The organization of the photosynthetic apparatus of Rhodobacter sphaeroides: Studies of antenna mutants using singlet-singlet quenching

Abstract

The organization of the light-harvesting system of the purple bacterium Rhodobacter sphaeroides was studied by use of mutants containing only one of the two light-harvesting pigment-protein complexes present in the wild type: B875 (LH1) and B800–850 (LH2). These mutants thus enabled us to study for the first time the properties of these complexes separately in their native environment. By measuring the yield of fluorescence caused by 35 ps, 532 nm flashes of varying energy density, the number of bacteriochlorophyll (BChl) molecules which constitute a domain for energy transfer was obtained as well as the rate constants for energy transfer. It appears that LH1 consists of clusters of about 100 BChl molecules, which, at room temperature, form large assemblies for energy transfer. This aggregation may be mediated by the presence of reaction centres. The LH2 complex also shows a strong tendency to aggregate to form large domains, although a fundamental structural unit of 30 connected BChl 850 and 15 BChl 800 molecules appears to exist. We also present evidence that in these mutants the LH1 and LH2 complexes are spectrally inhomogeneous and each contain an additional, long-wavelength-absorbing BChl.