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Electron transport pathways in spinach chloroplasts. Reduction of the primary acceptor of Photosystem II by reduced nicotinamide adenine dinucleotide phosphate in the dark
Authors:John D Mills  David Crowther  Rudolf E Slovacek  Geoffrey Hind  Richard E McCarty
Institution:Biology Department, Brookhaven National Laboratory, Upton, NY 11973 U.S.A.
Abstract:Addition of NADPH to osmotically lysed spinach chloroplasts results in a reduction of the primary acceptor (Q) of Photosystem II. This reduction of Q reaches a maximum of 50% in chloroplasts maintained under weak illumination and requires added ferredoxin and Mg2+. The reaction is inhibited by (i) an antibody to ferredoxin-NADP+ reductase (EC 1.6.7.1), (ii) treatment of chloroplasts with N-ethylmaleimide in the presence of NADPH, (iii) disulfodisalicylidenepropanediamine, (iv) antimycin, and (v) acceptors of non-cyclic electron transport. Uncouplers of phosphorylation do not affect NADPH-driven reduction of Q.It is proposed that electron flow from NADPH to Q may occur in the dark by a pathway utilising portions of the normal cyclic and non-cyclic electron carrier sequences. The possible in vivo role for such a pathway in redox poising of cyclic electron transport and hence in controlling the ATP/NADPH supply ratio is discussed.
Keywords:Antimycin A  Chloroplast  Cyclic electron flow  Electron transport  Ferredoxin  Fluorescence yield  sulfo-DSPD  disulfodisalicylidenepropanediamine  Tricine  MalNEt  PS I  Photosystem I  PS II  Photosystem II
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