Mechanisms of excitation trapping in reaction centers of purple bacteria

The contradiction between two groups of experimental data, which fails to be resolved within the framework of the widely accepted model of excitation migration and trapping (at least in case of purple bacteria), is discussed in the introduction to this review. Three directions of studies intended to resolve this conflict are reviewed in the three further sections: II. Exciton models; III. Water-polarization (water-latch) mechanism of excitation trapping; IV. Quantum-mechanical models. The maximum efficiency of these models in resolving the contradiction mentioned above was assessed. The advantages and disadvantages of the mechanisms described in sections II, III, and IV are discussed in the last section of this review. It is concluded that none of these mechanisms taken alone is able to solve this problem. Therefore, the fundamental problem of the primary excitation conversion in reaction centers remains unsolved and requires additional experimental research.     

Excitation transfer between photosynthetic units: the 1964 experiment

We review here the background and the experiments that led to the concept of excitation energy transfer among photosystem (PS) II units. On the basis of a kinetic analysis of oxygen evolution and chlorophyll a fluorescence yield, the authors showed, in 1964, that the PS II photochemical reaction involved in the formation of oxygen is not a first-order process. We concluded that excitation energy localized in a `photosynthetic unit' including a reduced primary acceptor is transferred with a high probability to neighboring PS II units. Here, the beginnings and the original data of this topic are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

The size of the photosynthetic unit in purple bacteria

Pigment analysis was performed by means of normal phase HPLC on a number of bacteriochlorophyll a and b containing species of purple bacteria that contain a core antenna only. At least 99% of the bacteriochlorophyll in Rhodobacter sphaeroides R26, Rhodopseudomonas viridis and Thiocapsa pfennigii was esterified with phytol (BChl a _p and BChl b _p, respectively). Rhodospirillum rubrum contained only BChl a esterified with geranyl-geraniol (BChl a _GG). Rhodospirillum sodomense and Rhodopseudomonas marina contained, in addition to BChl a _p, small amounts of BChl a _GG, and presumably also of BChl a esterified with dihydro and tetrahydro geranyl-geraniol (2,10,14-phytatrienol and probably 2,14-phytadienol). In all species bacteriopheophytin (BPhe) esterified with phytol was present. The BChl/BPhe ratio indicated that in these species a constant number of 25 ± 3 antenna BChls is present per reaction centre. This number supports a model in which the core antenna consists of 12 - heterodimers surrounding the reaction centre. Determination of the in vivo extinction coefficient of BChl in the core-reaction centre complex yielded a value of ca. 140 mM^–1 cm^–1 for BChl a containing species and of 130 mM^–1 cm^–1 for Rhodopseudomonas viridis.Abbreviations BChl bacteriochlorophyll - BPhe bacteriopheophytin - GG geranyl-geraniol - LHI and LHII core and peripheral antenna complexes - P phytol - RC reaction centre Dedicated to the memory of Professor D.I. Arnon.     

The purple bacterial photosynthetic unit

Now is a very exciting time for researchers in the area of the primary reactions of purple bacterial photosynthesis. Detailed structural information is now available for not only the reaction center (Lancaster et al. 1995, in: Blankenship RE et al. (eds) Anoxygenic Photosynthetic Bacteria, pp 503–526), but also LH2 from Rhodopseudomonas acidophila (McDermott et al. 1995, Nature 374: 517–521) and LH1 from Rhodospirillum rubrum (Karrasch et al. 1995. EMBO J 14: 631–638). These structures can now be integrated to produce models of the complete photosynthetic unit (PSU) (Papiz et al., 1996, Trends Plant Sci, in press), which opens the door to a much more detailed understanding of the energy transfer events occurring within the PSU.Abbreviations Bchl bacteriochlorophyll - LH light-harvesting - PSU photosynthetic unit Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences     

Excitation energy trapping in anoxygenic photosynthetic bacteria*

Various aspects of excitation energy conversion in anoxygenic photosynthetic bacteria are surveyed. This minireview discusses different models that have been proposed during the past 60 years to describe excitation energy transfer from an antenna molecule to the reaction center. First, a simple one-dimensional model was suggested, but over time the models became more detailed when structural and dynamic information was included. This review focuses mainly on the picture of purple bacteria and green sulfur bacteria developed during the past decades. This revised version was published online in June 2006 with corrections to the Cover Date.     

The cytochrome bc 1 complexes of photosynthetic purple bacteria

Complete nucleotide sequences are now available for the pet (fbc) operons coding for the three electron carrying protein subunits of the cytochrome bc ₁ complexes of four photosynthetic purple non-sulfur bacteria. It has been demonstrated that, although the complex from one of these bacteria may contain a fourth subunit, three subunit complexes appear to be fully functional. The ligands to the three hemes and the one [2Fe-2S] cluster in the complex have been identified and considerable progress has been made in mapping the two quinone-binding sites present in the complex, as well as the binding sites for quinone analog inhibitors. Hydropathy analyses and alkaline phosphatase fusion experiments have provided considerable insight into the likely folding pattern of the cytochrome b peptide of the complex and identification of the electrogenic steps associated with electron transport through the complex has allowed the orientation within the membrane of the electron-carrying groups of the complex to be modeled.     

High-order fluorescence and exciton interaction in photosynthetic bacteria

We have observed fluorescence at visible wavelengths from chromatophores of photosynthetic bacteria excited with infrared radiation which we attribute to bacteriochlorophyll of the antenna system. The fluorescence is prompt (no delay greater than 5 ns). Its spectrum shows peaks at 445, 530 (broad) and 600 nm when excited with either 694 or 868 nm. Quantum yield is of the order of 10^?9. The dependence on intensity indicates generation by mainly third-order processes which could involve triplet states in combination with excited singlets. Second-order single-singlet fusion could also contribute. The high-order fluorescence can also be explained as arising from absorption of a second photon by singlet excited states.     

Role of HiPIP as electron donor to the RC-bound cytochrome in photosynthetic purple bacteria

High-Potential Iron-Sulfur Proteins (HiPIP) are small electron carriers, present only in species of photosynthetic purple bacteria having a RC-bound cytochrome. Their participation in the photo-induced cyclic electron transfer was recently established for Rubrivivax gelatinosus, Rhodocyclus tenuis and Rhodoferax fermentans (Schoepp et al. 1995; Hochkoeppler et al. 1996a, Menin et al. 1997b). To better understand the physiological role of HiPIP, we extended our study to other selected photosynthetic bacteria. The nature of the electron carrier in the photosynthetic pathway was investigated by recording light-induced absorption changes in intact cells. In addition, EPR measurements were made in whole cells and in membrane fragments in solution or dried immobilized, then illuminated at room temperature. Our results show that HiPIP plays an important role in the reduction of the photo-oxidized RC-bound cytochrome in the following species: Ectothiorhodospira vacuolata, Chromatium vinosum, Chromatium purpuratum and Rhodopila globiformis. In Rhodopseudomonas marina, the HiPIP is not photo-oxidizible in whole cells and in dried membranes, suggesting that this electron carrier is not involved in the photosynthetic pathway. In Ectothiorhodospira halophila, the photo-oxidized RC-bound cytochrome is reduced by a high midpoint potential cytochrome c, in agreement with midpoint potential values of the two iso-HiPIPs (+ 50 mV and + 120 mV) which are too low to be consistent with their participation in the photosynthetic cyclic electron transfer.     

On the involvement of the water-polaron mechanism in energy trapping by reaction centers of purple bacteria

A locus for binding a mobile water molecule was searched for in the immediate vicinity of the special pair in the reaction center. Using the PROTEUS PC-program (a part of the GRASP package) atomic structures of the reaction centers were analyzed in purple bacteria Rhodopseudomonas viridis and Rhodobacter sphaeroides. In both structures the loci for binding mobile water molecules were found at the distance of about 4.5 Å from the middle of the special pair in the reaction center. The reorientation of a hydrogen atom of this water molecule in the electric field of the excited special pair required energy of no less than 40 MeV that corresponded to predictions of the water-polarization model of trapping of electron excitation which was developed by M. V. Fok and one of the authors of this article.     

Interactions between photosynthetic pigments in organisms and in model systems

The review summarizes results concerning photosynthetic systems with chlorophylls and carotenoids obtained by means of spectral methods such as polarized radiation, photoacoustic spectroscopy, delayed luminescence, thermal deactivation, and leading to construction of model systems.     

Energy migration as related to the mutual position and orientation of donor and acceptor molecules in LH1 and LH2 antenna complexes of purple bacteria

Many approaches to discovering the interaction energy of molecular transition dipoles use the well-known coefficient xi(phi, psi (1) psi (2)) = (cos phi - 3 cos psi (1) cos psi (2))(2), where phi, Psi (1), and Psi (2) are inter-dipole angles. Unfortunately, this formula often yields rather approximate results, in particular, when it is applied to closely positioned molecules. This problem is of great importance when dealing with energy migration in photosynthetic organisms, because the major part of excitation transfers in their chlorophyllous antenna proceed between closely positioned molecules. In this paper, the authors introduce corrected values of the orientation factor for several types of mutual orientation of molecules exchanging with electronic excitations for realistic ratios of dipole lengths and spacing. The corrected magnitudes of interaction energies of neighboring bacteriochlorophyll molecules in LH2 and LH1 light-absorbing complexes are calculated for the class of photosynthetic purple bacteria. Some advantageous factors are revealed in their mutual positions and orientations in vivo.     

Phototaxis and other sensory phenomena in purple photosynthetic bacteria

Abstract: The mechanisms employed by purple photosynthetic bacteria to convert light to utilizable chemical energy have been a major focus of research over the past 50 years. Utilization of light by photosynthetic bacteria for other purposes, however, has received relatively little attention. The recent discovery of phototaxis by Rhodospirillum centenum provides new opportunities for biochemical and molecular biological analysis of sensory processes in purple bacteria.     

Nitrogen-fixing symbiosis between photosynthetic bacteria and legumes

Rhizobia having photosynthetic systems form nitrogen-fixing nodules on the stem and/or root of some species of the legumes Aeschynomene and Lotononis. This review is focused on the recent knowledge about the physiology, genetics and role of the photosystem in these bacteria. Photosynthetic electron transport seems to involve reaction centers, soluble cytochrome c2 and cytochrome bc1. Anaerobically, the electron transport system becomes over-reduced. The photosynthesis genes have been partially characterized; their organization is classical but their regulation is unusual as it is activated by far-red light via a bacteriophytochrome. This original mechanism of regulation seems well adapted to promote photosynthesis during stem symbiosis. Photosynthesis plays a major role in the efficiency of stem nodulation. It is also observed that infrared light stimulates nitrogen fixation in nodules containing photosynthetic bacteroids, suggesting that photosynthesis may additionally provides energy for nitrogen fixation, allowing for more efficient plant growth. Other aspects of these bacteria are discussed, in particular their taxonomic position and nodulation ability, the role of carotenoids and the potential for application of photosynthetic rhizobia in rice culture. This revised version was published online in June 2006 with corrections to the Cover Date.     

Phylogenetic analysis of photosynthetic genes of Rhodocyclus gelatinosus: Possibility of horizontal gene transfer in purple bacteria

Nucleotide sequences of the genes coding for the M and cytochrome subunits of the photosynthetic reaction center of Rhodocyclus gelatinosus, a purple bacterium in the subdivision, were determined. The deduced amino acid sequences of these proteins were compared with those of other photosynthetic bacteria. Based on the homology of these two photosynthetic proteins, Rc. gelatinosus was placed in the subdivision of purple bacteria, which disagrees with the phylogenetic trees based on 16S rRNA and soluble cytochrome c ₂. Horizontal transfer of the genes which code for the photosynthetic apparatus in purple bacteria can be postulated if the phylogenetic trees based on 16S rRNA and soluble cytochrome c ₂ reflect the real history of purple bacteria.Abbreviations LH I light harvesting complex I - RC reaction center     

The two-electron gate in photosynthetic bacteria

This paper gives a historical and personal account of the author's work in Rod Clayton's laboratory, when he observed the first evidence of the two-electron gate in bacterial reaction center. Colin Wraight had independently discovered this phenomenon at the same time. The high similarity between the acceptor side of Photosystem II (PS II) and of bacterial reaction centers was one of the first proofs for a profound homology between these two photosystems. This revised version was published online in June 2006 with corrections to the Cover Date.     

Algal photosynthetic aeration increases the capacity of bacteria to degrade organics in wastewater

Wastewater treatment is an energy-intensive process and a net emitter of greenhouse gas emissions. A large fraction of these emissions is due to intensive aeration of aerobic bacteria to facilitate break-down of organic compounds. Algae can generate dissolved oxygen at levels in excess of saturation, and therefore hold the potential to partially displace or complement mechanical aeration in wastewater treatment processes. The objective of this study was to develop an internally consistent experimental and modeling approach to test the hypothesis that algal photosynthetic aeration can speed the removal of organic constituents by bacteria. This framework was developed using a simplified wastewater treatment process consisting of a model bacteria (Escherichia coli), a model algae (Auxenochlorella protothecoides), and a single carbon source that was consumable by bacteria only. This system was then tested both with and without the presence of algae. A MATLAB model that considered mass transfer and biological kinetics was used to estimate the production and consumption of O₂ and CO₂ by algae and bacteria. The results indicated that the presence of algae led to 18–66% faster removal of COD by bacteria, and that roughly one-third of biochemical oxygen demand was offset by algal photosynthetic aeration.     

Discrepancy between experimental and theoretical excitation transfer rates in LH2 bacteriochlorophyll-protein complexes of purple bacteria

Discrepancy is revealed between the values of excitation transfer times measured experimentally, and those calculated, for the atomic structures of B800 → B850 bacteriochlorophylls within the LH2 light-harvesting pigment–protein complex of the purple bacterium Rhodopseudomonas acidophila. The value 2.9–3.2 ps for the B800 → B850 excitation transfer, calculated on the basis of atomic structure of LH2, is about 4-times longer than that measured for this bacterium (0.7 ps). This discrepancy appears common in at least two purple bacteria. Possible sources responsible for this discrepancy are discussed. It may either signify some drawback/s/ in our notions about the precise in vivo structure of LH2 complexes, for example, possible changes of LH2 structure during crystallization, or it may reflect our ignorance of some mechanisms involved in excitation migration.     

Chemosensory and photosensory perception in purple photosynthetic bacteria utilize common signal transduction components.

The chemotaxis gene cluster from the photosynthetic bacterium Rhodospirillum centenum contains five open reading frames (ORFs) that have significant sequence homology to chemotaxis genes from other bacteria. To elucidate the functions of each ORF, we have made various mutations in the gene cluster and analyzed their phenotypic defects. Deletion of the entire che operon (delta che), as well as nonpolar disruptions of cheAY, cheW, and cheR, resulted in a smooth-swimming phenotype, whereas disruption of cheB resulted in a locked tumbly phenotype. Each of these mutants was defective in chemotactic response. Interestingly, disruption of cheY resulted in a slight increase in the frequency of tumbling/reversal with no obvious defects in chemotactic response. In contrast to observations with Escherichia coli and several other bacteria, we found that all of the che mutant cells were capable of differentiating into hyperflagellated swarmer cells when plated on a solid agar surface. When viewed microscopically, the smooth-swimming che mutants exhibited active surface motility but were unable to respond to a step-down in light intensity. Both positive and negative phototactic responses were abolished in all che mutants, including the cheY mutant. These results indicate that eubacterial photosensory perception is mediated by light-generated signals that are transmitted through the chemotaxis signal transduction cascade.     

Pigment organization and energy transfer in the green photosynthetic bacterium Chloroflexus aurantiacus. III. Energy transfer in whole cells

The transfer of excitation energy in intact cells of the thermophilic green photosynthetic bacterium Chloroflexus aurantiacus was studied both at low temperature and under more physiological conditions. Analysis of excitation spectra measured at 4K indicates that the minor fraction of bacteriochlorophyll a present in the chlorosome functions as an intermediate in energy transfer between the main light-harvesting pigment BChl c and the membrane-bound B808-866 antenna complex. This supports the hypothesis that BChl a is associated with the base plate which connects the chlorosome with the membrane. The overall efficiency for energy transfer from the chlorosome to the membrane is only 15% at 4K. High efficiencies of close to 100% are observed above 40°C near the temperature where the cultures are grown. Cooling to 20°C resulted in a sudden drop of the transfer efficiency which appeared to originate in the chlorosome. This decrease may be related to a lipid phase transition. Further cooling mainly affected the efficiency of transfer between the chlorosome and the membrane. This effect can only partially be explained by a decreased Förster overlap between the chlorosomal BChl a and BChl a 808 associated with the membrane-bound antenna system. The temperature dependence of the fluorescence yield of BChl a 866 also appeared to be affected by lipid phase transitions, suggesting that this fluorescence can be used as a native probe of the physical state of the membrane.