Photosystem II - mediated cyclic photophosphorylation.

DCMU-sensitive synthesis of ATP can be shown to continue in KCNtreated chloroplasts after cessation of 02 evolution. The catalyst for this reaction, k-phenylenediamine , also stimulates synthesis of ATP in NH OH-treated chloroplasts, but at much higher rates. This ATP synthesis can be o served 2 in the presence of the quinone antagonist dibromothymoquinone, and under the appropriate conditions it is completely sensitive to DCMU. Since neither uptake nor evolution of 02 can be observed during illumination, these results are interpreted as evidence for catalysis of cyclic photophosphorylation by photosystem I