Ferredoxin Catalyzed Cyclic Photophosphorylation: Reversal of Dibromothymoquinone Inhibition by N,N,N',N'-Tetram ethyl- /j-phenylenediamine

Conditions for washed, spinach thylakoid membranes to catalyze cyclic phosphorylation using ferredoxin as the cofactor for electron transfer have been re-examined. It was found necessary to ‘'redox poise” the system; however, the best method to accomplish the poising seemed to be the use of a reductant (glucose-6-phosphate) and not by optimizing the rate of phosphorylation with 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU). Under these conditions, ferredoxin catalyzed cyclic phosphorylation was found to be sensitive to the inhibitors antimycin A and dibromothymo­ quinone (DBMIB). The inhibition of ferredoxin catalyzed cyclic phosphorylation by DBMIB, but not by antimycin A, was completely reversed by N, N, N', N'-tetramethyl-p-phenylenediamine (TM PD). These data are taken as further support for the function of plastoquinone in ferredoxin catalyzed cyclic phosphorylation. The effect of TMPD in reversing the DBMIB inhibition is inter­ preted as the formation of a TMPD bypass on the internal side of the thylakoid membrane around the DBMIB site of inhibition.