Diaminobenzidine an electron donor to photosystem 1 and to photosystem 2 in chloroplasts.

1. 3,3'-Diaminobenzidine was shown to serve as an electron donor to photosystem 1 in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. In Tris-treated chloroplasts diaminobenzidine serves as an electron donor to photosystem 1 and to photosystem 2; the latter is sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea. 2. Addition of diaminobenzidine to Tris-treated chloroplasts causes an increase in fluorescence yield. 3. Diaminobenzidine-dependent electron transport mediated by photosystem 2 is coupled to synthesis of ATP even in the absence of an electron acceptor. This phosphorylation which is presumably supported by cyclic electron flow, is sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea. 4. Diaminobenzidine-dependent ATP formation, in Tris-treated chloroplasts exhibits the red-drop phenomenon. 5. The diaminobenzidine-induced cyclic photophosphorylation (mediated by photosystem 2) is resistant to a large extent to KCN-treatment which is known to inhibit reactions catalyzed by photosystem 1. On the other hand ATP formation supported by electron transport from diaminobenzidine to methyl viologen [in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea] is largely inhibited by KCN-treatment. This observation suggests that there are two coupling sites of ATP formation, one catalyzed by diaminobenzidine as a donor to photosystem 1 (in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea), and the other supported by diaminobenzidine which acts both as a donor to photosystem 2 (in Tris-treated chloroplasts) and as an acceptor (in its oxidized form) from a carrier located between the two photosystems.