Restoration by tetramethyl- p-phenylenediamine of photosynthesis in dibromothymoquinone-inhibited cells of the cyanobacterium Synechococcus sp.

Abstract

Inhibition of electron flow from water to methyl viologen by 2,5-dibromo-3-methyl-6-isopropyl- p-benzoquinone (DBMIB) was reversed by the addition of N, N, N′, N′-tetramethyl- p-phenylenediamine (TMPD) to cells of a thermophilic cyanobacterium Synechococcus sp. The restored activity is not a simple Photosystem I reaction because it was suppressed by 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU). TMPD was also found to restore photosynthetic oxygen evolution in the DBMIB-poisoned cells. That the restored oxygen evolution is coupled with CO 2 reduction was indicated by its sensitivity not only to electron-transport inhibitors such as DCMU and phenylmercuric acetate, but also to an inhibitor of CO 2 fixation (KCN), uncouplers (carbonyl cyanide m-chlorophenylhydrazone, methylamine and ammonium chloride) and an energy-transfer inhibitor ( N, N′-dicyclohexylcarbodiimide). In the DBMIB-poisoned cells, TMPD accelerated only P-700 reduction, while leaving the reduction kinetics of cytochrome c-553 and cytochrome f inhibited, indicating that P-700 is the site of electron entry from TMPD to Photosystem I. It is concluded that TMPD restores photosynthetic electron transport by accepting electrons in the plastoquinone region and in turn donating them directly to P-700, and thereby bypassing cytochrome c-553 and DBMIB-blocked cytochrome b 6- f complexes.