The structure of the photoreceptor unit of Rhodopseudomonas viridis

Abstract

The thylakoid membrane of Rhodopseudomonas viridis contains extensive, regular arrays of photoreceptor complexes arranged on a hexagonal lattice with a repeat distance of ˜130 Å. 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 Å. 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.