Phosphorylation coupled to non-cyclic electron flow in photosystem I

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Electron flow pathways in islated chloroplasts are associated with one or with both photosystems, and may be of a cyclic or a non-cyclic nature. Photoinduction of such electron flow paths is accompanied by the synthesis of ATP [I] . The localization of the sites of ATP formation along these electron flow pathways is a problem of great interest. A site of phosphorylation prior to the reduction of cytochrome fhas been established. It was suggested that this site is functional in the electron flow from water to ferricyanide or to NADP+ [2]. In contrast, controversy exists as to the phosphorylation accompanying the electron flow from ascorbate-DCIP to NADP’, in the presence of DCMU. This arises from a simultaneous cyclic phosphorylation reaction, which accompanies the noncyclic electron flow [3-51 . It was recently shown that in chloroplasts ATP inhibited both phosphorylation and the coupled electron flow [6]. Furthermore, this inhibition of electron flow was released in the presence of an uncoupler. Thus this specific effect of ATP on coupled electron flow can be used to determine whether the electron flow from ascorbate-DCIP to NADP+ is indeed coupled to phosphorylation. The present data support the concept of a phosphorylation site in the electron flow path from ascorbate to NADP’. In view of the evidence available that cytochrome f does not participate in this electron flow [2,7], it is concluded that two phosphorylation sites may operate in the electron flow from water to NADP’.