The structure of the photoreceptor unit of Rhodopseudomonas viridis.

The thylakoid membrane of Rhodopseudomonas viridis contains extensive, regular arrays of photoreceptor complexes arranged on a hexagonal lattice with a repeat distance of 130 A. Single membrane sheets were obtained by mild treatment of the thylakoid fraction with the detergent Triton X-100. Heavy metal shadowing and electron microscopy of isolated thylakoids indicated a strong asymmetry of the membrane, showing a smooth plasmic and a rough exoplasmic side. Fourier processing of rotary-shadowed specimens showed the different surface relief on both sides of the membrane. Structural units on both sides were roughly circular and showed 6-fold symmetry at a resolution close to 20 A. The structural unit was characterised by a central core that seemed to extend through the membrane, protruding on the exoplasmic side. The core was surrounded by a ring showing 12 subunits on the plasmic side. Rotary-shadowed as well as negatively-stained membranes indicated a handedness of the structure. Treatment of thylakoid vesicles with higher detergent concentrations yielded a fraction of particles showing the same features as Fourier maps of the structural units. The isolated particles therefore appeared to represent structurally intact units of photosynthesis.