PARENT SESSION
Posters P4Ab Type II reaction centres: Acceptor side. Abstracts (272-288)

A functional study of isolated RC-LH1 complexes from R. sphaeroides : excitation transfer, electron transfer and quinone distribution. Comayras Frédéric^*,1, Jungas Colette¹, Lavergne Jérôme¹, ¹ UMR 6191 CNRS-CEA-Université de la Méditerranée, Saint-Paul-lez-Durance, France

ABSTRACT- Using mild detergent solubilization of the intracytoplasmic membrane and sucrose gradient centrifugation, one can isolate complexes of the reaction center together with its open ring of core LH1 antenna (RC-LH1 complexes). These complexes come up as monomers and dimers; the latter are probably representing the native structure in the membrane. They retain a fraction ( 25%) of the ubiquinone pool (thus 4-7 quinones/RC). We examined several functional aspects in this material. 1 . The connectivity of energy transfer between the two LH1 rings was investigated by measuring the dependence of the fluorescence yield on the fraction of closed RCs. This relationship was linear in monomers and hyperbolic in dimers. Analysis of the latter indicates that the LH1-LH1 excitation transfer causes no rate limitation. Interestingly, the result obtained for isolated RC-LH1 dimers is very similar to that obtained in chromatophores, suggesting that antenna connectivity in the membrane is predominantly due to intra-dimer transfer. 2 . The quinone pool retained in the complexes is rapidly photoreduced (in the presence of an added electron donor). Under saturating light, the full turnover of one quinone (i.e. its double reduction and replacement by a fresh quinone) requires about 5.3 ms (to be compared with 1.6 ms in the native membrane). Experiments with a variable fraction of inhibited RCs show that in dimeric complexes, the pool is readily shared by the two RCs. 3 . From the photoreduction kinetics of the pool under light-limiting conditions, one can determine the distribution law of the quinones over the complexes. This distribution was found systematically broader than predicted by Poisson's law, indicating a cooperative binding of the quinones to the complexes.

KEY WORDS: quinone, core-complex, antenna, purple bacteria