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A very high rate for cyclic electron flow (CEF) around PSI (~180 ssup-/sup1 or 210 ssup-1/supin minimum medium or in the presence of a carbon source respectively) is measured in the presence of methyl viologen (MV) in intact cells of Chlamydomonas reinhardtii under anaerobic conditions. The observation of an efficient CEF in the presence of methyl viologen is in agreement with the previous results reports of Asada et al. in broken chloroplasts (Plant Cell Physiol. 31(4) (1990) 557-564). From the analysis of the P700 and PC absorbance changes, we propose that a confinement between 2 PC molecules, 1 PSI and 1 cytbsub6/subf corresponding to a functional supercomplex is responsible for these high rates of CEF. Supercomplex formation is also observed in the absence of methyl viologen, but with lower maximal CEF rate (about 100 ssup-1/sup) suggesting that this compound facilitates the mediation of electron transfer from PSI acceptors to the stromal side of cytbsub6/subf. Further analysis of CEF in mutants of Chlamydomonas defective in state transitions shows the requirement of a kinase-driven transition to state 2 to establish this functional supercomplex configuration. However, a movement of the LHCII antennae is not involved in this process. We discuss the possible involvement of auxiliary proteins, among which is a small cytbsub6/subf-associated polypeptide, the PETO protein, which is one of the targets of the STT7 kinase.

International audience; Jacques Breton spent his 39 years of professional life at Saclay, a center of the French Atomic Energy Commission. He studied photosynthesis with various advanced biophysical tools, often developed by himself and his numerous coworkers, obtaining a large number of new information on the structure and the functioning of antenna and of reaction centers of plants and bacteria: excitation migration in the antenna, orientation of molecules, rate of primary reactions, binding of pigments and electron transfer cofactors. Although it is much too short to illustrate his impressive work, we hope that this contribution will help maintaining the souvenir of Jacques Breton as an active and enthusiastic person, full of qualities, devoted to research and to his family as well. We include personal comments from N. E. Geacintov, A. Dobek, W. Leibl, M. Vos and W. W. Parson.

Rhodopseudomonas palustris is a purple photosynthetic bacterium that accumulates in the inner membrane the photosynthetic pigment spirilloxanthin, formed from lycopene. Here, we describe the procedures used to successfully engineer Rps. palustris strains to reroute the production of lycopene toward the synthesis of ß-carotene or canthaxanthin. The crtCD genes specifically involved in spirilloxanthin were replaced by crtY and crtW genes from Bradyrhizobium ORS278 to synthesize ß-carotene and (or) canthaxanthin, two pigments of industrial interest. Since the synthesis of canthaxanthin depends on the presence of oxygen, the procedure to optimize their production is also proposed. By modulating the light and oxygen during the growth process, a single species of photosynthetic bacteria, with an efficient growth rate, produces various carotenoids of economical interest.

International audience; Rhodopseudomonas palustris is a purple photosynthetic bacterium that accumulates in the inner membrane the photosynthetic pigment spirilloxanthin, formed from lycopene. Here, we describe the procedures used to successfully engineer Rps. palustris strains to reroute the production of lycopene toward the synthesis of ß-carotene or canthaxanthin. The crtCD genes specifically involved in spirilloxanthin were replaced by crtY and crtW genes from Bradyrhizobium ORS278 to synthesize ß-carotene and (or) canthaxanthin, two pigments of industrial interest. Since the synthesis of canthaxanthin depends on the presence of oxygen, the procedure to optimize their production is also proposed. By modulating the light and oxygen during the growth process, a single species of photosynthetic bacteria, with an efficient growth rate, produces various carotenoids of economical interest.

Deletion of two of the major electron carriers, the reaction center-bound tetrahemic cytochrome and the HiPIP, involved in the light-induced cyclic electron transfer pathway of the purple photosynthetic bacterium, Rubrivivax gelatinosus, significantly impairs its anaerobic photosynthetic growth. Analysis on the light-induced absorption changes of the intact cells of the mutants shows, however, a relatively efficient photo-induced cyclic electron transfer. For the single mutant lacking the reaction center-bound cytochrome, we present evidence that the electron carrier connecting the reaction center and the cytochrome bc1 complex is the High Potential Iron–sulfur Protein. In the double mutant lacking both the reaction center-bound cytochrome and the High Potential Iron–sulfur Protein, this connection is achieved by the high potential cytochrome c8. Under anaerobic conditions, the halftime of re-reduction of the photo-oxidized primary donor by these electron donors is 3 to 4 times faster than the back reaction between P+ and the reduced primary quinone acceptor. This explains the photosynthetic growth of these two mutants. The results are discussed in terms of evolution of the type II RCs and their secondary electron donors.

Photosynthetic electron transfer has been examined in whole cells, isolated membranes and in partially purified reaction centers (RCs) of Roseicyclus mahoneyensis, strain ML6 and Porphyrobacter meromictius, strain ML31, two species of obligate aerobic anoxygenic phototrophic bacteria. Photochemical activity in strain ML31 was observed aerobically, but the photosynthetic apparatus was not functional under anaerobic conditions. In strain ML6 low levels of photochemistry were measured anaerobically, possibly due to incomplete reduction of the primary electron acceptor (Q(A)) prior to light excitation, however, electron transfer occurred optimally under low oxygen conditions. Photoinduced electron transfer involves a soluble cytochrome c in both strains, and an additional reaction center (RC)-bound cytochrome c in ML6. The redox properties of the primary electron donor (P) and Q(A) of ML31 are similar to those previously determined for other aerobic phototrophs, with midpoint redox potentials of +463 mV and -25 mV, respectively. Strain ML6 showed a very narrow range of ambient redox potentials appropriate for photosynthesis, with midpoint redox potentials of +415 mV for P and +94 mV for Q(A). Cytoplasm soluble and photosynthetic complex bound cytochromes were characterized in terms of apparent molecular mass. Fluorescence excitation spectra revealed that abundant carotenoids not intimately associated with the RC are not involved in photosynthetic energy conservation.

The purple photosynthetic bacterium Rubrivivax gelatinosus has, at least, four periplasmic electron carriers, i.e., HiPIP, two cytochromes c 8 with low- and high-midpoint potentials, and cytochrome c 4 as electron donors to the photochemical reaction center. The quadruple mutant lacking all four electron carrier proteins showed extremely slow photosynthetic growth. During the long-term cultivation of this mutant under photosynthetic conditions, a suppressor strain recovering the wild-type growth level appeared. In the cells of the suppressor strain, we found significant accumulation of a soluble c -type cytochrome that has not been detected in wild-type cells. This cytochrome c has a redox midpoint potential of about + 280 mV and could function as an electron donor to the photochemical reaction center in vitro . The amino acid sequence of this cytochrome c was 65% identical to that of the high-potential cytochrome c 8 of this bacterium. The gene for this cytochrome c was identified as nirM on the basis of its location in the newly identified nir operon, which includes a gene coding cytochrome cd 1 -type nitrite reductase. Phylogenetic analysis and the well-conserved nir operon gene arrangement suggest that the origin of the three cytochromes c 8 in this bacterium is NirM. The two other cytochromes c 8 , of high and low potentials, proposed to be generated by gene duplication from NirM, have evolved to function in distinct pathways.

In aerobic anoxygenic phototrophs, oxygen is required for both the formation of the photosynthetic apparatus and an efficient cyclic electron transfer. Mutants of Bradyrhizobium sp. ORS278 affected in photosystem synthesis were selected by a bacteriochlorophyll fluorescence-based screening. Out of the 9,600 mutants of a random Tn5 insertion library, 50 clones, corresponding to insertions in 28 different genes, present a difference in fluorescence intensity compared to the WT. Besides enzymes and regulators known to be involved in photosystem synthesis, 14 novel components of the photosynthesis control are identified. Among them, two genes, hsIU and hsIV, encode components of a protein degradation complex, probably linked to the renewal of photosystem, an important issue in Bradyrhizobia which have to deal with harmful reactive oxygen species. The presence of homologs in non-photosynthetic bacteria for most of the regulatory genes identified during study suggests that they could be global regulators, as the RegA-RegB system.

The recent sequence analysis of the photosynthetic and plant-symbiotic Bradyrhizobium sp. strain BTAi1 revealed the unexpected presence of a pucBA operon encoding the apoproteins of peripheral light-harvesting (LH) complexes. This pucBA operon is found close to a bacteriophytochrome gene ( BphP3 B BTAi1 ) and a two-component transcriptional regulator gene (TF BTAi1 gene). In this study, we show that BphP3 B BTAi1 acts as a bona fide bacteriophytochrome and controls, according to light conditions, the expression of the pucBA operon found in its vicinity. This light regulatory pathway is very similar to the one previously described for chromo-BphP4 Rp in Rhodopseudomonas palustris and conducts the synthesis of a peripheral LH complex. This LH complex presents a single absorption band at low temperature, centered at 803 nm. Fluorescence emission analysis of intact cells indicates that this peripheral LH complex does not act as an efficient light antenna. One putative function of this LH complex could be to evacuate excess light energy in order to protect Bradyrhizobium strain BTAi1, an aerobic anoxygenic photosynthetic bacterium, against photooxidative damage during photosynthesis.

Since the first discovery of a bacteriophytochrome in Rhodospirillum centenum, numerous bacteriophytochromes have been identified and characterized in other anoxygenic photosynthetic bacteria. This review is focused on the biochemical and biophysical properties of bacteriophytochromes with a special emphasis on their roles in the synthesis of the photosynthetic apparatus.

A new type of membrane-bound cytochrome c was found in a marine purple photosynthetic bacterium, Rhodovulum sulfidophilum. This cytochrome c was significantly accumulated in cells growing under anaerobic photosynthetic conditions and showed an apparent molecular mass of approximately 100 kDa when purified and analyzed by SDS-PAGE. The midpoint potential of this cytochrome c was 369 mV. Flash-induced kinetic measurements showed that this new cytochrome c can work as an electron donor to the photosynthetic reaction center. The gene coding for this cytochrome c was cloned and analyzed. The deduced molecular mass was nearly equal to 50 kDa. Its C-terminal heme-containing region showed the highest sequence identity to the water-soluble cytochrome c(2), although its predicted secondary structure resembles that of cytochrome c(y). Phylogenetic analyses suggested that this new cytochrome c has evolved from cytochrome c(2). We, thus, propose its designation as cytochrome c(2m). Mutants lacking this cytochrome or cytochrome c(2) showed the same growth rate as the wild type. However, a double mutant lacking both cytochrome c(2) and c(2m) showed no growth under photosynthetic conditions. It was concluded that either the membrane-bound cytochrome c(2m) or the water-soluble cytochrome c(2) work as a physiological electron carrier in the photosynthetic electron transfer pathway of Rvu. sulfidophilum.

Bacterial cytochrome bc1-complex encoded by the petABC operon consists of three subunits, the Rieske iron-sulphur protein, the b-type cytochrome, and the c1-type cytochrome. Disruption of the petA gene of Rubrivivax gelatinosus is not lethal under photosynthetic growth conditions. However, deletion of both petA and petB results in a photosynthesis-deficient strain, suggesting the presence of a second gene encoding a Rieske protein and rescuing a functional cytochrome bc1-complex in the PETA1 mutant. The corresponding petA2 gene was identified and the PETA2 mutant could also grow under photosynthetic conditions. The double mutant PETA12, however, was unable to grow photosynthetically. The presence of a photo-induced cyclic electron transfer was tested by monitoring the kinetics of cytochrome photo-oxidation on intact cells; the data confirm the capacity of petA2 to replace petA1 in the bc1-complex to support photosynthesis. Soluble forms of both PetA1 and PetA2 Rieske proteins were purified from Escherichia coli and found to contain correctly inserted [2Fe-2S] clusters. Electron paramagnetic resonance (EPR) spectroscopy and midpoint potential measurements showed typical [2Fe-2S] signals and E(m) values of +275 mV for both Rieske proteins. The high amino acid sequence similarity and the obtained midpoint potential values argue for a functional role of these proteins in the cytochrome bc1-complex. The presence of duplicated Rieske genes is not restricted to R. gelatinosus. Phylogenetic trees of Rieske genes from Rubrivivax and other proteobacteria as well as from cyanobacteria were reconstructed. On the basis of the phylogenetic analyses, differing evolutionary origins of duplicated Rieske genes in proteo- and cyanobacteria are proposed.

The kinetics of reoxidation of the primary acceptor Q(a) has been followed by measuring the changes in the fluorescence yield induced by a series of saturating flashes in intact cells of Rhodobacter sphaeroides in anaerobic conditions. At 0 degrees C, about half of Q(a)(-) is reoxidized in about 200 ms while reoxidation of the remaining fraction is completed in several seconds to minutes. The fast phase is associated with the transfer of ubiquinone formed at site Q(o) of the cytochrome bc(1) complex while the slowest phase is associated with the diffusion of ubiquinone present in the membrane prior to the flash excitation. The biphasic kinetics of Q(a)(-) oxidation is interpreted assuming that the electron chain is organized in supercomplexes that associate two RCs and one cyt bc(1) complex, which allows a fast transfer of quinone formed at the level of cyt bc(1) complex to the RCs. In agreement with this model, the fast phase of Q(a)(-) reoxidation is inhibited by myxothiazol, a specific inhibitor of cyt bc(1). The PufX-deleted mutant displays only the slowest phase of Q(a)(-) oxidation; it is interpreted by the lack of supramolecular organization of the photosynthetic chain that leads to a larger average distance between cyt bc(1) and RCs.

The two closely related bacteria Bradyrhizobium and Rhodopseudomonas palustris show an unusual mechanism of regulation of photosystem formation by light thanks to a bacteriophytochrome that antirepresses the regulator PpsR. In these two bacteria, we found out, unexpectedly, that two ppsR genes are present. We show that the two Bradyrhizobium PpsR proteins exert antagonistic effects in the regulation of photosystem formation with a classical repressor role for PpsR2 and an unexpected activator role for PpsR1. DNase I footprint analysis show that both PpsR bind to the same DNA TGTN12ACA motif that is present in tandem in the bchC promoter and the crtED intergenic region. Interestingly, the cycA and aerR promoter regions that contain only one conserved palindrome are recognized by PpsR2, but not PpsR1. Further biochemical analyses indicate that PpsR1 only is redox sensitive through the formation of an intermolecular disulfide bond, which changes its oligomerization state from a tetramer to an octamer under oxidizing conditions. Moreover, PpsR1 presents a higher DNA affinity under its reduced form in contrast to what has been previously found for PpsR or its homolog CrtJ from the Rhodobacter species. These results suggest that regulation of photosystem synthesis in Bradyrhizobium involves two PpsR competing for the binding to the same photosynthesis genes and this competition might be modulated by two factors: light via the antagonistic action of a bacteriophytochrome on PpsR2 and redox potential via the switch of PpsR1 oligomerization state.

The photosynthetic cyclic electron transfer of the purple bacterium Rubrivivax gelatinosus, involving the cytochrome bc(1) complex and the reaction center, can be carried out via two pathways. A high potential iron-sulfur protein (HiPIP) acts as the in vivo periplasmic electron donor to the reaction center (RC)-bound cytochrome when cells are grown under anaerobic conditions in the light, while cytochrome c is the soluble electron carrier for cells grown under (8)aerobic conditions in the dark. A spontaneous reversion of R. gelatinosus C244, a defective mutant in synthesis of the RC-bound cytochrome by insertion of a Km(r) cassette leading to gene disruption with a slow growth rate, restores the normal photosynthetic growth. This revertant, designated C244-P1, lost the Km(r) cassette but synthesized a RC-bound cytochrome with an external 77-amino acid insertion derived from the cassette. We characterized the RC-bound cytochrome of this mutant by EPR, time-resolved optical spectroscopy, and structural analysis. We also investigated the in vivo electron transfer rates between the two soluble electron donors and this RC-bound cytochrome. Our results demonstrated that the C244-P1 RC-bound cytochrome is still able to receive electrons from HiPIP, but it is no longer reducible by cytochrome c(8). Combining these experimental and theoretical protein-protein docking results, we conclude that cytochrome c(8) and HiPIP bind the RC-bound cytochrome at two distinct but partially overlapping sites.

The cytochrome bound to the photosynthetic reaction center of Rhodovulum sulfidophilum presents two unusual characteristics with respect to the well characterized tetraheme cytochromes. This cytochrome contains only three hemes because it lacks the peptide motif CXXCH, which binds the most distal fourth heme. In addition, we show that the sixth axial ligand of the third heme is a cysteine (Cys-148) instead of the usual methionine ligand. This ligand exchange results in a very low midpoint potential (-160 +/- 10 mV). The influence of the unusual cysteine ligand on the midpoint potential of this distal heme was further investigated by site-directed mutagenesis. The midpoint potential of this heme is upshifted to +310 mV when cysteine 148 is replaced by methionine, in agreement with the typical redox properties of a His/Met coordinated heme. Because of the large increase in the midpoint potential of the distal heme in the mutant, both the native and modified high potential hemes are photooxidized at a redox poise where only the former is photooxidizable in the wild type. The relative orientation of the three hemes, determined by EPR measurements, is shown different from tetraheme cytochromes. The evolutionary basis of the concomitant loss of the fourth heme and the down-conversion of the third heme is discussed in light of phylogenetic relationships of the Rhodovulum species triheme cytochromes to other reaction center-associated tetraheme cytochromes.

The synthesis of the photosynthetic apparatus of different strains of Rhodopseudomonas palustris has been studied as a function of the oxygen concentration and far-red light. For strain CEA001, only a small amount of photosynthetic apparatus is synthesized in the dark for oxygen concentration higher than 8% whereas synthesis is strongly enhanced by far-red light illumination. This enhancement is due to the action of a bacteriophytochrome (ORF2127/ORF2128), which antagonizes the repressor PpsR. On the contrary, a large fraction of photosystem is synthesized in the dark and far-red illumination induces no enhancement in strain CGA009. This difference in phenotype of strain CGA009 is explained by a single point-mutation R428C in the helix-turn-helix DNA binding motif of PpsR, rendering it inactive. In addition, a frame-shift mutation had occurred in the gene encoding bacteriophytochrome (ORF2127/ORF2128), conducting to a truncated inactive sensor. We propose that these mutations occurred in culture. Bacteria have developed a sophisticated regulatory process to synthesize their photosynthetic apparatus when light is available. This process is a critical advantage for the bacteria under natural conditions since they optimize their development depending on the available energy resources. On the contrary, under laboratory growth conditions where there is no substrate limitation, there is no crucial need for such a regulation and deleterious mutations affecting this process are of no importance.

High potential iron-sulfur protein (HiPIP), a small soluble redox protein, has been shown to serve in vivo as electron donor to the photosynthetic reaction centre (RC) in Rubrivivax gelatinosus [Biochemistry 34 (1995) 11736]. The results of time-resolved optical spectroscopy on membrane-fragments from this organism indicates that the photooxidized RC is re-reduced by HiPIP even in the absence of the soluble fraction. This implies that a significant fraction of HiPIP can firmly bind to the membrane in a conformation able to interact with the RCs. Salt treatment of the membrane-fragments abolishes these re-reduction kinetics, demonstrating the presence of HiPIP on the membrane due to association with the RC rather than due to simple trapping in hypothetical chromatophores. The existence of such a functional complex in membranes is confirmed and its structure further examined by electron paramagnetic resonance (EPR) performed on membrane-fragments. Orientation-dependent EPR spectra of HiPIP were recorded on partially ordered membranes, oxidized either chemically or photochemically. Whereas hardly any preferential orientation of the HiPIP was seen in the chemically oxidised sample, a subpopulation of HiPIP showing specific orientations could be photooxidised. This fraction arises from the electron transfer complex between HiPIP and the RC.

A gene encoding the high-potential iron-sulfur protein (HiPIP) was cloned from the purple photosynthetic bacterium Rubrivivax gelatinosus. An insertional disruption of this gene by a kanamycin resistance cartridge resulted in a significant decrease in the growth rate under photosynthetic growth conditions. Flash-induced kinetic measurements showed that the rate of reduction of the photooxidized reaction center is greatly diminished in the mutant depleted in the HiPIP. On the other hand, mutants depleted in the low- and high-potential cytochromes c(8), the two other soluble electron carriers, which have been shown to donate an electron to the reaction center in Rvi. gelatinosus, showed growth rates similar to those of the wild type under both photosynthetic and respiratory growth conditions. It was concluded that HiPIP is the major physiological electron donor to the reaction center in Rvi. gelatinosus cells grown under photosynthetic conditions.

Plants use a set of light sensors to control their growth and development in response to changes in ambient light. In particular, phytochromes exert their regulatory activity by switching between a biologically inactive red-light-absorbing form (Pr) and an active far-red-light absorbing form (Pfr)1,2. Recently, biochemical and genetic studies have demonstrated the occurrence of phytochrome-like proteins in photosynthetic and non-photosynthetic bacteria3,4,5,6,7—but little is known about their functions. Here we report the discovery of a bacteriophytochrome located downstream from the photosynthesis gene cluster in a Bradyrhizobium strain symbiont of Aeschynomene. The synthesis of the complete photosynthetic apparatus is totally under the control of this bacteriophytochrome. A similar behaviour is observed for the closely related species Rhodopseudomonas palustris, but not for the more distant anoxygenic photosynthetic bacteria of the genus Rhodobacter, Rubrivivax or Rhodospirillum. Unlike other (bacterio)phytochromes, the carboxy-terminal domain of this bacteriophytochrome contains no histidine kinase features. This suggests a light signalling pathway involving direct protein–protein interaction with no phosphorelay cascade. This specific mechanism of regulation may represent an important ecological adaptation to optimize the plant–bacteria interaction.

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.

The photosynthetic apparatus in the bacterium Rhodobacter sphaeroides is mostly present in intracytoplasmic membrane invaginations. It has long been debated whether these invaginations remain in topological continuity with the cytoplasmic membrane, or form isolated chromatophore vesicles. This issue is revisited here by functional approaches. The ionophore gramicidin was used as a probe of the relative size of the electro-osmotic units in isolated chromatophores, spheroplasts, or intact cells. The decay of the membrane potential was monitored from the electrochromic shift of carotenoids. The half-time of the decay induced by a single channel in intact cells was about 6 ms, thus three orders of magnitude slower than in isolated chromatophores. In spheroplasts obtained by lysis of the cell wall, the single channel decay was still slower (~23 ms) and the sensitivity toward the gramicidin concentration was enhanced 1,000-fold with respect to isolated chromatophores. These results indicate that the area of the functional membrane in cells or spheroplasts is about three orders of magnitude larger than that of isolated chromatophores. Intracytoplasmic vesicles, if present, could contribute to at most 10 % of the photosynthetic apparatus in intact cells of Rba. sphaeroides. Similar conclusions were obtained from the effect of a ∆pH-induced diffusion potential in intact cells. This caused a large electrochromic response of carotenoids, of similar amplitude as the light-induced change, indicating that most of the system is sensitive to a pH change of the external medium. A single internal membrane and periplasmic space may offer significant advantages concerning renewal of the photosynthetic apparatus and reallocation of the components shared with other bioenergetic pathways.

A mutant of the phototrophic species belonging to the β-proteobacteria, Rubrivivax gelatinosus, lacking the photosynthetic growth ability was constructed by the removal of genes coding for the L, M, and cytochrome subunits of the photosynthetic reaction center complex. The L, M, and cytochrome genes derived from five other species of proteobacteria, Acidiphilium rubrum, Allochromatium vinosum, Blastochloris viridis, Pheospirillum molischianum, and Roseateles depolymerans, and the L and M subunits from two other species, Rhodobacter sphaeroides and Rhodopseudomonas palustris, respectively, have been introduced into this mutant. Introduction of the genes from three of these seven species, Rte. depolymerans, Ach. vinosum, and Psp. molischianum, restored the photosynthetic growth ability of the mutant of Rvi. gelatinosus, although the growth rates were 1.5, 9.4, and 10.7 times slower, respectively, than that of the parent strain. Flash-induced kinetic measurements for the intact cells of these three mutants showed that the photo-oxidized cytochrome c bound to the introduced reaction center complex could be rereduced by electron donor proteins of Rvi. gelatinosus with a t1/2 of less than 10 ms. The reaction center core subunits of photosynthetic proteobacteria appear to be exchangeable if the sequence identities of the LM core subunits between donor and acceptor species are high enough, i.e., 70 % or more.

Some leguminous species of the genus Aeschynomene are specifically stem-nodulated by photosynthetic bradyrhizobia. To study the effect of bacterial photosynthesis during symbiosis, we generated a photosynthesis-negative mutant of the Bradyrhizobium sp. strain ORS278 symbiont of Aeschynomene sensitiva . The presence of a functional photosynthetic unit in bacteroids and the high expression of the photosynthetic genes observed in stem nodules demonstrate that the bacteria are photosynthetically active during stem symbiosis. Stem inoculation by the photosynthetic mutant gave a 50% decrease in stem-nodule number, which reduced nitrogen fixation activity and plant growth in the same proportion. These results indicate an important role of bacterial photosynthesis in the efficiency of stem nodulation.

The orientation of the chromophores in the cytochrome bc 1 of Rhodospirillum rubrum, Rhodobacter sphaeroides, and beef heart mitochondria is reported. The combination of redox-resolved absorption spectrophotometry and linear dichroism experiments at low temperature allows the determination of the orientation of the three hemes with respect to the membrane plane. The orientations of theb H-and b L-hemes of theR. sphaeroides and beef heart mitochondrial complexes are similar to those determined by crystallographic studies of the mitochondrial cytochrome bc 1. On the other hand the orientations of the b-hemes of the R. rubrum complex lead to the conclusion that theb H-heme is more perpendicular to the membrane plane than the b L-heme. This could be explained by a specific organization of the b-hemes due to subunit composition of the complex or, alternatively, to a different spatial position of the heme transitions with respect to the porphyrin macrocycle compared with the other complexes. Moreover, our results demonstrate a different orientation of the hemec 1 of the three studied complexes in comparison to crystallographic studies. This difference may arise from the above hypothesis on the transitions of the heme or from flexibility of this subunit in function of its redox state.

The organization of chromophores in the cytochrome b 6 f fromChlamydomonas reinhardtii has been studied spectroscopically. Linear dichroism (LD) measurements, performed on the complex co-reconstituted into vesicles with photosynthetic reaction centers as an internal standard, allow the determination of the orientations of the chromophore with respect to the membrane plane. The orientations of the b H- andb L-hemes are comparable to those determined crystallographically on the cytochrome bc 1. The excitonic CD signal, resulting from the interaction betweenb-hemes, is similar to that reported for the cytochromebc 1. LD and CD data are consistent with the differences between the b 6 f andbc 1 leaving the orientation of theb-hemes unaffected. By contrast, the LD data yield a different orientation for the heme f as compared either to the heme c 1 in the crystallographic structures or to the heme f as studied by electron paramagnetic resonance. This difference could either result from incorrect assumptions regarding the orientations of the electronic transitions of the f-heme or may point to the possibility of a redox-dependent movement of cytochrome f. The chlorophyll a was observed in a well defined orientation, further corroborating a specific binding site for it in theb 6 f complex.

A detailed analysis of the periplasmic electron carriers of the photosynthetic bacterium Ectothiorhodospira sp. has been performed. Two low mid-point redox potential electron carriers, cytochrome c' and cytochrome c, are detected. A high potential iron-sulfur protein is the only high mid-point redox potential electron transfer component present in the periplasm. Analysis of light-induced absorption changes shows that this high potential iron-sulfur protein acts in vivo as efficient electron donor to the photo-oxidized high potential heme of the Ectothiorhodospira sp. reaction center.

In several strains of the photosynthetic bacterium Rubrivivax gelatinosus, the synthesis of a high midpoint potential cytochrome is enhanced 4-6-fold in dark aerobically grown cells compared with anaerobic photosynthetic growth. This observation explains the conflicting reports in the literature concerning the cytochrome c content for this species. This cytochrome was isolated and characterized in detail from Rubrivivax gelatinosus strain IL144. The redox midpoint potential of this cytochrome is +300 mV at pH 7. Its molecular mass, 9470 kDa, and its amino acid sequence, deduced from gene sequencing, support its placement in the cytochrome c8 family. The ratio of this cytochrome to reaction center lies between 0.8 and 1 for cells of Rvi. gelatinosus grown under dark aerobic conditions. Analysis of light-induced absorption changes shows that this high-potential cytochrome c8 can act in vivo as efficient electron donor to the photooxidized high-potential heme of the Rvi. gelatinosus reaction center.

In this work, we have examined, using Fourier-transform Raman (FT-R) spectroscopy, the bacteriochlorophyll a (BChl a) binding sites in light-harvesting (LH) antennae from different species of the Proteobacteria that exhibit unusal absorption properties. While the LH1 complexes from Erythromicrobium (E.) ramosum (RC-B871) and Rhodospirillum centenum (B875) present classic FT-R spectra in the carbonyl high-frequency region, we show that in the blue-shifted LH1 complex, absorbing at 856 nm, from Roseococcus thiosulfatophilus, as well as in the B798-832 LH2 from E. ramosum, or in the B830 complex from the obligate phototrophic bacterium Chromatium purpuratum, some H-bonds between the acetyl carbonyl of the BChl a and the surrounding protein are missing. The molecular mechanisms responsible for the unusual absorption of these complexes are thus similar to those responsible for tuning of the absorption of the LH2 complexes between 850 and 820 nm. Furthermore, our results suggest that the binding pocket of the monomeric BChl in the LH2 from E. ramosum is different from that of Rps. acidphila or Rb. sphaeroides. The FT-R spectra of Chromatium purpuratum indicate that, in contrast with every LH2 complex previously studied by FT-R spectroscopy, no free-from-interaction keto groupings exist in this complex.

The B-band excited resonance Raman (RR) spectra (100-1700 cm-1) of the bacterial cytochrome bc1 complex purified from Rhodospirillum rubrum are reported. Four redox states, i.e., the persulfate-oxidized, "as prepared", and ascorbate- and dithionite-reduced states of the complex, were investigated with the laser excitations at 406.7, 413.1, and 441.6 nm. Following the different absorption properties of the b- and c-type hemes and the different resonance enhancements of the vibrational modes of oxidized and reduced hemes, RR contributions from the b- and c-type hemes were characterized. For the nu2, nu10, and nu8 porphyrin vibrational modes, individual contributions of hemes c1, bH, and bL were determined. The data show that the macrocycle conformation of the three hemes of the cytochrome bc1 complex is different. In particular, the frequencies assigned to ferrous heme bL (1580, 1610, and 352 cm-1, respectively) reveal that its porphyrin is more strongly distorted than that of ferrous heme bH (1584, 1614, and 344 cm-1, respectively). The frequencies of the nu11 modes (1543, 1536, and 1526 cm-1 for ferrous heme c1, heme bH, and heme bL, respectively) confirm that the axial histidylimidazole ligands of heme bL have a marked anionic character. Strong differences in the peripheral interactions of the three hemes with the proteins were also detected through the frequency differences of the nu5, nu13, nu14, and nu42 modes. Considering that hemes bH and bL are inserted into a four-helice bundle, the RR data are interpreted in the frame of a strong protein constraint on heme bL.

Reaction centers (RC) from the species Erythrobacter (Eb.) litoralis, Erythromonas (Em.) ursincola and Sandaracinobacter (S.) sibiricus have been purified by LDAO treatment of light-harvesting-reaction center complexes and DEAE chromatography. The content and overall organisation of the RCs' chromophores, determined by linear dichroism (LD) and absorption spectroscopy, are similar to those isolated from anaerobic photosynthetic bacteria. The redox properties of the primary electron donor are pH-independent and very similar to those determined for anaerobic photosynthetic bacteria with midpoint potential values equal to 445 (± 10), 475 and 510 mV for Eb. litoralis, S. sibiricus and Em. ursincola, respectively. The RC purified from Eb. litoralis does not contain bound cytochrome (cyt), whereas RCs isolated from S. sibiricus and Em. ursincola possess a tetraheme cyt c. Each of these tetraheme cyts contains two high potential hemes and two low potential hemes. Their redox properties are very similar, with midpoint potentials equal to 385 (± 10), 305, 40, -40 mV for Em. ursincola and 355, 285, 30, -48 mV for S. sibiricus. At physiological pH, the midpoint potential of the primary electron acceptor (QA) varies with a slope of -60 mV/pH unit. The reduced form of QA presents pK values of 9, 9.8, 10.5 for S. sibiricus, Em. ursincola and Eb. litoralis, respectively. The main difference observed between RCs isolated from anaerobic photosynthetic and from obligate aerobic bacteria is the Emvalues of QA which are 65 to 120 mV higher in the last case. This difference is proposed to be a major reason for the inability of these species to grow under anaerobic photosynthetic conditions.

Both the soluble cytochrome c2 and the membrane-bound cytochrome cy act as secondary electron carriers in photoinduced cyclic electron transfer chain of Rhodobacter capsulatus [Jenney and Daldal (1993) EMBO J 12: 1283–1292]. In this work, we have studied the kinetics of electron transfer between these secondary electron donors and the reaction center in intact cells of two mutants, MT-G4/S4 and MT-GS18 deleted in cytochrome c2 and in cytochrome c2 plus cytochrome bc1 complex, respectively. In the MT-G4/S4 mutant, only about one third of the primary electron donor is reduced by cytochrome cy in less than five ms. The remaining fraction is reduced in several seconds, although about 90% of the photoxidized cytochrome cy is reduced in less than 10 ms by the cytochrome bc1 complex. This implies that cytochrome cy is not in thermodynamic equilibrium with the large fraction of primary donors which are slowly reduced. As shown by energy transfer measurements, the reaction centers connected to cytochrome cy and the disconnected reaction centers are localized in the same membrane region. We propose that the movement of cyt cy is restricted to a small membrane domain which includes a single cytochrome bc1 complex. The kinetics of cytochrome cy photooxidation in the MT-G4/S4 mutant in the presence of myxothiazol presents a fast phase (t1/2 ∼ 3 µs) followed by a slower phase (t1/2 ∼ 20 µs). In the case of the double mutant MT-GS18, the kinetics of electron transfer between cytochrome cy and the reaction center is highly multiphasic and much slower than those observed for the MT-G4/S4 mutant. In particular, the amplitude of the fast phase is decreased by more than a factor 2 and the 20-µs phase is not observed. This implies an important structural role of the cytochrome bc1 complex in the interaction between reaction center and cytochrome cy, and their formation in supercomplex. The more problable stoichiometry of electron carriers in this supercomplex is 2 reaction centers, 2 cytochrome cy and 1 cytochrome bc1 complex.

We have studied the electron transfer reactions from the tetraheme cytochrome of Rhodopseudomonas viridis to the oxidized primary donor in whole cells with a new high sensitivity spectrophotometer. In this apparatus the monochromatic detecting flashes are provided by a YAG pumped Optical Parametric Oscillator, allowing a 10 ns time resolution. When four hemes are reduced the observed electron transfer reaction sequence is the following: first the low-potential c552 heme (the number refers to the maximum absorption wavelength in the alpha-band region) is oxidized with a half time of 130 ns, in agreement with previous reports of measurements performed with purified reaction centers. Then, the electron hole is transferred to the low potential c554 heme with a half time of 2.6 µs. When only the two high potential hemes are reduced the observed electron transfer sequence is the following: oxidation of the high potential c559 heme in the hundreds of ns time range (410 ns), reduction of this heme by the high potential c556 heme in the µs time range (2.7 µs). This confirms the first steps of electron transfer observed in isolated reaction centers. However, in the microsecond time domain, the overall amount of oxidized hemes increases suggesting that, in vivo, the equilibrium constant between the P+/P and the c559ox/c559red couples is significantly lower than expected from the difference in their midpoint potentials.

We have analyzed the rate of electron transfer between the tetrahemic cytochrome and the primary electron donor in isolated reaction centers of Roseobacter denitrificans as a function of the ambient redox potential. Three different phases are observed: a slow phase (half-time > ms), and two fast phases with half-times of 5 µs and 380 ns. The slow phase is present at high redox potential, it corresponds to the kinetics of charge recombination between the photo-oxidized primary electron acceptor P+ and the reduced primary acceptor (QA−). The 5 µs phase titrates with the reduction of the highest potential heme (HP1). This phase corresponds to the electron transfer between heme HP1 and P+. At redox potentials where the second high potential heme HP2 becomes reduced, the 5 µs phase disappears and is replaced by the 380 ns phase, which is therefore related to the electron transfer between the high potential heme HP2 and P+. To explain the large difference in the rate of oxidation of HP1 and HP2 we propose a tentative model where the heme HP2 is closest to P.

The photosynthetic apparatus and the electron carriers of seven species of five different genera of obligate aerobic phototrophic bacteria have been characterized by biochemical and biophysical techniques. A tetrahemic reaction center (RC) bound cytochrome (cyt) was found in Erythromonas (Em.) ursincola, Sandaracinobacter (S.) sibiricus and Roseococcus (R.) thiosulfatophilus, but not in Erythromicrobium (E.) ezovicum, Erythromicrobium ramosum, Erythromicrobium hydrolyticum and Erythrobacter (Eb.) litoralis. In none of the studied species, photochemical activity was observed under anaerobic conditions. Under aerobic conditions, the photoinduced cyclic electron transfer involves a soluble c-type cyt for the seven species. The cyt content of soluble and membrane fractions is highly dependent upon the species. The Erythromicrobium species (E. ezovicum, E. ramosum and E. hydrolyticum) contains a major soluble cyt while the other species possess several soluble cyts, up to four in the case of Eb. litoralis. These cyts have been characterized in terms of midpoint potential and apparent molecular mass. The presence of cyt bc1 complexes has been clearly detected in Eb. litoralis, E. hydrolyticum, E. ezovicum and E. ramosum. These last three species also contain a high midpoint potential (350 mV) membrane-bound cyt c of unknown function.

We demonstrate the participation of a cytochrome c8 and a high-potential iron-sulfur protein (HiPIP) in the photoinduced electron transfer in whole cells of Rhodocyclus tenuis depending on the redox state or background continuous illumination. At high redox potentials (above 350 mV) or under a strong background illumination (5 W m-2), the cytochrome c8 acts as the physiological electron donor to the photo-oxidized high-potential hemes of the tetraheme cytochrome bound to the reaction center. For redox potentials ranging from 200 to 310 mV or under weak background illumination (1. 25 W m-2), the electron carrier is the HiPIP. The electron transfer between cyt c8 and HiPIP and the tetraheme cytochrome has half-times of 300 and 480 micros, respectively. A slow electrogenic phase of the membrane potential is linked to their rereduction. This phase is sensitive to a specific inhibitor of the cyt bc1 complex, indicating involvement of cyt c8 and HiPIP in the photoinduced cyclic electron transfer at these two redox conditions.

The three phases of the flash-induced membrane potential rise were measured in Rhodobacter sphaeroides, at temperatures between 16°C and −30°C, in the liquid or the frozen state. We show that phase II, which is myxothiazol-insensitive, includes phase IIa and phase IIb, completed at −8°C in about 2 ms and 10 ms, respectively. Phase IIa is very likely associated with the protonation of the doubly-reduced quinone acceptor QB= (Drachev, L.A., Mamedov, A., Mulkidjanian, A.Y., Semenov, A.Y., Shinkarev, V.P. and Verkhovsky, M.J. (1988) FEBS Lett. 233, 315–318); phase IIb is associated with the oxidation of cytochrome c2 (Jackson, J.B. and Dutton, P.L., 1973, Biochim. Biophys. Acta 325, 102–113). Freezing the sample does not modify the kinetics of phase IIa but slows down phase IIb by a factor of 2. The amplitude of phase III, which is exclusively related to electron and proton transfer within the cytochrome b/c1 complex, is temperature-independent between room temperature and −16°C. In the frozen state, the rate of phase III is mainly limited by the electron transfer from cytochrome c2 to the reaction centers and not by the movement of cytochrome c2 between the two membrane complexes. These results are interpreted assuming that one cytochrome c2 is trapped in a supercomplex formed by the association of two reaction centers and one cytochrome b/c1 complex.

Shewanella oneidensisMR-1 reduces selenite and tellurite preferentially under anaerobic conditions. The Se(0) and Te(0) deposits are located extracellularly and intracellularly, respectively. This difference in localization and the distinct effect of some inhibitors and electron acceptors on these reduction processes are taken as evidence of two independent pathways.

Illumination of intact cells of Rhodobacter sphaeroides under anaerobic conditions has a dual effect on the redox state of the quinone pool. A large oxidation of the quinone pool is observed during the first seconds following the illumination. This oxidation is suppressed by the addition of an uncoupler in agreement with a light-induced reverse electron transfer at the level of the complex I, present both in the non-invaginated part of the membrane and in the chromatophores. At longer dark times, this illumination increases the reducing power of the cells leading to a significant reduction of the others reaction centers (RCs). From the observation that a significant proportion of RCs could be reduced by the preillumination without affecting the numbers of charge separation for the RCs, we conclude that there is no rapid thermodynamic equilibrium between the quinones present in the non-invaginated part of the membrane and those localized in the chromatophores. Under anaerobic conditions where the chromatophores quinone pool is fully reduced, we deduce, on the basis of flash-induced fluorescence kinetics, that the reduced RCs are exclusively reoxidized by the quinone generated at the Q o site of the cyt bc 1 complex. The supramolecular association between a dimeric RC-LHI complex and one cyt bc 1 complex allows the confinement of a quinone between the RC-LHI directly associated to the cyt bc 1 complex.

Flash-induced kinetics of the membrane potential increase related to electron transfer within the cytochrome (cyt) b/c1 complex (Phase III) and that of cyt c1+c2 reduction have been measured as a function of myxothiazol concentration in isolated chromatophores and whole cells of Rhodobacter sphaeroides. Upon addition of nonsaturating concentrations of myxothiazol, kinetics of Phase III display two phases, Phase IIIa and Phase IIIb. The amplitude of Phase IIIa, completed in about 10 ms, is proportional to the fraction of non-inhibited cyt b/c1 complexes, while its half-time is independent of the myxothiazol concentration. A fast cyt c1+c2 reduction phase is correlated to Phase IIIa. These experiments demonstrate that, in a range of time of several ms, diffusion of cyt c2 is restricted to domains formed by a supercomplex including two reaction centers (RCs) and a single cyt b/c1 complex, as proposed by Joliot et al. (Biochim Biophys Acta 975: 336-345, 1989). Phase IIIb, completed in about 100 ms, shows that positive charges or inhibitor molecules are exchanged between supercomplexes in this range of time. These exchanges occur within domains including 2 to 3 supercomplexes, i.e. in membrane domains smaller than a single chromatophore. These conclusions apply to both isolated chromatophores and whole cells.

We have found that the only high redox potential electron transfer component in the soluble fraction of Rubrivivax gelatinosus TG-9 is a high-potential iron-sulfur protein (HiPIP). We demonstrated the participation of this HiPIP in the photoinduced electron transfer both in vivo and in vitro. First, the addition of HiPIP to purified membranes enhanced the rate of re-reduction of the photooxidized reaction center. Second, the photooxidation of HiPIP was observed in intact cells of Ru. gelatinosus TG-9 under anaerobic conditions by EPR and absorption spectroscopies. Analysis of flash-induced absorption changes showed that the equilibration of positive equivalents between the reaction center and HiPIP occurs in less than 1 ms after flash excitation. The complete re-reduction of the photooxidized reaction center is achieved in tens of milliseconds. The turnover of a cyt bc1 is also involved in this reaction, as shown by a slow electrogenic phase of the membrane potential linked to this process.

Some aerobic photosynthetic bacteria produce a cocktail of carotenoids, some of them being of a high economic value. A good example is the photosynthetic Bradyrhizobium sp. strain ORS278, which synthesizes, in addition to the photosynthetic carotenoid spirilloxanthin, large amounts of canthaxanthin. Here, we describe the procedures that have been successfully used to isolate the different crt genes involved in the synthesis of both carotenoids in this bacteria. The synthesis of these carotenoids is stimulated under far-red light by a bacteriophytochrome. The procedure we developed to study the effect of the light on carotenoids synthesis is also described. Finally, we describe a procedure to genetically transform photosynthetic Bradyrhizobium strain ORS278 for improvement of canthaxanthin production.

Some aerobic photosynthetic bacteria produce a cocktail of carotenoids, some of them being of a high economic value. A good example is the photosynthetic Bradyrhizobium sp. strain ORS278, which synthesizes, in addition to the photosynthetic carotenoid spirilloxanthin, large amounts of canthaxanthin. Here, we describe the procedures that have been successfully used to isolate the different crt genes involved in the synthesis of both carotenoids in this bacteria. The synthesis of these carotenoids is stimulated under far-red light by a bacteriophytochrome. The procedure we developed to study the effect of the light on carotenoids synthesis is also described. Finally, we describe a procedure to genetically transform photosynthetic Bradyrhizobium strain ORS278 for improvement of canthaxanthin production.

International audience; Tropical aquatic species of the legume genus Aeschynomene are stem- and root-nodulated by bradyrhizobia strains that exhibit atypical features such as photosynthetic capacities or the use of a nod gene-dependent (ND) or a nod gene-independent (NI) pathway to enter into symbiosis with legumes. In this study we used a comparative genomics approach on nine Aeschynomene symbionts representative of their phylogenetic diversity. We produced draft genomes of bradyrhizobial strains representing different phenotypes: five NI photosynthetic strains (STM3809, ORS375, STM3847, STM4509 and STM4523) in addition to the previously sequenced ORS278 and BTAi1 genomes, one photosynthetic strain ORS285 hosting both ND and NI symbiotic systems, and one NI non-photosynthetic strain (STM3843). Comparative genomics allowed us to infer the core, pan and dispensable genomes of Aeschynomene bradyrhizobia, and to detect specific genes and their location in Genomic Islands (GI). Specific gene sets linked to photosynthetic and NI/ND abilities were identified, and are currently being studied in functional analyses.

Two pools of cytochrome c t can be observed in whole cells of Rhodobacter sphaeroides forma sp. denitrificans upon excitation by continuous light or saturating flashes. The first pool is connected only to the photosynthetic chain. The second one is preferentially coupled to the respiratory and the denitrifying chains. This second pool is in large excess compared to the first when cells are grown under denitrifying and/or aerobic conditions. These two pools equilibrate in less than 50 ms at pH lower than 7.5, but not at higher pH or in the presence of glycerol or divalent cations. For the first pool, the rate of electron transfer between cytochrome c 1 and cytochrome c 2 is not affected by the medium viscosity. Measurements of cytochrome c t re-reduction in the presence of subsaturating concentrations of myxothiazol show that a given cytochrome c 2 can only react with a single bc 1 complex. This is interpreted in terms of a supramolecular organization of the photosynthetic electron transfer components. Under conditions where the synthesis of the photosynthetic chain is repressed, i.e., addition of nitrate or dark semi-aerobic conditions, the LHII/LHI ratio decreases. This induces the formation of tubular membranes. Freeze-etching pictures of these tubes show a well-ordered dimeric organization of the membrane proteins.

International audience; Ramlibacter tataouinensis TTB310(T) (strain TTB310), a betaproteobacterium isolated from a semi-arid region of South Tunisia (Tataouine), is characterized by the presence of both spherical and rod-shaped cells in pure culture. Cell division of strain TTB310 occurs by the binary fission of spherical "cyst-like" cells ("cyst-cyst" division). The rod-shaped cells formed at the periphery of a colony (consisting mainly of cysts) are highly motile and colonize a new environment, where they form a new colony by reversion to cyst-like cells. This unique cell cycle of strain TTB310, with desiccation tolerant cyst-like cells capable of division and desiccation sensitive motile rods capable of dissemination, appears to be a novel adaptation for life in a hot and dry desert environment. In order to gain insights into strain TTB310's underlying genetic repertoire and possible mechanisms responsible for its unusual lifestyle, the genome of strain TTB310 was completely sequenced and subsequently annotated. The complete genome consists of a single circular chromosome of 4,070,194 bp with an average G+C content of 70.0%, the highest among the Betaproteobacteria sequenced to date, with total of 3,899 predicted coding sequences covering 92% of the genome. We found that strain TTB310 has developed a highly complex network of two-component systems, which may utilize responses to light and perhaps a rudimentary circadian hourglass to anticipate water availability at the dew time in the middle/end of the desert winter nights and thus direct the growth window to cyclic water availability times. Other interesting features of the strain TTB310 genome that appear to be important for desiccation tolerance, including intermediary metabolism compounds such as trehalose or polyhydroxyalkanoate, and signal transduction pathways, are presented and discussed.

Kinetics of primary donor and cytochromes c2 and c1 photooxidation during continuous illumination in Rhodobacter sphaeroides Ga whole cells are nearly monophasic at low pH ( 8.5) these kinetics are clearly biphasic, whatever the excitation light intensity, demonstrating the occurrence of two compartments out of equilibrium. Light-induced difference spectra show that the ‘fast’ compartment contains cytochrome c2 and cytochrome c1 in equal amount, while the ‘slow’ compartment includes a large excess of cytochrome c2. The ratio between cytochromes involved in the ‘fast’ and ‘slow’ compartments increases depending on the growth conditions (1.7 for cells grown semiaerobically, 1.2 and 0.8 for cells grown at high and low light intensities, respectively). Only the cytochrome involved in the ‘slow’ compartment is coupled to the respiratory chain. This series of results indicates that the ‘fast’ compartment corresponds to the intracytoplasmic part of the membrane, while the ‘slow’ compartment to the cytoplasmic region. For the ‘fast’ compartment the stoichiometry is independent of the growth conditions. The analysis of the slow phase of the carotenoid bandshift versus the fraction of cytochromes bc1 complex inhibited by myxothiazol shows that the diffusion of cytochrome c2 within the ‘fast’ compartment is restricted to domains including a single cytochromes bc1 complex, very likely associated with two reaction centers and one cytochrome c2 to form a supercomplex. In the ‘slow’ compartment, cytochrome c2 is a diffusible species and the stoichiometry between reaction centers and secondary donors highly depends upon the growth conditions. At high pH or in presence of MgCl2 the cytochrome c2 involved in the supercomplex cannot exchange with free cytochrome c2 in excess. Therefore, the free cytochrome c2 can only be photooxidized by the small number of reaction centers present in the cytoplasmic part of the membrane. At low pH, charge exchange occurs via the diffusion of cytochrome c2 between the two compartments.

Reaction centers (RCs) with one antenna complex attached (RC/LH1 complexes) were isolated from the alkaliphilic and moderately halophilic, phototrophic purple sulfur bacterium Ectothiorhodospira (E.) mobilis and analyzed with respect to quinone composition and charge recombination kinetics, using time-resolved (low- and room-temperature) spectroscopy and thin-layer chromatography. The RCs contain menaquinone as primary and ubiquinone as secondary quinone electron acceptor, QA and QB, respectively. QB is lost during isolation of the RC/LH1 complexes. P+Q−A charge recombination kinetics, which were shown to be pH independent, were only slightly temperature dependent, indicating that this recombination proceeds via the direct (electron tunnelling) route. At room temperature, its average lifetime was 34.5 ms. These decay kinetics were shown to be monophasic at room temperature and biphasic at low temperature. Addition of an excess of UQ6 to RC/LH1 complexes resulted in retardation of the P+ recovery, due to charge recombination of the state P+QAQ−B. Functional reconstitution of QB was also evident from flash-induced binary oscillations at 450 nm, which could be observed in RC/LH1 complexes in the presence of an excess of UQ6. This reconstitution of QB activity was, however, incomplete and decreased with increasing pH. The P+QAQ−B decay kinetics were pH independent; the average lifetime was about 6 s at room temperature. The apparent equilibrium constant K2 between the states Q−AQB and QAQ−B, and consequently the free energy difference between these states, were relatively large and pH independent. K2 was on average 167, whereas the mean value of the free energy difference between the two states was 130 meV. Finally, a scheme is presented for the kinetics and free energy changes of the electron transfer steps in isolated E. mobilis RCs.