The photosynthetic apparatus of Rhodobacter sphaeroides.

Functional and ultrastructural studies have indicated that the components of the photosynthetic apparatus of Rhodobacter sphaeroides are highly organized. This organization favors rapid electron transfer that is unimpeded by reactant diffusion. The light-harvesting complexes only partially surround the photochemical reaction center, which ensures an efficient shuttling of quinones between the photochemical reaction center and the bc1 complex.     

Supramolecular organization of the photosynthetic chain in chromatophores and cells of Rhodobacter sphaeroides

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.Abbreviations cyt cytochrome - MOPS 3-(N-morpholino)propane sulfonic acid - PMS phenazine methosulfate - P primary donor - Rb. Rhodobacter - RC reaction center     

Functional consequences of the organization of the photosynthetic apparatus in Rhodobacter sphaeroides: II. A study of PufX- membranes

In the bacterium R. sphaeroides, the polypeptide PufX is indispensable for photosynthetic growth. Its deletion is known to have important consequences on the organization of the photosynthetic apparatus. In the wild-type strain, complexes between the reaction center (RC) and the antenna (light-harvesting complex 1 (LH1)) are associated in dimers, and LH1 does not fully encircle the RC. In the absence of PufX, the complexes become monomeric, and the LH1 ring closes around the RC. We analyzed the functional consequences of PufX deletion. Some effects can be ascribed to the monomerization of the RC.LH1 complexes: the number of RCs that share a common antenna for excitation transfer or a common quinone pool become smaller. We examined the kinetic effects of the closed LH1 ring on quinone turnover: diffusion across LH1 entails a delay of approximately 1 ms, and the barrier appears to be located directly against the quinone-binding (secondary quinone acceptor (Q(B))) pocket. The diffusion of ubiquinol from the RC to the cytochrome bc1 complex is approximately 2-fold slower in the mutant, suggesting an increased distance between the two complexes. The properties of the Q(B) pocket (binding of inhibitors, stabilization of Q(B-), and rate of Q(B)-H2 formation) appear to be modified in the mutant. Another specificity of PufX- is the accumulation of closed centers in the Q(A-) (where Q(A) is the primary quinone acceptor) state as the secondary acceptor pool becomes reduced, which is probably the origin of photosynthetic incompetence. We suggest that this is related to the Q(B) pocket alterations. The malfunction of the reaction center is probably due to a faulty association with LH1 that is prevented in the PufX-containing structure.     

Hydrogen release by recombinant strains of Rhodobacter sphaeroides using a modified photosynthetic apparatus

Hydrogen release by recombinant strains of Rhodobacter sphaeroides pRK puf DD13 without a peripheral light-harvesting antenna complex and pRK puf deltaLM1 which is able to synthesize both antenna complexes, both of which were grown in conditions of nitrogen limitation, has been studied. The velocity of hydrogen release depended on light intensity. At high cell concentration (0.91 g l(-1)) of pRK puf DD 13, velocity was maximal at 2270 W m(-2) and was equal to 144.7 ml l(-1) h(-1) that evidences to an opportunity to increase the volume velocity of hydrogen release by application of the strains with low content of pigments.     

Osmoregulated periplasmic glucans of the free-living photosynthetic bacterium Rhodobacter sphaeroides.

The osmoregulated periplasmic glucans (OPGs) produced by Rhodobacter sphaeroides, a free-living organism, were isolated by trichloracetic acid treatment and gel permeation chromatography. Compounds obtained were characterized by compositional analysis, matrix-assisted laser desorption ionization mass spectrometry and nuclear magnetic resonance. R. sphaeroides predominantly synthesizes a cyclic glucan containing 18 glucose residues that can be substituted by one to seven succinyl esters residues at the C6 position of some of the glucose residues, and by one or two acetyl residues. The glucans were subjected to a mild alkaline treatment in order to remove the succinyl and acetyl substituents, analyzed by MALDI mass spectrometry and purified by high-performance anion-exchange chromatography. Methylation analysis revealed that this glucan is linked by 17 1,2 glycosidic bonds and one 1,6 glycosidic bond. Homonuclear and (1)H/(13)C heteronuclear NMR experiments revealed the presence of a single alpha-1,6 glycosidic linkage, whereas all other glucose residues are beta-1,2 linked. The different anomeric proton signals allowed a complete sequence-specific assignment of the glucan. The structural characteristics of this glucan are very similar to the previously described OPGs of Ralstonia solanacearum and Xanthomonas campestris, except for its different size and the presence of substituents. Therefore, similar OPGs are synthesized by phytopathogenic as well as free-living bacteria, suggesting these compounds are intrinsic components of the Gram-negative bacterial envelope.     

A quorum-sensing system in the free-living photosynthetic bacterium Rhodobacter sphaeroides.

Rhodobacter sphaeroides is a free-living, photoheterotrophic bacterium known for its genomic and metabolic complexity. We have discovered that this purple photosynthetic organism possesses a quorum-sensing system. Quorum sensing occurs in a number of eukaryotic host-associated gram-negative bacteria. In these bacteria there are two genes required for quorum sensing, the luxR and luxI homologs, and there is an acylhomoserine lactone signal molecule synthesized by the product of the luxI homolog. In R. sphaeroides, synthesis of a novel homoserine lactone signal, 7,8-cis-N-(tetradecenoyl)homoserine lactone, is directed by a luxI homolog termed cerI. Two open reading frames immediately upstream of cerI are proposed to be components of the quorum-sensing system. The first of these is a luxR homolog termed cerR, and the second is a small open reading frame of 159 bp. Inactivation of cerI in R. sphaeroides results in mucoid colony formation on agar and formation of large aggregates of cells in liquid cultures. Clumping of CerI mutants in liquid culture is reversible upon addition of the acylhomoserine lactone signal and represents a phenotype unlike those controlled by quorum sensing in other bacteria.     

Functional consequences of the organization of the photosynthetic apparatus in Rhodobacter sphaeroides. I. Quinone domains and excitation transfer in chromatophores and reaction center.antenna complexes

The purpose of this study was to gain information on the functional consequences of the supramolecular organization of the photosynthetic apparatus in the bacterium Rhodobacter sphaeroides. Isolated complexes of the reaction center (RC) with its core antenna ring (light-harvesting complex 1 (LH1)) were studied in their dimeric (native) form or as monomers with respect to excitation transfer and distribution of the quinone pool. Similar issues were examined in chromatophore membranes. The relationship between the fluorescence yield and the amount of closed centers is indicative of a very efficient excitation transfer between the two monomers in isolated dimeric complexes. A similar dependence was observed in chromatophores, suggesting that excitation transfer in vivo from a closed RC.LH1 unit is also essentially directed to its partner in the dimer. The isolated complexes were found to retain 25-30% of the endogenous quinone acceptor pool, and the distribution of this pool among the complexes suggests a cooperative character for the association of quinones with the protein complexes. In chromatophores, the decrease in the amount of photoreducible quinones when inhibiting a fraction of the centers implies a confinement of the quinone pool over small domains, including one to six reaction centers. We suggest that the crowding of membrane proteins may not be the sole reason for quinone confinement and that a quinone-rich region is formed around the RC.LH1 complexes.     

The flagellar muramidase from the photosynthetic bacterium Rhodobacter sphaeroides

We have characterized open reading frame RSP0072, which is located within the flgG operon in Rhodobacter sphaeroides. The amino acid sequence analysis of this gene product showed the presence of a soluble lytic transglycosylase domain. The deletion of the N-terminal region (90 amino acids) of the product of RSP0072 yields a leaky nonmotile phenotype, as determined by swarm assays in soft agar. Electron micrographs revealed the lack of flagella in mutant cells. The purified wild-type protein showed lytic activity on extracts of Micrococcus luteus. In contrast, no lytic activity was observed when the residues E57 or E83 were replaced by alanine. Affinity blotting suggests that the protein encoded by RSP0072 interacts with the flagellar rod-scaffolding protein FlgJ, which lacks the muramidase domain present in FlgJ of many bacteria. We propose that the product of RSP0072 is a flagellar muramidase that is exported to the periplasm via the Sec pathway, where it interacts with FlgJ to open a gap in the peptidoglycan layer for the subsequent penetration of the nascent flagellar structure.     

Photoresponses in Rhodobacter sphaeroides: role of photosynthetic electron transport.

Rhodobacter sphaeroides responds to a decrease in light intensity by a transient stop followed by adaptation. There is no measurable response to increases in light intensity. We confirmed that photosynthetic electron transport is essential for a photoresponse, as (i) inhibitors of photosynthetic electron transport inhibit photoresponses, (ii) electron transport to oxidases in the presence of oxygen reduces the photoresponse, and (iii) the magnitude of the response is dependent on the photopigment content of the cells. The photoresponses of cells grown in high light, which have lower concentrations of light-harvesting photopigment and reaction centers, saturated at much higher light intensities than the photoresponses of cells grown in low light, which have high concentrations of light-harvesting pigments and reaction centers. We examined whether the primary sensory signal from the photosynthetic electron transport chain was a change in the electrochemical proton gradient or a change in the rate of electron transport itself (probably reflecting redox sensing). R. sphaeroides showed no response to the addition of the proton ionophore carbonyl cyanide 4-trifluoromethoxyphenylhydrazone, which decreased the electrochemical proton gradient, although a behavioral response was seen to a reduction in light intensity that caused an equivalent reduction in proton gradient. These results strongly suggest that (i) the photosynthetic apparatus is the primary photoreceptor, (ii) the primary signal is generated by a change in the rate of electron transport, (iii) the change in the electrochemical proton gradient is not the primary photosensory signal, and (iv) stimuli affecting electron transport rates integrate via the electron transport chain.     

The assembly and organisation of photosynthetic membranes in Rhodobacter sphaeroides.

Recent AFM data demonstrate that mature photosynthetic membranes of R. sphaeroides are composed of rows of dimeric RC-LH1-PufX complexes with some LH2 complexes 'sandwiched' between these rows of core complexes, and others in discrete LH2-only domains which might form the light-responsive complement of the LH2 antenna. The present work applies membrane fractionation, radiolabelling and LDS-PAGE techniques to investigate the response of R. sphaeroides to lowered light intensity. The kinetics underlying this adaptation to low light conditions were revealed by radiolabelling with the bacteriochlorophyll (bchl) biosynthetic precursor, delta-aminolevulinate, which allowed us to measure only the bchls synthesised after the light intensity shift. We show that (1) the increase in LH2 antenna size is mainly restricted to the mature ICM membrane fraction, and the antenna composition of the precursor upper pigmented band (UPB) membrane remains constant, (2) the precursor UPB membrane is enriched in bchl synthase, the terminal enzyme of the bchl biosynthetic pathway, and (3) the LH2 and the complexes of intermediate migration in LDS-PAGE exhibit completely different labelling kinetics. Thus, new photosynthetic complexes, mainly LH2, are synthesised and assembled at the membrane initiation UPB sites, where the LH2 rings pack between the rows of dimeric cores fostering new LH2-LH1 interactions. Mature membranes also assemble new LH2 rings, but in this case the 'sandwich' regions between the rows of core dimers are already fully occupied and the bulk antenna pool is the favoured location for these new LH2 complexes.     

Denitrification of PVA-immobilized denitrifying photosynthetic bacterium,Rhodobacter sphaeroides

A newly isolated denitrifying strain, Rhodobacter sphaeroides NII2 was immobilized in polyvinyl alcohol (PVA) gel, and the properties of the cells in the gel were examined. The immobilized cells had low or almost no denitrification activity, but the cells were activated by incubation in light with culture medium for denitrification containing 0.5% nitrate and no other nitrogen source. Cells grown in the dark were activated by incubation at an earlier stage and to a higher rate than the light-grown cells. The activation was markedly enhanced in the PVA gel with a low cell concentration. The immobilized cells consumed nitrate with a temporary accumulation of NO₂ and evolved nitrogen gas. The immobilized cells could use various organic compounds as electron donors for denitrification. Thus, the immobilized cells were applied to a continuous treatment of synthetic wastewater using an aparatus devised by this laboratory. The results showed an efficient removal of NO₃-N from the test water.     

Supramolecular organization of the photosynthetic chain in mutants of Rhodobacter capsulatus deleted in cytochrome c 2

Both the soluble cytochrome c₂ and the membrane-bound cytochrome c_y 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 c_y in less than five ms. The remaining fraction is reduced in several seconds, although about 90% of the photoxidized cytochrome c_y is reduced in less than 10 ms by the cytochrome bc₁ complex. This implies that cytochrome c_y 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 c_y and the disconnected reaction centers are localized in the same membrane region. We propose that the movement of cyt c_y is restricted to a small membrane domain which includes a single cytochrome bc₁ complex. The kinetics of cytochrome c_y photooxidation in the MT-G4/S4 mutant in the presence of myxothiazol presents a fast phase (t^1/2 3 µs) followed by a slower phase (t^1/2 20 µs). In the case of the double mutant MT-GS18, the kinetics of electron transfer between cytochrome c_y 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 c_y, and their formation in supercomplex. The more problable stoichiometry of electron carriers in this supercomplex is 2 reaction centers, 2 cytochrome c_y and 1 cytochrome bc₁ complex.     

Rhodobacter sphaeroides spd mutations allow cytochrome c2-independent photosynthetic growth.

In Rhodobacter sphaeroides, cytochrome c2 (cyt c2) is a periplasmic redox protein required for photosynthetic electron transfer. cyt c2-deficient mutants created by replacing the gene encoding the apoprotein for cyt c2 (cycA) with a kanamycin resistance cartridge are photosynthetically incompetent. Spontaneous mutations that suppress this photosynthesis deficiency (spd mutants) arise at a frequency of 1 to 10 in 10(7). We analyzed the cytochrome content of several spd mutants spectroscopically and by heme peroxidase assays. These suppressors lacked detectable cyt c2, but they contained a new soluble cytochrome which was designated isocytochrome c2 (isocyt c2) that was not detectable in either cycA+ or cycA mutant cells. When spd mutants were grown photosynthetically, isocyt c2 was present at approximately 20 to 40% of the level of cyt c2 found in photosynthetically grown wild type cells, and it was found in the periplasm with cytochromes c' and c554. These spd mutants also had several other pleiotropic phenotypes. Although photosynthetic growth rates of the spd mutants were comparable to those of wild-type strains at all light intensities tested, they contained elevated levels of B800-850 pigment-protein complexes. Several spd mutants contained detectable amounts of isocyt c2 under aerobic conditions. Finally, heme peroxidase assays indicated that, under anaerobic conditions, the spd mutants may contain another new cytochrome in addition to isocyt c2. These pleiotropic phenotypes, the frequency at which the spd mutants arise, and the fact that a frameshift mutagen is very effective in generating the spd phenotype suggest that some spd mutants contain a mutation in loci which regulate cytochrome synthesis.     

Introduction of new carotenoids into the bacterial photosynthetic apparatus by combining the carotenoid biosynthetic pathways of Erwinia herbicola and Rhodobacter sphaeroides.

Carotenoids have two major functions in bacterial photosynthesis, photoprotection and accessory light harvesting. The genes encoding many carotenoid biosynthetic pathways have now been mapped and cloned in several different species, and the availability of cloned genes which encode the biosynthesis of carotenoids not found in the photosynthetic genus Rhodobacter opens up the possibility of introducing a wider range of foreign carotenoids into the bacterial photosynthetic apparatus than would normally be available by producing mutants of the native biosynthetic pathway. For example, the crt genes from Erwinia herbicola, a gram-negative nonphotosynthetic bacterium which produces carotenoids in the sequence of phytoene, lycopene, beta-carotene, beta-cryptoxanthin, zeaxanthin, and zeaxanthin glucosides, are clustered within a 12.8-kb region and have been mapped and partially sequenced. In this paper, part of the E. herbicola crt cluster has been excised and expressed in various crt strains of Rhodobacter sphaeroides. This has produced light-harvesting complexes with a novel carotenoid composition, in which the foreign carotenoids such as beta-carotene function successfully in light harvesting. The outcome of the combination of the crt genes in R. sphaeroides with those from E. herbicola has, in some cases, resulted in an interesting rerouting of the expected biosynthetic sequence, which has also provided insights into how the various enzymes of the carotenoid biosynthetic pathway might interact. Clearly this approach has considerable potential for studies on the control and organization of carotenoid biosynthesis, as well as providing novel pigment-protein complexes for functional studies.     

Structure of the membrane-bound protein photosynthetic reaction center from Rhodobacter sphaeroides

The structure of the photosynthetic reaction center (RC) from Rhodobacter sphaeroides was determined at 3.1-A resolution by the molecular replacement method, using the Rhodopseudomonas viridis RC as the search structure. Atomic coordinates were refined with the difference Fourier method and restrained least-squares refinement techniques to a current R factor of 22%. The tertiary structure of the RC complex is stabilized by hydrophobic interactions between the L and M chains, by interactions of the pigments with each other and with the L and M chains, by residues from the L and M chains that coordinate to the Fe2+, by salt bridges that are formed between the L and M chains and the H chain, and possibly by electrostatic forces between the ends of helices. The conserved residues at the N-termini of the L and M chains were identified as recognition sites for the H chain.     

Oxygen-insensitive synthesis of the photosynthetic membranes of Rhodobacter sphaeroides: a mutant histidine kinase.

Two new loci, prrB and prrC, involved in the positive regulation of photosynthesis gene expression in response to anaerobiosis, have been identified in Rhodobacter sphaeroides. prrB encodes a sensor histidine kinase that is responsive to the removal of oxygen and functions through the response regulator PrrA. Inactivation of prrB results in a substantial reduction of photosynthetic spectral complexes as well as in the inability of cells to grow photosynthetically at low to medium light intensities. Together, prrB and prrA provide the major signal involved in synthesis of the specialized intracytoplasmic membrane (ICM), harboring components essential to the light reactions of photosynthesis. Previously, J. K. Lee and S. Kaplan (J. Bacteriol. 174:1158-1171, 1992) identified a mutant which resulted in high-level expression of the puc operon, encoding the apoproteins giving rise to the B800-850 spectral complex, in the presence of oxygen as well as in the synthesis of the ICM under conditions of high oxygenation. This mutation is shown to reside in prrB, resulting in a leucine-to-proline change at position 78 in mutant PrrB (PRRB78). Measurements of mRNA levels in cells containing the prrB78 mutation support the idea that prrB is a global regulator of photosynthesis gene expression. Two additional mutants, PRRB1 and PRRB2, which make two truncated forms of the PrrB protein, possess substantially reduced amounts of spectral complexes. Although the precise role of prrC remains to be determined, evidence suggests that it too is involved in the regulatory cascade involving prrB and prrA. The genetic organization of the photosynthesis response regulatory (PRR) region is discussed.     

Role of the N- and C-terminal regions of the PufX protein in the structural organization of the photosynthetic core complex of Rhodobacter sphaeroides.

The core complex of Rhodobacter sphaeroides is formed by the association of the light-harvesting antenna 1 (LH1) and the reaction center (RC). The PufX protein is essential for photosynthetic growth; it is located within the core in a 1 : 1 stoichiometry with the RC. PufX is required for a fast ubiquinol exchange between the Q(B) site of the RC and the Qo site of the cytochrome bc1 complex. In vivo the LH1-PufX-RC complex is assembled in a dimeric form, where PufX is involved as a structural organizer. We have modified the PufX protein at the N and the C-terminus with progressive deletions. The nine mutants obtained have been characterized for their ability for photosynthetic growth, the insertion of PufX in the core LH1-RC complex, the stability of the dimers and the kinetics of flash-induced reduction of cytochrome b561 of the cytochrome bc1 complex. Deletion of 18 residues at the N-terminus destabilizes the dimer in vitro without preventing photosynthetic growth. The dimer (or a stable dimer) does not seem to be a necessary requisite for the photosynthetic phenotype. Partial C-terminal deletions impede the insertion of PufX, while the complete absence of the C-terminus leads to the insertion of a PufX protein composed of only its first 53 residues and does not affect the photosynthetic growth of the bacterium. Overall, the results point to a complex role of the N and C domains in the structural organization of the core complex; the N-terminus is suggested to be responsible mainly for dimerization, while the C-terminus is thought to be involved mainly in PufX assembly.