ANTENNA ORGANIZATION IN THE PURPLE BACTERIUM RHODOPSEUDOMONAS-ACIDOPHILA STUDIED BY FLUORESCENCE INDUCTION

The photosynthetic membrane of the purple bacterium Rhodopseudomonas ( Rps.) acidophila is composed of reaction centers (RCs) which are surro unded by closely connected light harvesting complexes (LH1) and periph eral light-harvesting complexes (LH2). Both LH1 and LH2 - which bind t he antenna pigments between alpha-, beta-heterodimers - form rings com posed of an integer number of alpha-, beta-subunits. Here we use the s igmoidicity of fluorescence induction curves to probe the excitonic co nnectivity of RCs in order to gain information on the structural arran gement of these LH complexes in the natural chromatophore membrane. Th e data exclude models of the Rps. acidophila photosynthetic unit that assume aggregates of RC-LH1 complexes or linear chains of RC-LH1 compl exes to which LH2 complexes are attached on the periphery. Rather, the y 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) circumferenti ally.