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Plant and Cell Physiology, 1985, Vol. 26, No. 6 1057-1065
© 1985

Article

Comparison between Electron Transfers through Plastocyanin in Spinach Chloroplasts and Cytocbrome C2 in Rhodopseudomonas sphaeroides

Katsumi Matsuura1 and Shigeru Itoh

National Institute for Basic Biology Okazaki, Aichi 444, Japan

Photosynthetic electron transfers through the water-soluble peripheral membrane proteins of plastocyanin and cytochrome c2, were studied in spinach chloroplasts and the photosynthetic bacterium Rhodopseudomonas sphaeroides. In spinach chloroplasts, the rate of flash-induced oxidation of cytochrome f was highly dependent on the salt concentration in the suspending medium. The maximum rate with a half time of 200 µs was observed in the presence of 50 mas KCl or 5 mM MgCl2. The salt effect was similar to that on the reaction rate between P700 in thylakoid fragments and externally added plastocyanin. On the other hand, in intact cells of R. sphaeroides, in which cytochrome c2 is located in the periplasmic space exposed to the outer ionic environment, the rate of cytochrome c1 oxidation via cytochrome c2 was almost independent of salt concentration. This independence was a contrast to the strong dependence on salt concentration of reactions between isolated reaction centers and cytochrome c2 These results suggest that plastocyanin reacts collisionally with the photosystem I reaction center and cytochrome b6f complex in a manner that is controlled by the surface electrostatic potential. Cytochrome c2, on the other hand, reacts with the bacterial reaction center and cytochrome bc1 complex probably by forming a complex prior to activation of the reaction center.

1 Present address: Department of Biology, Faculty of Science, Tokyo Metropolitan University, Fukazawa 2-1-1, Setagaya, Tokyo 158, Japan.

(Received March 2, 1985; Accepted May 29, 1985)
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