Phosphorylation of nucleotides bound to chloroplast membranes and their role in photophosphorylation.

1. The initial rapid phosphorylation of membrane-bound ADP yields [gamma- 32P]-ATP. Long term illumination of chloroplasts resulted in the introduction of label in the beta position of ADP and ATP. 2. It is concluded that the initial acceptor in photophosphorylation is ADP and not AMP. The appearance of 32Pi in the beta position of the nucleotide fractions is probably the result of side reactions not directly involved in the photophosphorylation mechanism. 3. The inhibitor phlorizin affects similarly the phosphorylation of bound ADP and the net photophosphorylation reaction. 4. The uncoupler nigericin has different effects on the phosphorylation of membrane-bound ADP and the net photophosphorylation reaction. Dissipation of energy by low concentrations of the uncoupler affects primarily the step(s) of ATP release to the medium, while steps leading to the interconversion of Pi and ADP to form bound ATP are less sensitive and are inhibited by relatively higher uncoupler concentrations.