A chloroplast membrane conformational change activated by electron transport between the region of photosystem II and plastoquinone

Abstract

Various partial redox reactions involved in photosynthetic electron transport were studied in relation to the electron transport dependent incorporation of the water soluble chemical modifier, diazonium benzene sulfonic acid (DABS)* into chloroplast membranes. This electron transport dependent diazonium incorporation reflects a conformational change (unspecified at this time) in membrane components. The redox reaction(s) responsible for this conformational change was shown to be localized after the site of DCMU inhibition but before plastoquinone by the following evidence:

• 1. Electron transport from water to lipophilic “Class III” electron acceptors such as dimethyl benzoquinone and high concentrations of dibromothymoquinone potentiate the extra DABS binding to the membranes. These compounds are reduced prior to or at the plastoquinone site.

• 2. Electron transfer from water to silicomolybdate plus ferricyanide, a DCMU insensitive reaction, does not result in the incremental diazonium binding.

• 3. Photosystem I cyclic electron flow mediated by menadione (anaerobic), which requires participation of plastoquinone does not give the extra diazonium binding.

The exact redox step responsible for the conformational change is not known for certain, but there is a possibility that cytochrome b-559 may be involved. This is suggested by the observation that diazonium treatment of chloroplasts during illumination but not in darkness, causes the conversion of cytochrome b-559 from the high potential form to the low potential form.