On the role of cytochrome b 559 in oxygen evolution in photosynthesis

The role of cytochrome bss9 in photosynthesis has remained enigmatic despite considerable efforts to elucidate its function [ 11. Some experiments show that cytochrome bss9 is closely associated with the oxidizing side of photosystem II, others indicate redox activity between the two photosystems, while still others suggest participation in a cycle around photosystem II. Part of the enigma of cytochrome b 559 is that its redox changes measured under normal physiological conditions are too small and too slow to be consistent with its expected role in the electron transfer chain between photosystem II and photosystem I. In order to observe appreciable light-induced changes of cytochrome bss9 it has generally been necessary to modify the chloroplasts in ways which alter the redox properties of the cytochrome or to use nonphysiological conditions, such as low temperature, so that the physiological significance of many of the experimental approaches has been open to question. It was first proposed that cytochrome b 559 functioned in the electron transport chain between the two photosystems on the basis of the reversible photooxidation by photosystem I and photoreduc-