Chlorosomes: Structure, Function and Assembly (original) (raw)
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Biophysical Journal, 2004
Chlorosomes of green photosynthetic bacteria constitute the most efficient light harvesting complexes found in nature. In addition, the chlorosome is the only known photosynthetic system where the majority of pigments (BChl) is not organized in pigment-protein complexes but instead is assembled into aggregates. Because of the unusual organization, the chlorosome structure has not been resolved and only models, in which BChl pigments were organized into large rods, were proposed on the basis of freeze-fracture electron microscopy and spectroscopic constraints. We have obtained the first highresolution images of chlorosomes from the green sulfur bacterium Chlorobium tepidum by cryoelectron microscopy. Cryoelectron microscopy images revealed dense striations ;20 Å apart. X-ray scattering from chlorosomes exhibited a feature with the same ;20 Å spacing. No evidence for the rod models was obtained. The observed spacing and tilt-series cryoelectron microscopy projections are compatible with a lamellar model, in which BChl molecules aggregate into semicrystalline lateral arrays. The diffraction data further indicate that arrays are built from BChl dimers. The arrays form undulating lamellae, which, in turn, are held together by interdigitated esterifying alcohol tails, carotenoids, and lipids. The lamellar model is consistent with earlier spectroscopic data and provides insight into chlorosome self-assembly.
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 2013
Chlorosomes, the major antenna complexes in green sulphur bacteria, filamentous anoxygenic phototrophs, and phototrophic acidobacteria, are attached to the cytoplasmic side of the inner cell membrane and contain thousands of bacteriochlorophyll (BChl) molecules that harvest light and channel the energy to membranebound reaction centres. Chlorosomes from phototrophs representing three different phyla, Chloroflexus (Cfx.) aurantiacus, Chlorobaculum (Cba.) tepidum and the newly discovered "Candidatus (Ca.) Chloracidobacterium (Cab.) thermophilum" were analysed using PeakForce Tapping atomic force microscopy (PFT-AFM). Gentle PFT-AFM imaging in buffered solutions that maintained the chlorosomes in a near-native state revealed ellipsoids of variable size, with surface bumps and undulations that differ between individual chlorosomes. Cba. tepidum chlorosomes were the largest (133 × 57 × 36 nm; 141,000 nm 3 volume), compared with chlorosomes from Cfx. aurantiacus (120 × 44 × 30 nm; 84,000 nm 3) and Ca. Cab. thermophilum (99 × 40 × 31 nm; 65,000 nm 3). Reflecting the contributions of thousands of pigment-pigment stacking interactions to the stability of these supramolecular assemblies, analysis by nanomechanical mapping shows that chlorosomes are highly stable and that their integrity is disrupted only by very strong forces of 1000-2000 pN. AFM topographs of Ca. Cab. thermophilum chlorosomes that had retained their attachment to the cytoplasmic membrane showed that this membrane dynamically changes shape and is composed of protrusions of up to 30 nm wide and 6 nm above the mica support, possibly representing different protein domains. Spectral imaging revealed significant heterogeneity in the fluorescence emission of individual chlorosomes, likely reflecting the variations in BChl c homolog composition and internal arrangements of the stacked BChls within each chlorosome.
Biophysical Journal, 2006
Chlorosomes are the main light harvesting complexes of green photosynthetic bacteria. Recently, a lamellar model was proposed for the arrangement of pigment aggregates in Chlorobium tepidum chlorosomes, which contain bacteriochlorophyll (BChl) c as the main pigment. Here we demonstrate that the lamellar organization is also found in chlorosomes from two browncolored species (Chl. phaeovibrioides and Chl. phaeobacteroides) containing BChl e as the main pigment. This suggests that the lamellar model is universal among green sulfur bacteria. In contrast to green-colored Chl. tepidum, chlorosomes from the browncolored species often contain domains of lamellar aggregates that may help them to survive in extremely low light conditions. We suggest that carotenoids are localized between the lamellar planes and drive lamellar assembly by augmenting hydrophobic interactions. A model for chlorosome assembly, which accounts for the role of carotenoids and secondary BChl homologs, is presented.
Isolation and development of chlorosomes in the green bacterium Chloroflexus aurantiacus
Journal of …, 1981
Freeze-fracture electron microscopy was used to study further the changes in chlorosome structure during the development of the photosynthetic apparatus in Chloroflexus aurantiacus J-10-fl. During development, in response to decreased light intensity or lower oxygen tension, the number of chlorosomes per cell increased. The same conditions also led to a general thickening of chlorosomes but did not affect their length or width. The thickening of the chlorosomes paralleled increases in the bacteriochlorophyll c/bacteriochlorophyll a ratio. Semiaerobic induction of the photosynthetic apparatus did not produce a synchronous assembly of chlorosomes in all cells of a given culture. Even adjacent cells of a single filament showed great variations in the rate and extent of response. Parallel appearance of (i)-5-nm particles (in a lattice configuration) in the membrane attachment site, (ii) the crystalline baseplate material (with a periodicity of-6 nm) adjacent to the membrane attachment site, and (iii) the chlorosome envelope layer preceded addition of longitudinally oriented, rodlike elements (diameter, =6 rim) to the chlorosome core. It is estimated that each chlorosome can funnel energy into approximately 100 reaction centers. Chlorosomes could be isolated by a simple density gradient procedure only from cells grown at low light intensity. A bacteriochlorophyll a species absorbing at 790 nm was associated with isolated chlorosomes. Lithium dodecyl sulfate-polyacrylamide gel electrophoresis of chlorosomes showed only a few low-molecular-weight polypeptides (<15,000). Chlorosomes are the light-harvesting structures of the Chlorobiineae (4, 8, 9, 16). They are located in close association with the inner surface of the cytoplasmic membrane and contain all of the antenna bacteriochlorophyll (Bchl) c (d or e) within the cell (6, 17). The reaction center Bchl a is located in the cytoplasmic membrane (7; C.
Science Access, 2001
Chlorosome light-harvesting complexes from Chloroflexus aurantiacus have been examined to determine the bacteriochlorophyll content. Chlorosome isolates from Chloroflexus aurantiacus were used to determine the number of bacteriochlorophyll c molecules (Bchl c) present in each chlorosome. Single molecule fluorescence spectroscopy was used to determine the molar concentration of chlorosomes. In a sample with OD742 ~3, chlorosome concentration was determined to be 1.1x10 -8M and the chlorosomes were found to have a hydrodynamic radius of 22 nm. Pigment extraction and quantitation was used to determine the molar concentration of Bchl c present and a number of 4,500 ± 500 Bchl c per chlorosome was determined. Similar studies using chlorosomes from Chlorobium tepidum are currently in progress. Previously described chlorosome models are being used to determine possible structures using the number of bacteriochorophyll molecules per chlorosome. The results presented have significant implica...
Photosynthesis Research, 2014
The arrangement of core antenna complexes (B808-866-RC) in the cytoplasmic membrane of filamentous phototrophic bacterium Chloroflexus aurantiacus was studied by electron microscopy in cultures from different light conditions. A typical nearest-neighbor center-to-center distance of *18 nm was found, implying less protein crowding compared to membranes of purple bacteria. A mean RC:chlorosome ratio of 11 was estimated for the occupancy of the membrane directly underneath each chlorosome, based on analysis of chlorosome dimensions and core complex distribution. Also presented are results of single-particle analysis of core complexes embedded in the native membrane. Keywords Photosynthesis Á LH1 Á Chlorosome Á Macromolecular crowding Á Electron microscopy Á Chloroflexus aurantiacus Abbreviations B808-B866 The core antenna of Chloroflexus aurantiacus Cfx Chloroflexus FAP Filamentous anoxygenic phototrophs LH1, LH2 Light-harvesting complexes of purple bacteria P Primary donor of the reaction center Q A 1st quinone acceptor of the reaction center RC Reaction center
Biochemistry, 1987
The photosynthetic antenna of Chloroflexus aurantiacus includes bacteriochlorophyll (BChl) c740 and BChl a792, both of which occur in chlorosomes, and B808-866 (containing BChl aSo8 and BChl ~2866)~ which is membrane-located (subscripts refer to near-infrared absorption maxima in vivo). BChl a792 is thought to mediate excitation transfer from BChl c740 to BChl aSo8. Lifetimes of fluorescence from BChl c740 and BChl a792 were measured in isolated and membrane-bound chlorosomes in order to study energy transfer from these pigments. In both preparations, the lifetime of BChl c740 fluorescence was at or below the instrumental limit of temporal resolution (about 30-50 ps), implying extremely fast excitation transfer from this pigment. Attempts to disrupt excitation transfer from BChl c740, either by conversion of part of this pigment to a monomeric form absorbing at 671 nm or by partial destruction of BChl a792 by oxidation with K3Fe(CN),, had no discernible effects on the lifetime of BChl c740 fluorescence. Most (usually >90%) of the fluorescence from BChl a792 decayed with a lifetime of 93 f 21 ps in membrane-attached chlorosomes and 155 f 22 ps in isolated chlorosomes at room temperature. Assuming that the only difference between these preparations is the occurrence of excitation transfer from BChl a792 to B808-866, a 41% efficiency was calculated for this process. This value is lower than the 60% efficiency of excitation transfer from BChl c740 to B808-866 determined by comparison of fluorescence excitation and absorption spectra of membranes with attached chlorosomes and compares even less favorably with the 100% efficiency of excitation transfer found in whole cells by the same method. Furthermore, measurements at 77 K (on different samples) did not show an increased lifetime of BChl a792 fluorescence when isolated chlorosomes were compared with membrane-bound chlorosomes. These results imply either that BChl a792 is not an obligatory intermediate in energy transfer from BChl c740 to B808-866 or (more probably) that chlorosome isolation introduces new processes for quenching fluorescence from BChl a792.
Förster energy transfer in chlorosomes of green photosynthetic bacteria
Journal of Photochemistry and Photobiology B: Biology, 1992
Energy transfer properties of whole cells and chlorosome antenna complexes isolated from the green sulfur bacteria Chlorobium limicola (containing bacteriochlorophyll c), Chlorobium vibriofomze (containing bacteriochlorophyll d) and Pelodictyonphaeoclathhratiforme (containing bacteriochlorophyll e) were measured. The spectral overlap of the major chlorosome pigment (bacteriochlorophyll c, d or, e) with the bacteriochlorophyll a B795 chlorosome baseplate pigment is greatest for bacteriochlorophyll c and smallest for bacteriochlorophyll e. The absorbance and fluorescence spectra of isolated chlorosomes were measured, fitted to gaussian curves and the overlap factors with B795 calculated. Energy transfer times from the bacteriochlorophyll c, d or e to B79.5 were measured in whole cells and the results interpreted in terms of the Fkster theory of energy transfer.
Pigment-protein diversity in chlorosomes of green phototrophic bacteria
Archives of Microbiology, 1990
In order to compare and contrast the structure and function of the light-harvesting antennae (i.e. chlorosomes) of green bacteria, a procedure for isolating and characterizing them from green sulfur bacteria was developed. The chlorosomes from Chlorobium species with bacteriochlorophyll (Bchl) c or e were isolated by a two step sucrose density centrifugation in the presence of 2% miranol, a mild detergent, and 2 M sodium thiocyanate (NaSCN). Purified chlorosomes from two green sulfur bacteria, Chlorobium phaeobacteroides and Chlorobium tepidum, and the filamentous green bacterium Chloroflexus aurantiacus were analysed by spectrophotometry, SDS-polyacrylamide gel electrophoresis, and immunological procedures. Isolated chlorosomes from both Chlorobium species contain only two electrophoretically separable protein components with approximate molecular masses of 5 -7 . 5 and 34.5 kDa. In addition, they have a major light-harvesting antenna pigment (Bchl c or e), a minor Bchl a species, and carotenoids. ChloroJTexus aurantiacus antisera for the three major chlorosome proteins (5.6, 11, and 18 kDa), and the reaction center proteins (24 and 24.5 kDa) did not cross react with any Chlorobium proteins analyzed in this study. Chlorobium limicola f. thiosulfatophilum antisera against the 7.5 kDa chlorosome protein cross reacted strongly with the 5 -7.5 kDa protein from Cb. tepidum, weakly with the Cb. phaeobacteroides protein, but not at all to the 5.6 kDa chlorosome protein from Cf. aurantiacus. These results provide further evidence for the evolutionary divergence of the chlorosomes from green phototrophic bacteria (e. g., Chlorobium-type and Chloroflexus-type).
Biophysical Journal, 2003
The bacteriochlorophyll (Bchl) c content and organization was determined for Chlorobium (Cb.) tepidum chlorosomes, the light-harvesting complexes from green photosynthetic bacteria, using fluorescence correlation spectroscopy and atomic force microscopy. Single-chlorosome fluorescence data was analyzed in terms of the correlation of the fluorescence intensity with time. Using this technique, known as fluorescence correlation spectroscopy, chlorosomes were shown to have a hydrodynamic radius (Rh) of 25 6 3.2 nm. This technique was also used to determine the concentration of chlorosomes in a sample, and pigment extraction and quantitation was used to determine the molar concentration of Bchl c present. From these data, a number of ;215,000 6 80,000 Bchl c per chlorosome was determined. Homogeneity of the sample was further characterized by dynamic light scattering, giving a single population of particles with a hydrodynamic radius of 26.8 6 3.7 nm in the sample. Tapping-mode atomic force microscopy (TMAFM) was used to determine the x,y,z dimensions of chlorosomes present in the sample. The results of the TMAFM studies indicated that the average chlorosome dimensions for Cb. tepidum was 174 6 8.3 3 91.4 6 7.7 3 10.9 6 2.71 nm and an overall average volume 90,800 nm 3 for the chlorosomes was determined. The data collected from these experiments as well as a model for Bchl c aggregate dimensions was used to determine possible arrangements of Bchl c oligomers in the chlorosomes. The results obtained in this study have significant implications on chlorosome structure and architecture, and will allow a more thorough investigation of the energetics of photosynthetic light harvesting in green bacteria.