hydroquinone reducible) cytochrome b-559 was oxidized with low intensity far-red light in the presence of N-methyl phenazonium methosulfate or after preillumination with high intensity light. Using long flashes of red light, the half-reduction time of cytochrome b-559 was found to be 100 +/- 10 ms, compared to 6-10 ms for the photoreduction of the plastoquinone pool. Light saturation of the photoreduction of cytochrome b-559 occurred at a light intensity less than one-third of the intensity necessary for the saturation of ferricyanide reduction under identical illumination conditions. The photoreduction of cytochrome b-559 was accelerated in the presence of dibromothymoquinone with a t 1/2 = 25-35 ms. The addition of uncouplers, which caused stimulatory effect on ferricyanide reduction under the same experimental conditions resulted in a decrease in the rate of cytochrome b-559 reduction. The relatively slow photoreduction rate of cytochrome b-559 compared to the plastoquinone pool implies that electrons can be transferred efficiently from Photosystem II to plastoquinone without the involvement of cytochrome b-559 as an intermediate. These results indicate that it is unlikely that high potential cytochrome b-559 functions as an obligatory redox component in the main electron transport chain joining the two photosystems.