Photosynthesis relies on the delicate interplay between a specific set of membrane-bound pigment-protein complexes that harvest and transport solar energy, execute charge separation, and conserve the energy. We have investigated the organization of the light-harvesting (LH) and reaction-center (RC) complexes in native bacterial photosynthetic membranes of the purple bacterium Rhodobacter sphaeroides by using polarized light spectroscopy, linear dichroism (LD) on oriented membranes. These LD measurements show that in native membranes, which contain LH2 as the major energy absorber, the RC-LH1-PufX complexes are highly organized in a way similar to that which we found previously for a mutant lacking LH2. The relative contribution of LH1 and LH2 light-harvesting complexes to the LD spectrum shows that LH2 preferentially resides in highly curved parts of the membrane. Combining the spectroscopic data with our recent atomic force microscopy (AFM) results, we propose an organization for this photosynthetic membrane that features domains containing linear arrays of RC-LH1-PufX complexes interspersed with LH2 complexes and some LH2 found in separate domains. The study described here allows the simultaneous assessment of both global and local structural information on the organization of intact, untreated membranes. membrane organization ͉ photosynthesis ͉ polarized spectroscopy P hotosynthetic purple bacteria are among the oldest species known (1, 2). Within the class of photosynthetic organisms, they have specialized in growing on low-energy photons under dim light conditions (3). The main characteristic of the photosystems of these bacteria is their ''less is more'' design: They are constructed by the repetitive use of identical protein units as building blocks, resulting in highly symmetric pigment-protein complexes (''rings'') with carefully tuned light-absorption properties. Protein-bound chromophores, carotenoids and bacteriochlorophylls (BChls), are the light-interacting functional groups that carry out the photosynthetic process: the conversion of light energy into a stable charge separation. The incoming light is collected by specialized lightharvesting (LH) complexes, and, by means of intra-and intercomplex energy transfer, the excitation energy is transported to a reaction center (RC). Here charge separation takes place, which fuels subsequent electron-and proton-transfer reactions that build up the transmembrane electrochemical gradient that is eventually used for the formation of ATP.Photosynthetic purple bacteria may contain two different types of pigment-protein complexes involved in energy-and electrontransfer reactions: the RC-LH1 core complexes and the peripheral light-harvesting complex 2 (LH2). Both LH complexes are composed of two concentric rings of roughly circularly arranged ␣-helices that bind the carotenoid and BChl pigments. The peripheral LH2 feeds its excitations into LH1, which surrounds the RC, enabling efficient energy transfer from the LH1 BChls to the primary RC energy acceptor͞electron...