The preparation and characterisation of native photoreceptor units from the thylakoids of Rhodopseudomonas viridis

Isomerization kinetics of bacteriochlorophyll b and bacteriopheophytin b under acidic conditions

Bacteriochlorophyll (BChl) b has a unique π-conjugation system, in which the bacteriochlorin macrocycle is conjugated with the C8-ethylidene group. This π-system is converted easily to the chlorin macrocycle. However, the effects of the central magnesium in BChl b on this conversion are unclear. In this study, the isomerization kinetics of BChl b and its demetalated pigment, bacteriopheophytin (BPhe) b, was analyzed under weakly acidic conditions. BChl b exhibited faster acid-induced isomerization than BPhe b. These results were attributed to the stabilization of a cationic intermediate, whose C8-ethylidene group is protonated, during the isomerization of BChl b compared to BPhe b because of a difference in the electron densities of the π-conjugation systems between BChl b and BPhe b. High-performance liquid chromatography analyses indicated that BChl b was primarily isomerized to 3-acetyl Chl a, followed by demetalation. The reaction order was due to the slower demetalation kinetics of metallobacteriochlorins than metallochlorins. These results will be helpful for handling unstable BChl b and BPhe b. The reaction properties of BChl b and BPhe b demonstrated here will be helpful for understanding the in vivo formation of BPhe b, which acts as the primary electron acceptor in photosynthetic reaction center complexes in BChl b-containing purple photosynthetic bacteria.

Excitation Energy Transfer from Bacteriochlorophyll b in the B800 Site to B850 Bacteriochlorophyll a in Light-Harvesting Complex 2

Control of the spectral overlap between energy donors and acceptors provides insight into excitation energy transfer (EET) mechanisms in photosynthetic light-harvesting proteins. Substitution of energy-donating B800 bacteriochlorophyll (BChl) a with other pigments in the light-harvesting complex 2 (LH2) of purple photosynthetic bacteria has been extensively performed; however, most studies on the B800 substitution have focused on the decrease in the spectral overlap integral with energy-accepting B850 BChl a by reconstitution of chlorophylls into the B800 site. Here, we reconstitute BChl b into the B800 site of the LH2 protein from Rhodoblastus acidophilus to increase the spectral overlap with B850 BChl a. BChl b in the B800 site had essentially the same hydrogen-bonding pattern as B800 BChl a, whereas it showed a red-shifted Q_y absorption band at 831 nm. The EET rate from BChl b to B850 BChl a in the reconstituted LH2 was similar to that of native LH2 despite the red shift of the Q_y band of the energy donor. These results demonstrate the importance of the contribution of the density of excitation states of the B850 circular assembly, which incorporates higher lying optically forbidden states, to intracomplex EET in LH2.

Proposal for verifying dipole properties of light-harvesting antennas

For light harvesters with a reaction center complex (LH1-RC complex) of three types, we propose an experiment to verify our analysis based upon antenna theories that automatically include the required structural information. Our analysis conforms to the current understanding of light-harvesting antennas in that we can explain known properties of these complexes. We provide an explanation for the functional roles of the notch at the light harvester, a functional role of the polypeptide called PufX or W at the opening, a functional role of the special pair, a reason that the cross section of the light harvester must not be circular, a reason that the light harvester must not be spherical, reasons for the use of dielectric bacteriochlorophylls instead of conductors to make the light harvester, a mechanism to prevent damage from excess sunlight, an advantage of the dimeric form, and reasons for the modular design of nature. Based upon our analysis we provide a mechanism for dimerization. We predict that the dimeric form of light-harvesting complexes is favored under intense sunlight. We further comment upon the classification of the dimeric or S-shape complexes. The S-shape complexes should not be considered as the third type of light harvester but simply as a composite form.

Complete Genome Sequence of the Bacteriochlorophyll b-Producing Photosynthetic Bacterium Blastochloris viridis

We report the complete genome sequence of the purple photosynthetic bacterium Blastochloris viridis belonging to α-Proteobacteria. This is the first completed genome sequence of a phototroph producing bacteriochlorophyll b. The genome information will be useful for further analysis of the photosynthetic energy conversion system and bacteriochlorophyll pigment biosynthesis.

Rhodobacter sphaeroides mutants overexpressing chlorophyllide a oxidoreductase of Blastochloris viridis elucidate functions of enzymes in late bacteriochlorophyll biosynthetic pathways

In previous studies we have demonstrated that chlorophyllide a oxidoreductases (CORs) from bacteriochlorophyll (BChl) a-producing Rhodobacter species and BChl b-producing Blastochloris viridis show distinct substrate recognition and different catalytic hydrogenation reactions, and that these two types of CORs therefore cause committed steps for BChls a and b biosynthesis. In this study, COR genes from B. viridis were incorporated and overexpressed in a series of Rhodobacter sphaeroides mutants. We found that the following two factors are essential in making R. sphaeroides produce BChl b: the loss of functions of both intrinsic COR and 8-vinyl reductase (BciA) in the host R. sphaeroides strain; and expression of the BchYZ catalytic components of COR from B. viridis, not the complete set of COR (BchXYZ), in the host strain. In addition, we incorporated bchYZ of B. viridis into the R. sphaeroides mutant lacking BchJ and BciA, resulting in the strain accumulating both BChl a and BChl b. This is the first example of an anoxygenic photosynthetic bacterium producing BChls a and b together. The results suggest that BchJ enhances activity of the intrinsic COR. The physiological significance of BchJ in pigment biosynthetic pathways will be discussed.

Structure of the dimeric RC-LH1-PufX complex from Rhodobaca bogoriensis investigated by electron microscopy

Electron microscopy and single-particle averaging were performed on isolated reaction centre (RC)-antenna complexes (RC-LH1-PufX complexes) of Rhodobaca bogoriensis strain LBB1, with the aim of establishing the LH1 antenna conformation, and, in particular, the structural role of the PufX protein. Projection maps of dimeric complexes were obtained at 13 Å resolution and show the positions of the 2 × 14 LH1 α- and β-subunits. This new dimeric complex displays two open, C-shaped LH1 aggregates of 13 αβ polypeptides partially surrounding the RCs plus two LH1 units forming the dimer interface in the centre. Between the interface and the two half rings are two openings on each side. Next to the openings, there are four additional densities present per dimer, considered to be occupied by four copies of PufX. The position of the RC in our model was verified by comparison with RC-LH1-PufX complexes in membranes. Our model differs from previously proposed configurations for Rhodobacter species in which the LH1 ribbon is continuous in the shape of an S, and the stoichiometry is of one PufX per RC.

An unexpectedly branched biosynthetic pathway for bacteriochlorophyll b capable of absorbing near-infrared light

Chlorophyllous pigments are essential for photosynthesis. Bacteriochlorophyll (BChl) b has the characteristic C8-ethylidene group and therefore is the sole naturally occurring pigment having an absorption maximum at near-infrared light wavelength. Here we report that chlorophyllide a oxidoreductase (COR), a nitrogenase-like enzyme, showed distinct substrate recognition and catalytic reaction between BChl a- and b-producing proteobacteria. COR from BChl b-producing Blastochloris viridis synthesized the C8-ethylidene group from 8-vinyl-chlorophyllide a. In contrast, despite the highly conserved primary structures, COR from BChl a-producing Rhodobacter capsulatus catalyzes the C8-vinyl reduction as well as the previously known reaction of the C7 = C8 double bond reduction on 8-vinyl-chlorophyllide a. The present data indicate that the plasticity of the nitrogenase-like enzyme caused the branched pathways of BChls a and b biosynthesis, ultimately leading to ecologically different niches of BChl a- and b-based photosynthesis differentiated by more than 150 nm wavelength.

Inelastic neutron scattering study of light-induced dynamics of a photosynthetic membrane system

Inelastic neutron scattering was employed to study photoeffects on the molecular dynamics of membranes of the photosynthetic bacterium Rhodopseudomonas viridis. The main photoactive parts of this biomolecular system are the chlorophyll molecules whose dynamics were found to be affected under illumination by visible light in a twofold manner. First, vibrational modes are excited at energies of 12(2) and 88(21) cm(-1). Second, a partial "freezing" of rotational modes is observed at energies of 1.2(3) and 2.9(5) cm(-1). These results are attributed to a possible coupling between molecular motions and particular mechanisms in the photosynthetic process.

Circular symmetry of the light-harvesting 1 complex from Rhodospirillum rubrum is not perturbed by interaction with the reaction center

The effect of the interaction of the reaction center (RC) upon the geometrical arrangement of the bacteriochlorophyll a (BChla) pigments in the light-harvesting 1 complex (LH1) from Rhodospirillum rubrum has been examined using single molecule spectroscopy. Fluorescence excitation spectra at 1.8 K obtained from single detergent-solubilized as well as single membrane-reconstituted LH1-RC complexes showed predominantly (>70%) a single broad absorption maximum at 880-900 nm corresponding to the Q(y) transition of the LH1 complex. This absorption band was independent of the polarization direction of the excitation light. The remaining complexes showed two mutually orthogonal absorption bands in the same wavelength region with moderate splittings in the range of DeltaE = 30-85 cm(-1). Our observations are in agreement with simulated spectra of an array of 32 strongly coupled BChla dipoles arranged in perfect circular symmetry possessing only a diagonal disorder of <or=150 cm(-1). However, in contrast to LH1 complexes alone, excitation spectra that consist of a single absorption band were observed more frequently in the presence of the reaction center. Our results show that the interaction of the RC with the LH1 complex stabilizes the circular symmetric arrangement of the bacteriochlorophyll pigments and are in contradiction to recent studies by other groups using single molecule spectroscopy as well as cryoelectronmicroscopy and atomic force microscopy indicating that the RC induces an elliptical distortion of the LH1 complex. Possible reasons for this discrepancy are discussed.

Nanodissection and high-resolution imaging of the Rhodopseudomonas viridis photosynthetic core complex in native membranes by AFM. Atomic force microscopy

In photosynthesis, highly organized multiprotein assemblies convert sunlight into biochemical energy with high efficiency. A challenge in structural biology is to analyze such supramolecular complexes in native membranes. Atomic force microscopy (AFM) with high lateral resolution, high signal-to-noise ratio, and the possibility to nanodissect biological samples is a unique tool to investigate multiprotein complexes at molecular resolution in situ. Here we present high-resolution AFM of the photosynthetic core complex in native Rhodopseudomonas viridis membranes. Topographs at 10-A lateral and approximately 1-A vertical resolution reveal a single reaction center (RC) surrounded by a closed ellipsoid of 16 light-harvesting (LH1) subunits. Nanodissection of the tetraheme cytochrome (4Hcyt) subunit from the RC allows demonstration that the L and M subunits exhibit an asymmetric topography intimately associated to the LH1 subunits located at the short ellipsis axis. This architecture implies a distance distribution between the antenna and the RC compared with a centered location of the RC within a circular LH1, which may influence the energy transfer within the core complex. The LH1 subunits rearrange into a circle after removal of the RC from the core complex.

Characterization of expressed pigmented core light harvesting complex (LH 1) in a reaction center deficient mutant of Blastochloris viridis

The utility of photosynthetically defective mutants in the purple photosynthetic bacterium Blastochloris viridis (formerly Rhodopseudomonas viridis)was demonstrated with construction of a reaction-center deficient mutant, LH 1-H. This LH 1-H mutant has a photosynthetic apparatus in which most of the puf operon genes were deleted, resulting in an organism containing only the genes for the light harvesting polypeptides and the H subunit of the reaction center. This B. viridisstrain containing a truncation of the puf operon was characterized by gel electrophoresis, lipid-to-protein ratio analysis, optical spectroscopy, electron paramagnetic resonance and transmission electron microscopy. Optical and electron paramagnetic resonance spectroscopies revealed no photoactivity in this LH 1-H mutant consistent with the absence of intact reaction centers. Electron paramagnetic resonance evidence for assembled LH 1 complexes suggested that the interactions between light harvesting polypeptide complexes in membranes were largely unchanged despite the absence of their companion reaction center cores. The observed increase in the lipid-to-protein ratio was consistent with modified interactions between LH 1s, a view supported by transmission electron microscopy analysis of membrane fragments. The results show that B. viridis can serve as a practical system for investigating structure-function relationships in membranes and photosynthesis through the construction of photosynthetically defective mutants.

Model for the light-harvesting complex I (B875) of Rhodobacter sphaeroides

The light-harvesting complex I (LH-I) of Rhodobacter sphaeroides has been modeled computationally as a hexadecamer of alphabeta-heterodimers, based on a close homology of the heterodimer to that of light-harvesting complex II (LH-II) of Rhodospirillum molischianum. The resulting LH-I structure yields an electron density projection map that is in agreement with an 8.5-A resolution electron microscopic projection map for the highly homologous LH-I of Rs. rubrum. A complex of the modeled LH-I with the photosynthetic reaction center of the same species has been obtained by a constrained conformational search. This complex and the available structures of LH-II from Rs. molischianum and Rhodopseudomonas acidophila furnish a complete model of the pigment organization in the photosynthetic membrane of purple bacteria.

Two-dimensional crystallization of the light-harvesting I-reaction centre photounit from Rhodospirillum rubrum

A stoichiometric unit of the light-harvesting complex I and the reaction centre (LHI-RC complex) has been isolated from a carotenoid-less mutant of the purple non-sulphur bacterium Rhodospirillum rubrum by mild solubilization of photosynthetic membranes with the phospholipid detergent diheptylphosphatidylcholine. Dialysis of the isolated LHI-RC complexes in the presence of added dioleoyl-sn-phosphatidylcholine produced ordered two-dimensional crystals. Digital image processing revealed that the LHI-RC are packed together in a square lattice (a = b = 16.3 nm). The dimensions of the LHI ring are essentially identical with those determined from two-dimensional (2D) crystals of purified carotenoid-containing light-harvesting I complexes after analysis by cryo-electron microscopic techniques or from negatively stained 2D crystals of purified LHI complexes from a carotenoid-less mutant. Each LHI ring of the LHI-RC complex contains a central diffuse stain-excluding region, which is assigned to the reaction centre. The analysis of the LHI-RC 2D crystals strongly suggests that the geometry and subunit stoichiometry of the LHI ring is unaffected by the presence of a reaction centre that can probably assume various orientations within the ring.

Purification of an LHI-RC-complex of Rhodospirillum rubrum by solubilization of chromatophores with a short-chain lecithin

Chromatophores from Rhodospirillum rubrum were solubilized using the detergent 1,2-diheptanoyl-sn-phosphatidylcholine (DHPC). The solubilization curves are sigmoidal reaching a plateau at a detergent/protein ratio of 2-3 μmol/mg corresponding to 75-90% solubilized protein. The BChl-binding proteins are stable over a large range of DHPC/protein ratios. A complex of BChl-binding proteins containing both LHI- and RC-polypeptides (LHI-RC-complex) was purified using a two step procedure. RC photochemical activity as well as absorption and near-IR CD spectra showed the complex to be active and stable after purification in presence of DHPC.

Probing protein structural requirements for formation of the core light-harvesting complex of photosynthetic bacteria using hybrid reconstitution methodology

The α- and β-polypeptides of LH1 isolated from four different photosynthetic bacteria (Rhodospirillum rubrum, Rhodobacter sphaeroides, Rhodobacter capsulatus and Rhodopseudomonas viridis) were used for homologous and hybrid reconstitution experiments with bacteriochlorophyll a. Formation of B820-type subunit complexes and LH1-type complexes were evaluated. The β-polypeptides of R. rubrum, Rb. sphaeroides and Rb. capsulatus behaved similarly and formed B820-type subunit complexes in the absence of an α-polypeptide. The α- and β-polypeptides were both required to form a LH1-type complex with each of these three homologous systems. In hybrid experiments where the β-polypeptides were tested for reconstitution with α-polypeptides other than their homologous partners, half of the twelve possible combinations resulted in formation of both B820- and LH1-type complexes. Three of the combinations that did not result in formation of a LH1-type complex involved the β-polypeptide of R. rubrum. It is suggested that these latter results can be explained by charge repulsion between the Lys at position-17 (assigning the conserved His located nearest to the C-terminus as position 0) in the β-polypeptide of R. rubrum and each of the heterologous α-polypeptides tested, all of which have an Arg at this location. Conclusions that can be derived from these experimental results include: (1) the experimental data support the idea that a central core region of approximately 40 amino acids exists in each of the polypeptides, which contains sufficient information to allow formation of both the B820- and LH1-type complexes and (2) a specific portion of the N-terminal hydrophilic region of each polypeptide was found in which ion pairs between oppositely charged groups on the α- and β-polypeptides seem to stabilize complex formation.

Structure, function and organization of antenna polypeptides and antenna complexes from the three families of Rhodospirillaneae

Comparative primary structural analysis of polypeptides from antenna complexes from species of the three families of Rhodospirillaneae indicates the structural principles responsible for the formation of spectrally distinct light-harvesting complexes. In many of the characterized antenna systems the basic structural minimal unit is an alpha/beta polypeptide pair. Specific clusters of amino acid residues, in particular aromatic residues in the C-terminal domain, identify the antenna polypeptides to specific types of antenna systems, such as B880 (strong circular dichroism (CD)), B870 (weak CD), B800-850 (high), B800-850 (low) or B800-820. The core complex B880 (B1020) of species from Ectothiorhodospiraceae and Chromatiaceae apparently consists of four (alpha 1 alpha 2 beta 1 beta 2) or three (2 alpha beta 1 beta 2) chemically dissimilar antenna polypeptides respectively. There is good evidence that the so-called variable antenna complexes, such as the B800-850 (high), B800-850 (low) or B800-820 of Rp. acidophila, Rp. palustris and Cr. vinosum, are comprised of multiple forms of peripheral light-harvesting polypeptides. Structural similarities between prokaryotic and eukaryotic antenna polypeptides are discussed in terms of similar pigment organization. The structural basis for the strict organization of pigment molecules (bacteriochlorophyll (BChl) cluster) in the antenna system of purple bacteria is the hierarchical organization of the alpha- and beta-antenna polypeptides within and between the antenna complexes. On the basis of the three-domain structure of the antenna polypeptides with the central hydrophobic domain, forming a transmembrane alpha helix, possible arrangements of the antenna polypeptides in the three-dimensional structure of core and peripheral antenna complexes are discussed. Important structural and functional features of these polypeptides and therefore of the BChl cluster are the alpha/beta heterodimers, the alpha 2 beta 2 basic units and cyclic arrangements of these basic units. Equally important for the formation of the antenna complexes or the entire antenna are polypeptide-polypeptide, pigment-pigment and pigment-polypeptide interactions.

Involvement of the protein-protein interactions in the thermodynamics of the electron-transfer process in the reaction centers from Rhodopseudomonas viridis

Reaction centers from Rhodopseudomonas viridis were reconstituted into dimyristoylphosphatidylcholine (DMPC) and dielaidoylphosphatidylcholine (DEPC) liposomes. Freeze-fracture electron micrographs were performed on the samples frozen from temperatures above and below the phase transition temperatures of those lipids (Tc = 23 and 9.5 degrees C, in DMPC and DEPC, respectively). Above Tc, in the fluid conformation of the lipids, the reaction centers are randomly distributed in the vesicle membranes. Below Tc, aggregation of the proteins occurs. The Arrhenius plots of the rate constants of the charge recombination between P+ and QA- display a break at about 24 degrees C in DMPC vesicles and about 10 degrees C in DEPC vesicles (P represents the primary electron donor, a dimer of bacteriochlorophyll, and QA the primary quinone electron acceptor). This is in contrast to what was previously observed for the proteoliposomes of egg yolk phosphatidylcholine and for chromatophores [Baciou, L., Rivas, E., & Sebban, P. (1990) Biochemistry 29, 2966-2976], for which Arrhenius plots were linear. In DMPC and DEPC proteoliposomes, the activation parameters were very different on the two sides of Tc (delta H degrees for T less than Tc = 2.5 times delta H degrees for T greater than Tc), leading however, to the same delta G degrees values. Taking into account the structural and thermodynamic data, we suggest that, in vivo, protein-protein interactions play a role in the thermodynamic parameters associated with the energy stabilization process within the reaction centers.

P+QA- and P+QB- charge recombinations in Rhodopseudomonas viridis chromatophores and in reaction centers reconstituted in phosphatidylcholine liposomes. Existence of two conformational states of the reaction centers and effects of pH and o-phenanthroline

The P+QA- and P+QB- charge recombination decay kinetics were studied in reaction centers from Rhodopseudomonas viridis reconstituted in phosphatidylcholine bilayer vesicles (proteoliposomes) and in chromatophores. P represents the primary electron donor, a dimer of bacteriochlorophyll; QA and QB are the primary and secondary stable quinone electron acceptors, respectively. In agreement with recent findings for reaction centers isolated in detergent [Sebban, P., & Wraight, C.A. (1989) Biochim. Biophys. Acta 974, 54-65] the P+QA- decay kinetics were biphasic (kfast and kslow). Arrhenius plots of the kinetics were linear, in agreement with the hypothesis of a thermally activated process (probably via P+I-; I is the first electron acceptor, a bacteriopheophytin) for the P+QA- charge recombination. Similar activation free energies (delta G) for this process were found in chromatophores and in proteoliposomes. Significant pH dependences of kfast and kslow were observed in chromtophores and in proteoliposomes. In the pH range 5.5-11, the pH titration curves of kfast and kslow were interpreted in terms of the existence of three protonable groups, situated between I- and QA-, which modulate the free energy difference between P+I- and P+QA-. In proteoliposomes, a marked effect of o-phenanthroline was observed on two of the three pKs, shifting one of them by more than 2 pH units. On the basis of recent structural data, we suggest a possible interpretation for this effect, which is much smaller in Rhodobacter sphaeroides. The decay kinetics of P+QB- were also biphasic. Marked pH dependences of the rate constants and of the relative proportions of both phases were also detected for these decays. The major conclusion of this work comes from the biphasicity of the P+QB- decay kinetics. We had suggested previously that biphasicity of the P+QA- charge recombination in Rps. viridis comes from nonequilibrium between protonation states of the reaction centers due to comparable rates of the protonation events and charge recombination. This hypothesis does not hold since the P+QB- decays occur on a time scale (tau approximately 300 ms at pH 8) much longer than protonation events. This leads to the conclusion that kfast and kslow (for both P+QA- and P+QB-) are related to conformational states of the reaction centers, existing before the flash. In addition, the fast and slow decays of P+QB- are related to those measured for P+QA-, via the calculations of the QA-QB in equilibrium QAQB- apparent equilibrium constants, K2.(ABSTRACT TRUNCATED AT 400 WORDS)

Reaction center light harvesting B875 complexes from Rhodocyclus gelatinosus: characterization and identification of quinones

Reaction center-B875 pigment-protein complexes were purified from Rhodocyclus gelatinosus. The proteic components consist of 7-8 polypeptides among which some were identified by their apparent molecular weights: the light harvesting B875 polypeptides α and β of 8 and 6 kDa, reaction center L (23 kDa), M (28 kDa) and H (34 kDa), cytochrome c (43 kDa). Four c-type hemes were found per reaction center. Flash-induced absorbance changes showed the presence of both QA and QB in the complex. Charge recombination times were determined to be: 1.16±0.2 (n=30) for P(+)QAQB (-) and 7-10 ms for P(+)QA (-) in presence of herbicides. From quinone analysis on one hand and kinetics of charge recombination on the other hand, we proposed that in the reaction center of Rhodocyclus gelatinosus QA is menaquinone 8 and QB is ubiquinone 8.

Nobel lecture. A structural basis of light energy and electron transfer in biology

Aspects of intramolecular light energy and electron transfer will be discussed for three protein cofactor complexes, whose three-dimensional structures have been elucidated by x-ray crystallography: Components of light harvesting cyanobacterial phycobilisomes, the purple bacterial reaction centre, and the blue multi-copper oxidases. A wealth of functional data is available for these systems which allow specific correlations between structure and function and general conclusions about light energy and electron transfer in biological materials to be made.

Interactions of the bacteriochlorophylls in antenna bacteriochlorophyll-protein complexes of photosynthetic bacteria

Several models have been proposed for the arrangements of the bacteriochlorophylls in the antenna complexes of purple photosynthetic bacteria, but none of the models has accounted fully for the spectroscopic properties of the bacteriochlorophyll-protein complexes. We suggest a model involving strong exciton interactions within a bacteriochlorophyll dimer, and weaker interactions of each dimer with other, relatively distant dimers. The model is shown to account for the spectroscopic properties of the complexes, and to be consistent with other available information.

Reversible chemical cross-linking of the light-harvesting polypeptides of Rhodopseudomonas viridis

The topography of the light-harvesting polypeptides of Rhodopseudomonas viridis was investigated using cleavable chemical cross-linkers. To this end a set of succinimidyl esters and surface-specific sulfosuccinimidyl esters of different span widths were synthesized. The cross-linking reagents have been characterized using NMR and infrared spectroscopy and thin-layer chromatography. The cross-linking reaction was carried out under physiological conditions and the aggregates were analyzed by the methods of one- and two-dimensional polyacrylamide gel electrophoresis and by immunoblot analysis. We found cross-linkage between B1015-alpha and B1015-alpha, between B1015-alpha and B1015-beta and B1015-beta and B1015-beta. Aggregates of higher molecular mass were hetero-oligomers of B1015-alpha and B1015-beta containing three and four polypeptides, respectively. The results obtained in this work indicate a very tight contact among the light-harvesting polypeptides. We assume that the light-harvesting polypeptides are localized alternately as dimers of B1015-alpha and B1015-beta around the reaction centre core.