Cyt b-559 (Fd) participating in cyclic electron transport in spinach chloroplasts: Evidence for kinetic connection with the Cyt b(6)/f complex

A small fraction of low potential Cyt b-559, amounting to only 13% of total Cyt b-559 in spinach chloroplasts, is analyzed with the help of a highly s elective, computer-controlled spectrophotometer, which simultaneously appli es 16 pulse modulated narrow band measuring beams with wavelengths in the c ytochrome a-band (500-600 nm) for recordings of time resolved difference sp ectra, This Cyt b-559 fraction remains oxidized upon dark incubation with a scorbate and is reduced upon illumination. It can be reduced by cyclic PSI in an antimycin A-sensitive reaction or in the course of antimycin A-insens itive linear electron transport via the Cyt b(6)/f complex. Reduction by NA DPH in the dark requires ferredoxin, Simultaneous recordings of Cyt b-563 a nd Cyt f reveal close kinetic connection between this Cyt b-559 fraction an d the low potential chain of the Cyt b(6)/f complex. These results confirm and extend previous observations of Miyake et al. 1995 (Plant Cell Physiol. 36: 743) in maize mesophyll thylakoids, which led to the hypothesis that C yt b-559 (Fd) occupies the position of the postulated ferredoxin-plastoquin one reductase (FQR) in cyclic electron transport.