Abstract

The photosynthetic membrane of the purple bacterium Rhodopseudomonas (Rps.) acidophila is composed of reaction centers (RCs) which are surrounded by closely connected light harvesting complexes (LH1) and peripheral light-harvesting complexes (LH2). Both LH1 and LH2 – which bind the antenna pigments between α-, β-heterodimers – form rings composed of an integer number of α-, β-subunits. Here we use the sigmoidicity of fluorescence induction curves to probe the excitonic connectivity of RCs in order to gain information on the structural arrangement of these LH complexes in the natural chromatophore membrane. The data exclude models of the Rps. acidophila photosynthetic unit that assume aggregates of RC-LH1 complexes or linear chains of RC-LH1 complexes to which LH2 complexes are attached on the periphery. Rather, they support the model suggested by Papiz et al. ((1996) Trends in Plant Science 1: 198–206) in which peripheral light-harvesting rings tightly surround each core complex (LH1-ring with the RC inside) circumferentially.

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