Light-induced absorbance changes of two cytochrome b components in the electron-transport system of spinach chloroplasts

Abstract

Light-induced absorbance changes of cytochromes b and f in chloroplasts have been measured in the cytochrome α-band region with a dual-wavelength spectrophotometer. The data indicate that there are b-type cytochromes at two different places in the electron-transport system. Whereas the cytochrome f changes induced by red (645 mμ) and far-red (715 mμ) light are typical of the two pigment system antagonism, the cytochrome b changes are more complex. With an actinic light intensity of 2–3·10 4 ergs·cm −2·sec −1 and no electron acceptor, the cytochrome b changes show two components, with difference spectra peaking at 563 and 560 mμ, respectively. Cytochrome b-563 is associated with a fast transient reduction upon illumination with far-red light. On turning off the far-red, cytochrome b-560 shows an oxidation which is reversible by red light. Cytochrome b-563 is oxidized in the dark and is not reduced by ascorbate, whereas cytochrome b-560, when oxidized, is ascorbate reducible. Both components are reduced by NADPH via a diaphorase. The cytochrome b-560 shows red, far-red reversibility at lower actinic light intensities and in the presence of carbonylcyanide m-chlorophenylhydrazone. It is concluded that cytochrome b-563 is reduced by Photosystem 1 and that cytochrome b-560 is in the electron transport chain between Photosystems 1 and 2.