The role of cytochrome b6 in cyclic electron transport: Evidence for an energy-coupling site in the pathway of cytochrome b6 oxidation in spinach chloroplasts

Abstract

The problem of selecting reference wavelengths appropriate for the study of reductive and oxidative absorbance changes in cytochromes b 6 and ƒ is discussed. Action spectra for cytochrome b-563 photoreduction are consistent with this cytochrome functioning in a System I cyclic pathway, but do not exclude the possibility that the cytochrome might also have another function in a DCMU-sensitive pathway connected with Photosystem II. The dark oxidation of photoreduced cytochrome b-563 is inhibited by the quinone analog 2,5-dibromo-3-methyl-6-isopropylbenzoquinone (DBMIB), and restored by subsequent addition of plastoquinone. Diaminodurol causes a partial reduction of cytochrome b-563 in the dark. The photooxidation of diaminodurol-reduced cytochrome b-563 by System I is stimulated by addition of NH 4Cl or ADP, and under the same conditions cytochrome ƒ becomes more reduced. The increase in the amplitude of cytochrome b-563 photooxidation caused by addition of NH 4Cl is suppressed by the quinone analog DBMIB. The rate and amplitude of the photooxidation of dithionite-reduced cytochrome b 6 are also stimulated by addition of NH 4Cl, and this photooxidation is also inhibited by DBMIB. It is concluded that in the absence of added cofactors of cyclic electron transport the pathway of cytochrome b-563 oxidation includes plastoquinone and the coupling site between plastoquinone and cytochrome ƒ. In the presence of DBMIB, phenazine methosulfate (PMS) restores the dark oxidation of photoreduced cytochrome b-563 and also causes a decrease in the amplitude of cytochrome ƒ photooxidation. When DCMU is substituted for DBMIB, PMS has no effect on the amplitude of cytochrome ƒ photooxidation. It is concluded that in cyclic phosphorylation one function of PMS is to transfer electrons from cytochrome b 6 to System I either through the plastoquinone-cytochrome ƒ coupling site or in the presence of DBMIB through a bypass of this site.