Three-dimensional structure of a photosynthetic membrane

Abstract

The photosynthetic membranes in a variety of organisms are involved in the conversion of energy from sunlight into chemically useful forms^1–3. Rhodopseudomonas viridis is a purple non-sulphur photosynthetic bacterium offering unique advantages for the study of photosynthetic membranes. Its internal photosynthetic membranes form large flat sheets which, as first described by Giesbrecht and Drews⁴, contain subunits organized into a regular repeating pattern. The membranes are ideally suited to the use of Fourier image analysis techniques, and have been studied in two dimensions^5,6. We have now determined the three-dimensional structure of photosynthetic membranes of R. viridis by electron microscopy. A large central structure protrudes from both surfaces of the membrane, and is surrounded by six smaller centres of mass. Comparison of these data with studies on the polypeptide composition of the membrane suggests a membrane in which a photosynthetic reaction centre is surrounded by light-harvesting bacteriochlorophyll protein complexes.