Uphill energy transfer in LH2-containing purple bacteria at room temperature

Uphill energy transfer in the LH2-containing purple bacteria Rhodopseudomon as acidophila, Rhodopseudomonas palustris, Rhodobacter sphaeroides, Chromat ium vinosum and Chromatium purpuratum was studied by stationary fluorescenc e spectroscopy at room temperature upon selective excitation of the B800 pi gments of LH2 and the B880 pigments of LH1 at 803 nm and 900 nm, respective ly. The resulting fluorescence spectra differed significantly at wavelength s shorter than the fluorescence maximum but agreed at longer wavelengths. T he absorption spectra of the species studied were decomposed into five band s at approx. 800, 820, 830, 850 and 880 nm using the shapes of the absorpti on spectra of the LH1-RC only species Rhodospirillum rubrum and the isolate d B800-850 complex from Rps. acidophila strain 10050 as guide spectra. This allowed a quantification of the number of pigments in each pigment group a nd, consequently, the antenna size of the photosynthetic unit assuming 36 b acteriochlorophyll a molecules in an LH1-RC complex. In most of the LH2-con taining purple bacterial strains the number of LH2 rings per LH1-RC was les s than the idealized number of eight (Papiz et al., Trends Plant Sci. 1 (19 96) 198-206), which was achieved only by C. purpuratum. Uphill energy trans fer was assayed by comparing the theoretical fluorescence spectrum obtained from a Boltzmann equilibrium with the measured fluorescence spectrum obtai ned by 900 nm excitation. The good match of both spectra in all the purple bacteria studied indicates that uphill energy transfer occurs practically u p to its thermodynamically maximal possible extent, All strains studied con tained a small fraction of either poorly connected or unconnected LH2 compl exes as indicated by higher fluorescence yields from the peripheral complex es than predicted by thermal equilibration or kinetic modeling. This impede s generally the quantitative analysis of blue-excited fluorescence spectra. (C) 1999 Elsevier Science B.V. All rights reserved.