The light dependent uptake of N-methylphenazinium cations by the thylakoids of isolated chloroplasts.

The absorption of N-methylphenazinium methylsulfate (MP+ methylsulfate) in suspensions of envelope-free chloroplasts is reversibly lowered in the light. When the electron transport system of the chloroplasts is inhibited by 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU), the photobleaching reflects an uptake of MP+ into the thylakoids. Its magnitude is a function of the composition and of the pH of the suspension medium and, most importantly, is controlled by the availability of permeant anions which apparently accompany MP+ into the thylakoid as counterions. Consequently, the rate of the bleaching is strongly dependent on the permeability of the thylakoid to the available anion. At pH 7.5, the thylakoids of DCMU poisoned pokeweed chloroplasts are able to hold at least 6 MP+/chlorophyll. It is proposed that, in the presence of MP+, the light reaction of Photosystem I in DCMU-inhibited chloroplasts causes a conformational change of the membranes which exposes nucleophilic sites inside the thylakoids. These sites appear to have a high affinity for MP+, but may bind protons or other cations under certain experimental conditions. The uptake of MP+ has a hypochromic effect on its absorption band in the near ultraviolet due to the resulting heterogeneous distribution of the dye cation between medium and chloroplast grana.