Primary photochemistry in the facultatively aerobic green photosynthetic bacterium Chloroflexus aurantiacus

Coherent intradimer dynamics in reaction centers of photosynthetic green bacterium Chloroflexus aurantiacus

Early-time dynamics of absorbance changes (light minus dark) in the long-wavelength Q_y absorption band of bacteriochlorophyll dimer P of isolated reaction centers (RCs) from thermophilic green bacterium Chloroflexus (Cfx.) aurantiacus was studied by difference pump-probe spectroscopy with 18-fs resolution at cryogenic temperature. It was found that the stimulated emission spectrum gradually moves to the red on the ~100-fs time scale and subsequently oscillates with a major frequency of ~140 cm⁻¹. By applying the non-secular Redfield theory and linear susceptibility theory, the coherent dynamics of the stimulated emission from the excited state of the primary electron donor, bacteriochlorophyll dimer P*, was modeled. The model showed the possibility of an extremely fast transition from the locally excited state P₁* to the spectrally different excited state P₂*. This transition is clearly seen in the kinetics of the stimulated emission at 880 and 945 nm, where mostly P₁* and P₂* states emit, respectively. These findings are similar to those obtained previously in RCs of the purple bacterium Rhodobacter (Rba.) sphaeroides. The assumption about the existence of the second excited state P₂* helps to explain the complicated temporal behavior of the ΔA spectrum measured by pump-probe spectroscopy. It is interesting that, in spite of the strong coupling between the P₁* and P₂* states assumed in our model, the form of the coherent oscillations is mainly defined by pure vibrational coherence in the excited states. A possible nature of the P₂* state is discussed.

Evolutionary Implications of Anoxygenic Phototrophy in the Bacterial Phylum Candidatus Eremiobacterota (WPS-2)

Genome-resolved environmental metagenomic sequencing has uncovered substantial previously unrecognized microbial diversity relevant for understanding the ecology and evolution of the biosphere, providing a more nuanced view of the distribution and ecological significance of traits including phototrophy across diverse niches. Recently, the capacity for bacteriochlorophyll-based anoxygenic photosynthesis has been proposed in the uncultured bacterial WPS-2 phylum (recently proposed as Candidatus Eremiobacterota) that are in close association with boreal moss. Here, we use phylogenomic analysis to investigate the diversity and evolution of phototrophic WPS-2. We demonstrate that phototrophic WPS-2 show significant genetic and metabolic divergence from other phototrophic and non-phototrophic lineages. The genomes of these organisms encode a new family of anoxygenic Type II photochemical reaction centers and other phototrophy-related proteins that are both phylogenetically and structurally distinct from those found in previously described phototrophs. We propose the name Candidatus Baltobacterales for the order-level aerobic WPS-2 clade which contains phototrophic lineages, from the Greek for "bog" or "swamp," in reference to the typical habitat of phototrophic members of this clade.

Reconstructing the Origin of Oxygenic Photosynthesis: Do Assembly and Photoactivation Recapitulate Evolution?

Due to the great abundance of genomes and protein structures that today span a broad diversity of organisms, now more than ever before, it is possible to reconstruct the molecular evolution of protein complexes at an incredible level of detail. Here, I recount the story of oxygenic photosynthesis or how an ancestral reaction center was transformed into a sophisticated photochemical machine capable of water oxidation. First, I review the evolution of all reaction center proteins in order to highlight that Photosystem II and Photosystem I, today only found in the phylum Cyanobacteria, branched out very early in the history of photosynthesis. Therefore, it is very unlikely that they were acquired via horizontal gene transfer from any of the described phyla of anoxygenic phototrophic bacteria. Second, I present a new evolutionary scenario for the origin of the CP43 and CP47 antenna of Photosystem II. I suggest that the antenna proteins originated from the remodeling of an entire Type I reaction center protein and not from the partial gene duplication of a Type I reaction center gene. Third, I highlight how Photosystem II and Photosystem I reaction center proteins interact with small peripheral subunits in remarkably similar patterns and hypothesize that some of this complexity may be traced back to the most ancestral reaction center. Fourth, I outline the sequence of events that led to the origin of the Mn4CaO5 cluster and show that the most ancestral Type II reaction center had some of the basic structural components that would become essential in the coordination of the water-oxidizing complex. Finally, I collect all these ideas, starting at the origin of the first reaction center proteins and ending with the emergence of the water-oxidizing cluster, to hypothesize that the complex and well-organized process of assembly and photoactivation of Photosystem II recapitulate evolutionary transitions in the path to oxygenic photosynthesis.

Electron paramagnetic resonance study of a photosynthetic microbial mat and comparison with Archean cherts

Organic radicals in artificially carbonized biomass dominated by oxygenic and non-oxygenic photosynthetic bacteria, Microcoleus chthonoplastes-like and Chloroflexus-like bacteria respectively, were studied by Electron Paramagnetic Resonance (EPR) spectroscopy. The two bacteria species were sampled in mats from a hypersaline lake. They underwent accelerated ageing by cumulative thermal treatments to induce progressive carbonization of the biological material, mimicking the natural maturation of carbonaceous material of Archean age. For thermal treatments at temperatures higher than 620 °C, a drastic increase in the EPR linewidth is observed in the carbonaceous matter from oxygenic photosynthetic bacteria and not anoxygenic photosynthetic bacteria. This selective EPR linewidth broadening reflects the presence of a catalytic element inducing formation of radical aggregates, without affecting the molecular structure or the microstructure of the organic matter, as shown by Raman spectroscopy and Transmission Electron Microscopy. For comparison, we carried out an EPR study of organic radicals in silicified carbonaceous rocks (cherts) from various localities, of different ages (0.42 to 3.5 Gyr) and having undergone various degrees of metamorphism, i.e. various degrees of natural carbonization. EPR linewidth dispersion for the most primitive samples was quite significant, pointing to a selective dipolar broadening similar to that observed for carbonized bacteria. This surprising result merits further evaluation in the light of its potential use as a marker of past bacterial metabolisms, in particular oxygenic photosynthesis, in Archean cherts.

SHKUROPATOV ANATOLI, SHUVALOV VLADIMIRA. WAVE PACKET MOTIONS COUPLED TO ELECTRON TRANSFER IN REACTION CENTERS OF CHLOROFLEXUS AURANTIACUS

Transient absorption difference spectroscopy with ~20 femtosecond (fs) resolution was applied to study the time and spectral evolution of low-temperature (90 K) absorbance changes in isolated reaction centers (RCs) of Chloroflexus (C.) aurantiacus. In RCs, the composition of the B-branch chromophores is different with respect to that of purple bacterial RCs by occupying the BB binding site of accessory bacteriochlorophyll by bacteriopheophytin molecule (ΦB). It was found that the nuclear wave packet motion induced on the potential energy surface of the excited state of the primary electron donor P* by ~20 fs excitation leads to a coherent formation of the states [Formula: see text] and [Formula: see text] (BA is a bacteriochlorophyll monomer in the A-branch of cofactors). The processes were studied by measuring coherent oscillations in kinetics of the absorbance changes at 900 nm and 940 nm (P* stimulated emission), at 750 nm and 785 nm (ΦB absorption bands), and at 1,020–1028 nm ([Formula: see text] absorption band). In RCs, the immediate bleaching of the P band at 880 nm and the appearance of the stimulated wave packet emission at 900 nm were accompanied (with a small delay of 10–20 fs) by electron transfer from P* to the B-branch with bleaching of the ΦB absorption band at 785 nm due to [Formula: see text] formation. These data are consistent with recent measurements for the mutant HM182L Rb. sphaeroides RCs (Yakovlev et al., Biochim Biophys Acta1757:369–379, 2006). Only at a delay of 120 fs was the electron transfer from P* to the A-branch observed with a development of the [Formula: see text] absorption band at 1028 nm. This development was in phase with the appearance of the P* stimulated emission at 940 nm. The data on the A-branch electron transfer in C. aurantiacus RCs are consistent with those observed in native RCs of Rb. sphaeroides. The mechanism of charge separation in RCs with the modified B-branch pigment composition is discussed in terms of coupling between the nuclear wave packet motion and electron transfer from P* to ΦB and BA primary acceptors in the B-branch and A-branch, respectively.

Investigation of the redox interaction between Mn-bicarbonate complexes and reaction centers from Rhodobacter sphaeroides R-26, Chromatium minutissimum, and Chloroflexus aurantiacus

The change in the dark reduction rate of photooxidized reaction centers (RC) of type II from three anoxygenic bacteria (Rhodobacter sphaeroides R-26, Chromatium minutissimum, and Chloroflexus aurantiacus) having different redox potentials of the P(+)/P pair and availability of RC for exogenous electron donors was investigated upon the addition of Mn(2+) and HCO(3)(-). It was found that the dark reduction of P(870)(+) from Rb. sphaeroides R-26 is considerably accelerated upon the combined addition of 0.5 mM MnCl(2) and 30-75 mM NaHCO(3) (as a result of formation of "low-potential" complexes [Mn(HCO(3))(2)]), while MnCl(2) and NaHCO(3) added separately had no such effect. The effect is not observed either in RC from Cf. aurantiacus (probably due to the low oxidation potential of the primary electron donor, P(865), which results in thermodynamic difficulties of the redox interaction between P(865)(+) and Mn(2+)) or in RC from Ch. minutissimum (apparently due to the presence of the RC-bound cytochrome preventing the direct interaction between P(870)(+) and Mn(2+)). The absence of acceleration of the dark reduction of P(870)(+) in the RC of Rb. sphaeroides R-26 when Mn(2+) and HCO(3)(-) were replaced by Mg(2+) or Ca(2+) and by formate, oxalate, or acetate, respectively, reveals the specificity of the Mn2+-bicarbonate complexes for the redox interaction with P(+). The results of this work might be considered as experimental evidence for the hypothesis of the participation of Mn(2+) complexes in the evolutionary origin of the inorganic core of the water oxidizing complex of photosystem II.

Kinetics and energetics of electron transfer in reaction centers of the photosynthetic bacterium Roseiflexus castenholzii

The kinetics and thermodynamics of the photochemical reactions of the purified reaction center (RC)-cytochrome (Cyt) complex from the chlorosome-lacking, filamentous anoxygenic phototroph, Roseiflexus castenholzii are presented. The RC consists of L- and M-polypeptides containing three bacteriochlorophyll (BChl), three bacteriopheophytin (BPh) and two quinones (Q(A) and Q(B)), and the Cyt is a tetraheme subunit. Two of the BChls form a dimer P that is the primary electron donor. At 285K, the lifetimes of the excited singlet state, P*, and the charge-separated state P(+)H(A)(-) (where H(A) is the photoactive BPh) were found to be 3.2±0.3 ps and 200±20 ps, respectively. Overall charge separation P*→→ P(+)Q(A)(-) occurred with ≥90% yield at 285K. At 77K, the P* lifetime was somewhat shorter and the P(+)H(A)(-) lifetime was essentially unchanged. Poteniometric titrations gave a P(865)/P(865)(+) midpoint potential of +390mV vs. SHE. For the tetraheme Cyt two distinct midpoint potentials of +85 and +265mV were measured, likely reflecting a pair of low-potential hemes and a pair of high-potential hemes, respectively. The time course of electron transfer from reduced Cyt to P(+) suggests an arrangement where the highest potential heme is not located immediately adjacent to P. Comparisons of these and other properties of isolated Roseiflexus castenholzii RCs to those from its close relative Chloroflexus aurantiacus and to RCs from the purple bacteria are made.

Structural analysis of alternative complex III in the photosynthetic electron transfer chain of Chloroflexus aurantiacus

The green photosynthetic bacterium Chloroflexus aurantiacus, which belongs to the phylum of filamentous anoxygenic phototrophs, does not contain a cytochrome bc or bf type complex which is found in all other known groups of phototrophs. This suggests that a functional replacement exists to link the reaction center photochemistry to cyclic electron transfer as well as respiration. Earlier work identified a potential substitute of the cytochrome bc complex, now named alternative complex III (ACIII), which has been purified from C. aurantiacus, identified, and characterized. ACIII functions as a menaquinol:auracyanin oxidoreductase in the photosynthetic electron transfer chain, and a related but distinct complex functions in respiratory electron flow to a terminal oxidase. In this work, we focus on elucidating the structure of photosynthetic ACIII. We found that ACIII is an integral membrane protein complex of approximately 300 kDa that consists of eight subunits of seven different types. Among them, there are four metalloprotein subunits, including a 113 kDa iron-sulfur cluster-containing polypeptide, a 25 kDa penta-heme c-containing subunit, and two 20 kDa monoheme c-containing subunits in the form of a homodimer. A variety of analytical techniques were employed in determining the ACIII substructure, including HPLC combined with ESI-MS, metal analysis, potentiometric titration, and intensity analysis of heme staining SDS-PAGE. A preliminary structural model of ACIII is proposed on the basis of the analytical data and chemical cross-linking in tandem with mass analysis using MALDI-TOF, as well as transmembrane and transit peptide analysis.

Characterization of a blue-copper protein, auracyanin, of the filamentous anoxygenic phototrophic bacterium Roseiflexus castenholzii

A blue-copper protein auracyanin of the filamentous anoxygenic phototroph Roseiflexus castenholzii was purified and characterized. Genomic sequence analysis showed that R. castenholzii has only one auracyanin, whereas Chloroflexus aurantiacus is known to have two auracyanins, A and B. Absorption spectrum of the Roseiflexus auracyanin was similar to that of auracyanin B of C. aurantiacus. On the other hand, ESR spectrum of the Roseiflexus auracyanin resembles that of auracyanin A of C. aurantiacus. These results suggest that the blue-copper protein auracyanin from R. castenholzii shares features with each of auracyanin A and B. Amino acid sequence alignment of auracyanins from filamentous anoxygenic phototrophs also demonstrated the chimeral feature of the primary structure of the Roseiflexus auracyanin, i.e., auracyanin A-like amino-terminal characteristics and auracyanin B-like one-residue spacing at the Cu-binding loop in the carboxyl-terminus.

Femtosecond phase of charge separation in reaction centers of Chloroflexus aurantiacus

Difference absorption spectroscopy with temporal resolution of approximately 20 fsec was used to study the primary phase of charge separation in isolated reaction centers (RCs) of Chloroflexus aurantiacus at 90 K. An ensemble of difference (light-minus-dark) absorption spectra in the 730-795 nm region measured at -0.1 to 4 psec delays relative to the excitation pulse was analyzed. Comparison with analogous data for RCs of HM182L mutant of Rhodobacter sphaeroides having the same pigment composition identified the 785 nm absorption band as the band of bacteriopheophytin Phi(B) in the B-branch. By study the bleaching of this absorption band due to formation of Phi(B)(-), it was found that a coherent electron transfer from P* to the B-branch occurs with a very small delay of 10-20 fsec after excitation of dimer bacteriochlorophyll P. Only at 120 fsec delay electron transfer from P* to the A-branch occurs with the formation of bacteriochlorophyll anion B(A)(-) absorption band at 1028 nm and the appearance of P* stimulated emission at 940 nm, as also occurs in native RCs of Rb. sphaeroides. It is concluded that a nuclear wave packet motion on the potential energy surface of P* after a 20-fsec light pulse excitation leads to the coherent formation of the P(+)Phi(B)(-) and P(+)B(A)(-) states.

A glimpse into the expanded genome content of Vibrio cholerae through identification of genes present in environmental strains

Vibrio cholerae has multiple survival strategies which are reflected both in its broad distribution in many aquatic environments and its high genotypic diversity. To obtain additional information regarding the content of the V. cholerae genome, suppression subtractive hybridization (SSH) was used to prepare libraries of DNA sequences from two southern California coastal isolates which are divergent or absent in the clinical strain V. cholerae O1 El Tor N16961. More than 1,400 subtracted clones were sequenced. This revealed the presence of novel sequences encoding functions related to cell surface structures, transport, metabolism, signal transduction, luminescence, mobile elements, stress resistance, and virulence. Flanking sequence information was determined for loci of interest, and the distribution of these sequences was assessed for a collection of V. cholerae strains obtained from southern California and Mexican environments. This led to the surprising observation that sequences related to the toxin genes toxA, cnf1, and exoY are widespread and more common in these strains than those of the cholera toxin genes which are a hallmark of the pandemic strains of V. cholerae. Gene transfer among these strains could be facilitated by a 4.9-kbp plasmid discovered in one isolate, which possesses similarity to plasmids from other environmental vibrios. By investigating some of the nucleotide sequence basis for V. cholerae genotypic diversity, DNA fragments have been uncovered which could promote survival in coastal environments. Furthermore, a set of genes has been described which could be involved in as yet undiscovered interactions between V. cholerae and eukaryotic organisms.

Structural and spectroscopic properties of a reaction center complex from the chlorosome-lacking filamentous anoxygenic phototrophic bacterium Roseiflexus castenholzii

The photochemical reaction center (RC) complex of Roseiflexus castenholzii, which belongs to the filamentous anoxygenic phototrophic bacteria (green filamentous bacteria) but lacks chlorosomes, was isolated and characterized. The genes coding for the subunits of the RC and the light-harvesting proteins were also cloned and sequenced. The RC complex was composed of L, M, and cytochrome subunits. The cytochrome subunit showed a molecular mass of approximately 35 kDa, contained hemes c, and functioned as the electron donor to the photo-oxidized special pair of bacteriochlorophylls in the RC. The RC complex appeared to contain three molecules of bacteriochlorophyll and three molecules of bacteriopheophytin, as in the RC preparation from Chloroflexus aurantiacus. Phylogenetic trees based on the deduced amino acid sequences of the RC subunits suggested that R. castenholzii had diverged from C. aurantiacus very early after the divergence of filamentous anoxygenic phototrophic bacteria from purple bacteria. Although R. castenholzii is phylogenetically related to C. aurantiacus, the arrangement of its puf genes, which code for the light-harvesting proteins and the RC subunits, was different from that in C. aurantiacus and similar to that in purple bacteria. The genes are found in the order pufB, -A, -L, -M, and -C, with the pufL and pufM genes forming one continuous open reading frame. Since the photosynthetic apparatus and genes of R. castenholzii have intermediate characteristics between those of purple bacteria and C. aurantiacus, it is likely that they retain many features of the common ancestor of purple bacteria and filamentous anoxygenic phototrophic bacteria.

Coupling of nuclear wavepacket motion and charge separation in bacterial reaction centers

The mechanism of the charge separation and stabilization of separated charges was studied using the femtosecond absorption spectroscopy. It was found that nuclear wavepacket motions on potential energy surface of the excited state of the primary electron donor P* leads to a coherent formation of the charge separated states P(+)B(A)(-), P(+)H(A)(-) and P(+)H(B)(-) (where B(A), H(B) and H(A) are the primary and secondary electron acceptors, respectively) in native, pheophytin-modified and mutant reaction centers (RCs) of Rhodobacter sphaeroides R-26 and in Chloroflexus aurantiacus RCs. The processes were studied by measurements of coherent oscillations in kinetics at 890 and 935 nm (the stimulated emission bands of P*), at 800 nm (the absorption band of B(A)) and at 1020 nm (the absorption band of B(A)(-)) as well as at 760 nm (the absorption band of H(A)) and at 750 nm (the absorption band of H(B)). It was found that wavepacket motion on the 130-150 cm(-1) potential surface of P* is accompanied by approaches to the intercrossing region between P* and P(+)B(A)(-) surfaces at 120 and 380 fs delays emitting light at 935 nm (P*) and absorbing light at 1020 nm (P(+)B(A)(-)). In the presence of Tyr M210 (Rb. sphaeroides) or M195 (C. aurantiacus) the stabilization of P(+)B(A)(-) is observed within a few picosseconds in contrast to YM210W. At even earlier delay (approximately 40 fs) the emission at 895 nm and bleaching at 748 nm are observed in C. aurantiacus RCs showing the wavepacket approach to the intercrossing between the P* and P(+)H(B)(-) surfaces at that time. The 32 cm(-1) rotation mode of HOH was found to modulate the electron transfer rate probably due to including of this molecule in polar chain connecting P(B) and B(A) and participating in the charge separation. The mechanism of the charge separation and stabilization of separated charges is discussed in terms of the role of nuclear motions, of polar groups connecting P and acceptors and of proton of OH group of TyrM210.

Discovery and characterization of electron transfer proteins in the photosynthetic bacteria

Research on photosynthetic electron transfer closely parallels that of other electron transfer pathways and in many cases they overlap. Thus, the first bacterial cytochrome to be characterized, called cytochrome c (2), is commonly found in non-sulfur purple photosynthetic bacteria and is a close homolog of mitochondrial cytochrome c. The cytochrome bc (1) complex is an integral part of photosynthetic electron transfer yet, like cytochrome c (2), was first recognized as a respiratory component. Cytochromes c (2) mediate electron transfer between the cytochrome bc (1) complex and photosynthetic reaction centers and cytochrome a-type oxidases. Not all photosynthetic bacteria contain cytochrome c (2); instead it is thought that HiPIP, auracyanin, Halorhodospira cytochrome c551, Chlorobium cytochrome c555, and cytochrome c (8) may function in a similar manner as photosynthetic electron carriers between the cytochrome bc (1) complex and reaction centers. More often than not, the soluble or periplasmic mediators do not interact directly with the reaction center bacteriochlorophyll, but require the presence of membrane-bound intermediates: a tetraheme cytochrome c in purple bacteria and a monoheme cytochrome c in green bacteria. Cyclic electron transfer in photosynthesis requires that the redox potential of the system be delicately poised for optimum efficiency. In fact, lack of redox poise may be one of the defects in the aerobic phototrophic bacteria. Thus, large concentrations of cytochromes c (2) and c' may additionally poise the redox potential of the cyclic photosystem of purple bacteria. Other cytochromes, such as flavocytochrome c (FCSD or SoxEF) and cytochrome c551 (SoxA), may feed electrons from sulfide, sulfur, and thiosulfate into the photosynthetic pathways via the same soluble carriers as are part of the cyclic system.

Exogenous quinones inhibit photosynthetic electron transfer in Chloroflexus aurantiacus by specific quenching of the excited bacteriochlorophyll c antenna

In the photosynthetic green filamentous bacterium Chloroflexus aurantiacus, excitation energy is transferred from a large bacteriochlorophyll (BChl) c antenna via smaller BChl a antennas to the reaction center. The effects of substituted 1,4-naphthoquinones on BChl c and BChl a fluorescence and on flash-induced cytochrome c oxidation were studied in whole cells under aerobic conditions. BChl c fluorescence in a cell suspension with 5.4 microM BChl c was quenched to 50% by addition of 0.6 microM shikonin ((R)-2-(1-hydroxy-4-methyl-3-pentenyl)-5,8-dihydroxy-1, 4-naphthoquinone), 0.9 microM 5-hydroxy-1,4-naphthoquinone, or 4 microM 2-acetyl-3-methyl-1,4-naphthoquinone. Between 25 and 100 times higher quinone concentrations were needed to quench BChl a fluorescence to a similar extent. These quinones also efficiently inhibited flash-induced cytochrome c oxidation when BChl c was excited, but not when BChl a was excited. The quenching of BChl c fluorescence induced by these quinones correlated with the inhibition of flash-induced cytochrome c oxidation. We concluded that the quinones inhibited electron transfer in the reaction center by specifically quenching the excitation energy in the BChl c antenna. Our results provide a model system for studying the redox-dependent antenna quenching in green sulfur bacteria because the antennas in these bacteria inherently exhibit a sensitivity to O(2) similar to the quinone-supplemented cells of Cfx. aurantiacus.

Oxygen uncouples light absorption by the chlorosome antenna and photosynthetic electron transfer in the green sulfur bacterium chlorobium tepidum

In photosynthetic green sulfur bacteria excitation energy is transferred from large bacteriochlorophyll (BChl) c chlorosome antennas via small BChl a antennas to the reaction centers which then transfer electrons from cytochrome c to low-potential iron-sulfur proteins. Under oxidizing conditions a reversible mechanism is activated in the chlorosomes which quenches excited BChl c. We used flash-induced cytochrome c oxidation to investigate the effect of this quenching on photosynthetic electron transfer in whole cells of Chlorobium tepidum. The extent of cytochrome c photooxidation under aerobic conditions decreased to approx. 3% of that under anaerobic conditions when BChl c was excited under light-limiting conditions. Photooxidation obtained by excitation of BChl a was similar under aerobic and anaerobic conditions. We interpret this drastic decrease in energy transfer from BChl c to the reaction center as a consequence of the quenching mechanism which is activated by O2. This reversible uncoupling of the chlorosome antenna might prevent formation of toxic reactive oxygen species from photosynthetically produced reductants under aerobic conditions. The green filamentous bacterium Chloroflexus aurantiacus also contains chlorosomes but energy transfer from the BChl c and BChl a antennas to the reaction center in this species was not affected by O2.

Photosynthetic electrogenic events in native membranes ofChloroflexus aurantiacus. Flash-induced charge displacements within the reaction center-cytochromec 554 complex

The thermophilic phototrophChloroflexus aurantiacus possesses a photosynthetic reaction center (RC) containing a pair of menaquinones as primary (QA) and secondary (QB) electron acceptors and a bacteriochlorophyll dimer (P) as a primary donor. A tetraheme cytochromec 554 with two high(H)- and two low(L)-potential hemes operates as an immediate electron donor for P. The following equilibrium Em,7 values were determined by ESR for the hemes in whole membrane preparations: 280 mV (H1), 150 mV (H2), 95 mV (L1) and 0 mV (L2) (Van Vliet et al. (1991) Eur. J. Biochem. 199: 317-323). Partial electrogenic reactions induced by a laser flash inChl. aurantiacus chromatophores adsorbed to a phospholipid-impregnated collodion film were studied electrometrically at pH 8.3. The photoelectric response included a fast phase of ΔΨ generation (τ < 10 ns, phase A). It was ascribed to the charge separation between P(+) and QA (-) as its amplitude decreased both at high and low Eh values (Em,high=360±10 mV, estimated Em,low∼\s-160 mV) in good agreement with Em values for P/P(+) and QA/QA (-) redox couples. A slower kinetic component appeared upon reduction of the cytochromec 554 hemes (phase C). With H1 reduced before the flash the amplitude of phase C was equal to 15-20% of that of phase A and its rise time was 1.2-1.3 μs: we attribute this phase to the electrogenic electron transfer from H1 to P(+). Pre-reduction of H2 decreased the τ value to about 700-800 ns and increased the amplitude of phase C to 30-35% of that of phase A. Pre-reduction of L1 further accelerated phase C (up to τ of 500 ns) and induced a reverse electrogenic phase with τ of 12 μs and amplitude equal to 10% of phase A. Upon pre-reduction of L2 the rise time of phase C was decreased to about 300 ns and its amplitude decreased by 30%. The acceleration in the onset of phase C is explained by the acceleration of the rate-limiting H1 ⇒ P electrogenic reaction after reduction of the other hemes due to their electrostatic influence; a P-H1-(L1-L2)-H2 alignment of redox centers with an approximately rhombic arrangement of the cytochromec 554 hemes is proposed. The observed reverse phase is ascribed to the post-flash charge redistribution between the hemes. Redox titration of the amplitude of phase C yielded the Em,8.3 values of H1, H2 and L2 hemes: 340±10 mV for H1, 160±20 mV for H2 and -40±40 mV for L2.

Origin and early evolution of photosynthesis

Photosynthesis was well-established on the earth at least 3.5 thousand million years ago, and it is widely believed that these ancient organisms had similar metabolic capabilities to modern cyanobacteria. This requires that development of two photosystems and the oxygen evolution capability occurred very early in the earth's history, and that a presumed phase of evolution involving non-oxygen evolving photosynthetic organisms took place even earlier. The evolutionary relationships of the reaction center complexes found in all the classes of currently existing organisms have been analyzed using sequence analysis and biophysical measurements. The results indicate that all reaction centers fall into two basic groups, those with pheophytin and a pair of quinones as early acceptors, and those with iron sulfur clusters as early acceptors. No simple linear branching evolutionary scheme can account for the distribution patterns of reaction centers in existing photosynthetic organisms, and lateral transfer of genetic information is considered as a likely possibility. Possible scenarios for the development of primitive reaction centers into the heterodimeric protein structures found in existing reaction centers and for the development of organisms with two linked photosystems are presented.

Temperature dependence of charge recombination from the P+QA- and P+QB- states in photosynthetic reaction centers isolated from the thermophilic bacterium Chloroflexus aurantiacus

The temperature dependence of charge recombination from the P+QA- and from the P+QB- states produced by a flash was studied in reaction centers isolated from the photosynthetic thermophilic bacterium Chloroflexus aurantiacus. P designates the primary electron donor; QA and QB the primary and secondary quinone electron acceptors respectively. In QB-depleted reaction centers the rate constant (kAP) for P+QA- recombination was temperature independent between 0-50 degrees C (17.6 +/- 0.7 s-1 at pH 8 and pH 10). The same value was obtained in intact membranes in the presence of o-phenanthroline. Upon lowering the temperature from 250 K to 160 K, kAP increased by a factor of two and remained constant down to 80 K. The overall temperature dependence of kAP was consistent with an activationless process. Ubiquinone (UQ-3) and different types of menaquinone were used for QB reconstitution. In UQ-3 reconstituted reaction centers charge recombination was monoexponential (rate constant k = 0.18 +/- 0.03 s-1) and temperature independent between 5-40 degrees C. In contrast, in menaquinone-3- and menaquinone-4-reconstituted reaction centers P+ rereduction following a flash was markedly biphasic and temperature dependent. In menaquinone-6-reconstituted reaction centers a minor contribution from a third kinetic phase corresponding to P+QA- charge recombination was detected. Analysis of these kinetics and of the effects of the inhibitor o-phenanthroline at high temperature suggest that in detergent suspensions of menaquinone-reconstituted reaction centers a redox reaction removing electrons from the quinone acceptor complex competes with charge recombination. Instability of the semiquinone anions is more pronounced when QB is a short-chain menaquinone. From the temperature dependence of P+ decay the activation parameters for the P+QB- recombination and for the competing side oxidation of the reduced menaquinone acceptor have been derived. For both reactions the activation enthalpies and entropies change markedly with menaquinone chain length but counterbalance each other, resulting in activation free energies at ambient temperature independent of the menaquinone tail. When reaction centers are incorporated into phospholipid vesicles containing menaquinone-8 a temperature-dependent, monophasic, o-phenanthroline-sensitive recombination from the P+QB- state is observed, which is consistent with the formation of stable semiquinone anions. This result seems to indicate a proper QB functioning in the two-subunit reaction center isolated from Chlorflexus aurantiacus when the complex is inserted into a lipid bilayer.

Membrane-bound cytochromes in Chloroflexus aurantiacus studied by EPR

The heme components of chlorosome-depleted membranes of the green-gliding bacterium Chloroflexus aurantiacus were studied by EPR spectroscopy. The four major species, which are present in approximately equimolar quantities, are characterized by the following gz values, redox midpoint potentials and orientations of heme planes with respect to the plane of the membrane: gz = 3.40, Em = +280 mV, 30 degrees; gz = 3.33, Em = 0 mV, 45 degrees; gz = 3.03, Em = +95 mV, 40-50 degrees and gz = 2.95, Em = +150 mV, 90 degrees. These four hemes were attributed to cytochrome c554, the membrane-bound immediate electron donor to the photosynthetic reaction centre in Chloroflexus. All hemes except that with the highest potential were able to undergo photooxidation at 4 K. The photooxidation of the lowest potential heme was stable, whereas that of the +95 mV and the +150 mV hemes reversed on increasing the temperature to 100 K in darkness, due to charge recombination. The ability to photooxidize these hemes at 4 K was lost upon aging of samples. The results demonstrate that a reaction-centre-associated tetraheme cytochrome subunit, analogous to that of purple bacteria, is also present in C. aurantiacus.

Energy transfer kinetics in whole cells and isolated chlorosomes of green photosynthetic bacteria

Time-resolved fluorescence spectroscopy and global data analysis techniques have been used to study the flow of excitations in antennae of the green photosynthetic bacteria Chloroflexus aurantiacus and Chlorobium vibrioforme f. thiosulfatophilum. The transfer of energy from bacteriochlorophyll (BChl) c in Chloroflexus or BChl d in Chlorobium to BChl a 795 was resolved in both whole cells and isolated chlorosomes. In Chloroflexus, the decay of excitations in BChl c occurs in ∼16 ps and a corresponding rise in BChl a emission at 805 nm is detected in global analyses. This band then decays in 46 ps in whole cells due to energy transfer into the membrane. The 805 nm fluorescence in isolated chlorosomes shows a fast decay component similar to that of whole cells, which is consistent with trapping by residual membrane antenna complexes. In Chlorobium, the kinetics are sensitive to the presence of oxygen. Under anaerobic conditions, BChl d decays in 66 ps while the lifetime shortens to 11 ps in aerobic samples. The effect is reversible and occurs in both whole cells and isolated chlorosomes. Emission from BChl a is similarly affected by oxygen, indicating that oxidant-induced quenching can occur from all chlorosome pigments.

Isolation and characterization of the membrane-bound cytochrome c-554 from the thermophilic green photosynthetic bacterium Chloroflexus aurantiacus

The membrane-bound photooxidizable cytochrome c-554 from Chloroflexus aurantiacus has been purified. The purified protein runs as a single heme staining band on SDS-PAGE with an apparent molecular mass of 43 000 daltons. An extinction coefficient of 28 ± 1 mM(-1) cm(-1) per heme at 554 nm was found for the dithionite-reduced protein. The potentiometric titration of the hemes takes place over an extended range, showing clearly that the protein does not contain a single heme in a well-defined site. The titration can be fit to a Nernst curve with midpoint potentials at 0, +120, +220 and +300 mV vs the standard hydrogen electrode. Pyridine hemochrome analysis combined with a Lowry protein assay and the SDS-PAGE molecular weight indicates that there are a minimum of three, and probably four hemes per peptide. Amino acid analysis shows 5 histidine residues and 29% hydrophobic residues in the protein. This cytochrome appears to be functionally similar to the bound cytochrome from Rhodopseudomonas viridis. Both cytochrome c-554 from C. aurantiacus and the four-heme cytochrome c-558-553 from R. viridis appear to act as direct electron donors to the special bacteriochlorophyll pair of the photosynthetic reaction center. They have a similar content of hydrophobic amino acids, but differ in isoelectric point, thermodynamic characteristics, spectral properties, and in their ability to be photooxidized at low temperature.

Oxidation-reduction thermodynamics of the acceptor quinone complex in whole-membrane fragments from Chloroflexus aurantiacus

Oxidation-reduction thermodynamic equilibria involving the quinone-acceptor complex have been examined in whole-membrane fragments from Chloroflexus aurantiacus. The primary quinone acceptor was titrated by monitoring the amount of cytochrome c554 photooxidized by a flash of light as a function of the redox potential. In contrast to previous data obtained in purified plasma membranes, in which the primary quinone acceptor exhibited a midpoint potential equal to -50 mV at pH 8.2, in whole-membrane fragments it titrated at -210 mV (pH 8.0), with a pH dependence of -60 mV/pH up to a pK value of 9.3. o-Phenanthroline, an inhibitor of electron transfer from the primary to the secondary quinone acceptor, shifted the Em/pH curve of the primary acceptor to higher redox potentials. The midpoint potential of the secondary quinone acceptor and its dependence on pH has been determined by comparing the kinetics of the charge recombination processes within the reaction center complex in the presence and in the absence of o-phenanthroline. It is concluded that both the primary and the secondary quinone acceptors interact with a proton, with pK values of 9.3 and of approximately 10.2 respectively. At physiological pH the electron appears to be stabilized on the secondary with respect to the primary quinone acceptor by approximately 60 meV.

Photosynthetic reaction centre of Chloroflexus aurantiacus. I. Primary structure of L-subunit

The L-subunit primary structure of the reaction centre from Chloroflexus aurantiacus composed of 310 amino acid residues has been determined by parallel analysis of the protein and corresponding DNA. Significant homology between this protein and L-subunits from reaction centres of purple bacteria is observed. This implies close similarity in the tertiary structure of these proteins.

Primary photochemistry of reaction centers from the photosynthetic purple bacteria

Photosynthetic organisms transform the energy of sunlight into chemical potential in a specialized membrane-bound pigment-protein complex called the reaction center. Following light activation, the reaction center produces a charge-separated state consisting of an oxidized electron donor molecule and a reduced electron acceptor molecule. This primary photochemical process, which occurs via a series of rapid electron transfer steps, is complete within a nanosecond of photon absorption. Recent structural data on reaction centers of photosynthetic bacteria, combined with results from a large variety of photochemical measurements have expanded our understanding of how efficient charge separation occurs in the reaction center, and have changed many of the outstanding questions.

Oxygen regulation of development of the photosynthetic membrane system in Chloroflexus aurantiacus

Oxygen levels which control induction of the assembly of the pigment-protein photosynthetic polypeptides in dark-grown Chloroflexus aurantiacus were determined. The induction signal by low-oxygen tension is not directly related to the respiratory competence of these photosynthetic cells. Cytochrome c554, the primary electron donor to P865+ of the reaction center, is not present in dark-grown respiratory cells but is induced in parallel with bacteriochlorophylls a and c and at similar oxygen partial pressure. The development of these components of the photosynthetic apparatus and its electron transport chain is completely independent of the presence of any detectable light or bacteriochlorophyll c or a pigments in C. aurantiacus.

Isolation of cytochrome bc 1 complexes from the photosynthetic bacteria Rhodopseudomonas viridis and Rhodospirillum rubrum

Cytochrome bc 1 complexes have been isolated from wild type Rhodopseudomonas viridis and Rhodospirillum rubrum and purified by affinity chromatography on cytochrome c-Sepharose 4B. Both complexes are largely free of bacteriochlorophyll and carotenoids and contain cytochromes b and c 1 in a 2:1 molar ratio. For the Rps. viridis complex, evidence has been obtained for two spectrally distinct b-cytochromes. The R. rubrum complex contains a Rieske iron-sulfur protein (present in approximately 1:1 molar ratio to cytochrome c 1) and catalyzes an antimycin A- and myxothiazol-sensitive electron transfer from duroquinol to equine cytochrome c or R. rubrum cytochrome c 2. Although an attempt to prepare a cytochrome bc 1 complex from the gliding green bacterium Chloroflexus aurantiacus was not successful, membranes isolated from phototrophically grown Cfl. aurantiacus were shown to contain a Rieske iron-sulfur protein and protoheme (the prosthetic group of b-type cytochromes).

Electron transport in green photosynthetic bacteria

Green bacteria make up two of the four families of anoxygenic photosynthetic prokaryotes. The two families have similar pigment compositions and membrane fine structure, and both contain a specialized antenna structure known as a chlorosome. The primary photochemistry and electron transport pathways of the two groups are, however, quite distinct. The anaerobic green bacteria (Chlorobiaceae) contain low-potential iron-sulfur proteins as early electron acceptors and can directly reduce NAD(+) in a manner reminiscent of Photosystem I of oxygenic organisms. The facultatively aerobic green bacteria (Chloroflexaceae) contain quinone-type acceptors and have an overall pattern of electron transport very similar to that found in purple bacteria. Many aspects of energy storage in green bacteria, especially photophosphorylation and the role of cytochrome b/c complexes in electron transport, remain poorly understood.