Localisation of reaction centre and light harvesting complexes in the photosynthetic unit of Rhodopseudomonas viridis

The six fold symmetry of the B880 light-harvesting complex and the structure of the photosynthetic membranes of Rhodopseudomonas marina

The reaction center free light-harvesting core complex of Rp. marina was purified by DEAE 52 ion exchange chromatography in the presence of the detergent OG. The protein complex was crystallised by microdialysis yielding two-dimensional crystals with a diameter of up to 10 microns. The crystals were negatively stained with uranyl acetate or prepared in vitrified ice and electron micrographs were taken. They exhibited a hexagonal lattice with a lattice constant of 102 +/- 3 A. The optical diffraction pattern of the best ordered areas of electron micrographs showed spots up to a resolution of 29 A. Image processing revealed a six fold symmetry of the ring like B880-complex. The protein ring is hexagonal with one subunit in each corner of the hexagon and two subunits forming the connection site to the neighbouring B880-complex in the crystal. In freeze fracture preparations of whole cells the intra-cytoplasmic photosynthetic membranes are seen to be organised into large stacks that affect the organisation of the photosynthetic complexes. Most notably, the stacked membrane regions exhibit hexagonally packed photosynthetic complexes with a repeat of approximately 100 A, which is very similar to the lattice of the artificial B880-complex crystals. The same quasi-crystalline structure appeared in the cytoplasmic membrane of the contact sites with the intra-cytoplasmic membrane stack, but was absent from the end membrane of the stack. Thus, membrane stacking appears to induce the formation of the crystalline arrays, presumably through interactions between the cytoplasmic surface domains of the photosynthetic complexes. Tight packing of the photosynthetic particles is not sufficient to induce the crystalline order. The intra cytoplasmic membranes form a continuum with the cytoplasmic membrane via their origins at the round invagination sites.

Probing the structure of the core light-harvesting complex (LH1) of Rhodopseudomonas viridis by dissociation and reconstitution methodology

A subunit complex was formed from the core light-harvesting complex (LH1) of bacteriochlorophyll(BChl)-b-containing Rhodopseudomonas viridis. The addition of octyl glucoside to a carotenoid-depleted Rps. viridis membrane preparation resulted in a subunit complex absorbing at 895 nm, which could be quantitatively dissociated to free BChl b and then reassociated to the subunit. When carotenoid was added back, the subunit could be reassociated to LH1 with a 25% yield. Additionally, the Rps. viridis α- and β-polypeptides were isolated, purified, and then reconstituted with BChl b. They formed a subunit absorbing near 895 nm, similar to the subunit formed by titration of the carotenoid depleted membrane, but did not form an LH1-type complex at 1015 nm. The same results were obtained with the β-polypeptide alone and BChl b. Isolated polypeptides were also tested for their interaction with BChl a. They formed subunit and LH1-type complexes similar to those formed using polypeptides isolated from BChl-a-containing bacteria but displayed 6-10 nm smaller red shifts in their long-wavelength absorption maxima. Thus, the larger red shift of BChl-b-containing Rps. viridis is not attributable solely to the protein structure. The β-polypeptide of Rps. viridis differed from the other β-polypeptides tested in that it could form an LH1-type complex with BChl a in the absence of the α- and γ-polypeptides. It apparently contains the necessary information required to assemble into an LH1-type complex. When the γ-polypeptide was tested in reconstitution with BChl a and BChl b with the α- and β-polypeptides, it had no effect; its role remains undetermined.

The light-harvesting core-complex and the B820-subunit from Rhodopseudomonas marina. Part II. Electron microscopic characterisation

Electron micrographs of photosynthetic membranes of the BChla-containing bacterium Rp. marina showed a quasi-crystalline structure. The photoreceptor units are arranged in a hexagonal lattice with a reaction center to reaction center distance of 102 +/- 3 A. Purified B880-complex was concentrated up to an OD880 of 60 which induced the formation of large protein vesicles. The protein complexes within these vesicles were highly ordered and showed a hexagonal lattice with the same center to center distance of 102 +/- 3 A as was observed in the native membranes. Image processing of the micrographs revealed a ring-like structure of the B880-complex at 26 A resolution and suggests that the B880-complex consists of 5 or 6 subunits. For the first time it can be shown that an isolated core-complex is in a stable, ring-like structure even without the reaction center which is supposed to be located in the middle of the B880-ring. The data indicate that the isolated B880-complex exhibits the same structure as in the native membrane.

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.

Isolation and protein chemical characterization of the B806-866 antenna complex of the green thermophilic bacterium Chloroflexus aurantiacus

The B806-866 antenna complex was isolated from cytoplasmic membranes of the green thermophilic bacterium Chloroflexus aurantiacus. The membranes were treated with 7 M urea at 50 degrees C, the B806-866 antenna complex was solubilized with a mixture of Noni-fjdet P-40 (octylphenoxypolyethoxyethanol (Sigma)) and sodium dodecylsulphate (2:1) and isolated by sucrose density gradient centrifugation. This antenna complex was characterized by reversed-phase chromatography (fast polypeptide and polynucleotide liquid chromatography), amino acid and sequence analyses. The B806-866 antenna of Chloroflexus aurantiacus consists of two polypeptides: the B806-866-alpha and B806-866-beta polypeptides in an apparent stoichiometric ratio of 1:1, which may be equivalent to the structural elementary unit found in the antenna systems of many species of Rhodospirillaceae.

Structure and transcription of the genes encoding the B1015 light-harvesting complex beta and alpha subunits and the photosynthetic reaction center L, M, and cytochrome c subunits from Rhodopseudomonas viridis

The genes encoding the beta and alpha subunits of the B1015 light-harvesting complex (LHC) and the L, M, and cytochrome c subunits of the photosynthetic reaction center from Rhodopseudomonas viridis are organized in an operon, in analogy to other nonsulfur purple bacteria, named the puf operon. In photoheterotrophically grown cells, two abundant puf operon mRNA species of 3,581 and 621 bases were present. The large transcript encoded the LHC beta, LHC alpha, and reaction center L, M, and cytochrome c polypeptides, whereas the small transcript only coded for the LHC beta and alpha polypeptides. Both transcripts share a common 5' end which is located 115 bases upstream from the initiation codon of the LHC beta gene. Two additional low-level transcripts of 3,718 and 758 bases with 5' ends 254 +/- 3 bases upstream from the LHC beta gene were detected. Analysis of the DNA sequence preceding the different 5' ends revealed DNA elements of striking homology. The 3' ends of the small transcripts were mapped within the alpha-L intercistronic DNA region downstream from a sequence capable of forming a very stable stem-loop when transcribed into RNA. The 3' termini of the large transcripts are located immediately downstream from the region coding the cytochrome c subunit in two areas resembling rho-independent transcription terminators. No open reading frames corresponding to pufQ and pufX from Rhodobacter capsulatus and Rhodobacter sphaeroides were present in the flanking DNA regions of the puf operon. In contrast, an open reading frame ending 191 base pairs upstream from the LHC beta gene showed 50% homology at the amino acid level to the available sequence of the bchA gene from R. capsulatus. The genes coding for the B1015 LHC subunits had C-terminal extensions of 13 (beta) and 10 (alpha) amino acids which were not present in the proteins isolated from intracytoplasmic membranes.

Transfer of excitation energy in photosynthesis: some thoughts

The aim of the present paper is to aid biologists understand the complex physical problems of intramolecular energy transfer, in particular, between antenna (bacterio) chlorophyll molecules in vivo.The author has attempted, in the first part of the paper, to explain complicated processes of excitation transfer in a language understandable to readers with knowledge in fundamentals of general physics, but not in molecular optics.The second part of this paper is a critical review relevant to the specifics of physical theories and their applicability to the problem of energy transfer in antenna (bacterio) chlorophylls ((B) Chls) to reaction centers (RCs) in the photosynthetic organisms.

The puf operon region of Rhodobacter sphaeroides

The puf operon of the purple nonsulfur photosynthetic bacterium, Rhodobacter sphaeroides, contains structural gene information for at least two functionally distinct bacteriochlorophyll-protein complexes (light harvesting and reaction center) which are present in a fixed ratio within the photosynthetic intracytoplasmic membrane. Two proximal genes (pufBA) specify subunits of a long wavelength absorbing (i.e., 875 nm) light harvesting complex which are present in the photosynthetic membrane in ≃15 fold excess relative to the reaction center subunits which are encoded by the pufLM genes. This review summarizes recent studies aimed at determining how expression of the R. sphaeroides puf operon region relates to the ratio of individual bacteriochlorophyll-protein complexes found within the photosynthetic membrane. These experiments indicate that puf operon expression may be regulated at the transcriptional, post-transcriptional, translation and post-translational levels. In addition, this review discusses the possible role(s) of newly identified loci upstream of pufB which may be involved in regulating either synthesis or assembly of individual bacteriochrlorophyll-protein complexes as well as the pufX gene, the most distal genetic element within the puf operon whose function is still unknown.