Studies on electron transport associated with Photosystem I. I. Functional site of plastocyanin: Inhibitory effects of HgCl 2 on electron transport and plastocyanin in chloroplasts

Abstract

1. Incubation of chloroplasts with HgCl 2 at a molar ratio of HgCl 2 to chlorophyll of about unity, induced a complete inhibition of the methyl viologen Hill reaction, as well as methyl viologen photoreduction with reduced 2,6-dichlorophenolindophenol (DCIP) as electron donor. Photooxidation of cytochrome ƒ was similarly sensitive towards HgCl 2, whereas photooxidation of P700 was resistant to the poison. Photoreduction of cytochrome ƒ and light-induced increase in fluorescence yield were enhanced by the HgCl 2 treatment of chloroplasts. 2. Photooxidation of reduced yeast cytochrome c catalyzed by Photosystem I particles in the presence of plastocyanin was also suppressed by preincubation with HgCl 2. The inhibition was recovered by adding more plastocyanin, but not by addition of the particles, to the reaction mixture. 3. Incubation of reduced plastocyanin with HgCl 2 caused a bleaching of the blue color of the protein, whereas the oxidized protein was resistant to HgCl 2. A similar contrasting difference in sensitivity towards HgCl 2 was found in the Hill reaction in the absence and presence of ferricyanide. 4. Sonic treatment released plastocyanin from the untreated chloroplasts, whereas no plastocyanin was detected in the extract of the HgCl 2-treated chloroplasts. However, plastocyanin could be reconstructed by dialyzing the extract of the HgCl 2-treated chloroplasts against cysteine solution and then CuSO 4 solution. 5. The role of plastocyanin in the photosynthetic electron transport is discussed.