Bacteriopheophytin g: Properties and some speculations on a possible primary role for bacteriochlorophylls b and g in the biosynthesis of chlorophylls

Electronic Structures of Radical-Pair-Forming Cofactors in a Heliobacterial Reaction Center

Photosynthetic reaction centers (RCs) are membrane proteins converting photonic excitations into electric gradients. The heliobacterial RCs (HbRCs) are assumed to be the precursors of all known RCs, making them a compelling subject for investigating structural and functional relationships. A comprehensive picture of the electronic structure of the HbRCs is still missing. In this work, the combination of selective isotope labelling of 13C and 15N nuclei and the utilization of photo-CIDNP MAS NMR (photochemically induced dynamic nuclear polarization magic-angle spinning nuclear magnetic resonance) allows for highly enhanced signals from the radical-pair-forming cofactors. The remarkable magnetic-field dependence of the solid-state photo-CIDNP effect allows for observation of positive signals of the electron donor cofactor at 4.7 T, which is interpreted in terms of a dominant contribution of the differential relaxation (DR) mechanism. Conversely, at 9.4 T, the emissive signals mainly originate from the electron acceptor, due to the strong activation of the three-spin mixing (TSM) mechanism. Consequently, we have utilized two-dimensional homonuclear photo-CIDNP MAS NMR at both 4.7 T and 9.4 T. These findings from experimental investigations are corroborated by calculations based on density functional theory (DFT). This allows us to present a comprehensive investigation of the electronic structure of the cofactors involved in electron transfer (ET).

Isomerization kinetics of bacteriochlorophyll b and bacteriopheophytin b under acidic conditions

Bacteriochlorophyll (BChl) b has a unique π-conjugation system, in which the bacteriochlorin macrocycle is conjugated with the C8-ethylidene group. This π-system is converted easily to the chlorin macrocycle. However, the effects of the central magnesium in BChl b on this conversion are unclear. In this study, the isomerization kinetics of BChl b and its demetalated pigment, bacteriopheophytin (BPhe) b, was analyzed under weakly acidic conditions. BChl b exhibited faster acid-induced isomerization than BPhe b. These results were attributed to the stabilization of a cationic intermediate, whose C8-ethylidene group is protonated, during the isomerization of BChl b compared to BPhe b because of a difference in the electron densities of the π-conjugation systems between BChl b and BPhe b. High-performance liquid chromatography analyses indicated that BChl b was primarily isomerized to 3-acetyl Chl a, followed by demetalation. The reaction order was due to the slower demetalation kinetics of metallobacteriochlorins than metallochlorins. These results will be helpful for handling unstable BChl b and BPhe b. The reaction properties of BChl b and BPhe b demonstrated here will be helpful for understanding the in vivo formation of BPhe b, which acts as the primary electron acceptor in photosynthetic reaction center complexes in BChl b-containing purple photosynthetic bacteria.

Differential sensitivity to oxygen among the bacteriochlorophylls g in the type-I reaction centers of Heliobacterium modesticaldum

The type-I, homodimeric photosynthetic reaction center (RC) of Heliobacteria (HbRC) is the only known RC in which bacteriochlorophyll g (BChl g) is found. It is also simpler than other RCs, having the smallest number of protein subunits and bound chromophores of any type-I RC. In the presence of oxygen, BChl g isomerizes to 8¹-hydroxychlorophyll a_F (Chl a_F). This naturally occurring process provides a way of altering the chlorophylls and studying the effect of these changes on energy and electron transfer. Transient absorbance difference spectroscopy reveals that triplet-state formation occurs in the antenna chlorophylls of HbRCs but does not provide site-specific information. Here, we report on an extended optically detected magnetic resonance (ODMR) study of the antenna triplet states in HbRCs with differing levels of conversion of BChl g to Chl a_F. The data reveal pools of BChl g molecules with different triplet zero-field splitting parameters and different susceptibilities to chemical oxidation. By relating the detailed spectroscopic characteristics derived from the ODMR data to the recently solved crystallographic structure, we have tentatively identified BChl g molecules in which the probability of triplet formation is high and sites at which BChl g conversion is more likely, providing useful information about the fate of the excitation in the complex.

A Review of Bacteriochlorophyllides: Chemical Structures and Applications

Generally, bacteriochlorophyllides were responsible for the photosynthesis in bacteria. Seven types of bacteriochlorophyllides have been disclosed. Bacteriochlorophyllides a/b/g could be synthesized from divinyl chlorophyllide a. The other bacteriochlorophyllides c/d/e/f could be synthesized from chlorophyllide a. The chemical structure and synthetic route of bacteriochlorophyllides were summarized in this review. Furthermore, the potential applications of bacteriochlorophyllides in photosensitizers, immunosensors, influence on bacteriochlorophyll aggregation, dye-sensitized solar cell, heme synthesis and for light energy harvesting simulation were discussed.

Absorption and Fluorescence Spectral Database of Chlorophylls and Analogues

Absorption spectra and fluorescence spectra are essential for use across the photosciences, yet such spectra along with the all-important values for molar absorption coefficient (ε) and fluorescence quantum yield (Φ_f ) often are found with great difficulty. Here, a literature survey concerning the vital class of chlorophyll compounds has led to identification of spectra for 150 members. Spectra in print form have been digitized (with baseline corrections) and assembled into a database along with literature references, solvent identity and values for ε and Φ_f (where available). The database encompasses photosynthetic tetrapyrroles wherein the chromophore is a porphyrin (e.g. chlorophyll c₁ , protochlorophyll a), chlorin (e.g. chlorophyll a, bacteriochlorophyll c) or bacteriochlorin (e.g. bacteriochlorophyll a). Altogether, the database contains 305 absorption spectra (from 19 porphyrins, 109 chlorins and 22 bacteriochlorins) and 72 fluorescence spectra (from 10 porphyrins, 30 chlorins and 4 bacteriochlorins). The spectral database should facilitate comparisons and quantitative calculations. All spectra are available in print form in the Supporting Information. The entire database in digital form is available with the PhotochemCAD program for free downloading and further use at http://www.photochemcad.com.

Using the Endogenous CRISPR-Cas System of Heliobacterium modesticaldum To Delete the Photochemical Reaction Center Core Subunit Gene

In Heliobacterium modesticaldum, as in many Firmicutes, deleting genes by homologous recombination using standard techniques has been extremely difficult. The cells tend to integrate the introduced plasmid into the chromosome by a single recombination event rather than perform the double recombination required to replace the targeted locus. Transformation with a vector containing only a homologous recombination template for replacement of the photochemical reaction center gene pshA produced colonies with multiple genotypes, rather than a clean gene replacement. To address this issue, we required an additional means of selection to force a clean gene replacement. In this study, we report the genetic structure of the type I-A and I-E CRISPR-Cas systems from H. modesticaldum, as well as methods to leverage the type I-A system for genome editing. In silico analysis of the CRISPR spacers revealed a potential consensus protospacer adjacent motif (PAM) required for Cas3 recognition, which was then tested using an in vivo interference assay. Introduction of a homologous recombination plasmid that carried a miniature CRISPR array targeting sequences in pshA (downstream of a naturally occurring PAM sequence) produced nonphototrophic transformants with clean replacements of the pshA gene with ∼80% efficiency. Mutants were confirmed by PCR, sequencing, optical spectroscopy, and growth characteristics. This methodology should be applicable to any genetic locus in the H. modesticaldum genome.IMPORTANCE The heliobacteria are the only phototrophic members of the largely Gram-positive phylum Firmicutes, which contains medically and industrially important members, such as Clostridium difficile and Clostridium acetobutylicum Heliobacteria are of interest in the study of photosynthesis because their photosynthetic system is unique and the simplest known. Since their discovery in the early 1980s, work on the heliobacteria has been hindered by the lack of a genetic transformation system. The problem of introducing foreign DNA into these bacteria has been recently rectified by our group; however, issues still remained for efficient genome editing. The significance of this work is that we have characterized the endogenous type I CRISPR-Cas system in the heliobacteria and leveraged it to assist in genome editing. Using the CRISPR-Cas system allowed us to isolate transformants with precise replacement of the pshA gene encoding the main subunit of the photochemical reaction center.

Probing the Electronic Structure of Bacteriochlorophyll Radical Ions-A Theoretical Study of the Effect of Substituents on Hyperfine Parameters

In reaction centers (RCs) of photosynthesis, a light-induced charge separation takes place creating radical cations and anions of the participating cofactors. In photosynthetic bacteria, different bacteriochlorophylls (BChl) are involved in this process. Information about the electronic structure of the BChl radical cations and anions can be obtained by measuring the electron spin density distribution via the electron-nuclear hyperfine interaction using EPR and ENDOR techniques. In this communication, we report isotropic hyperfine coupling constants (hfcs) of the BChl b and g radical cations and anions, calculated by density functional theory, and compare them with the more common radical ions of BChl a and with available experimental data. The observed differences in the computed hyperfine data are discussed in view of a possible distinction between these species by EPR/ENDOR methods. In addition, ¹⁴ N nuclear quadrupole coupling constants (nqcs) computed for BChl a, b, g, and also for Chl a in their charge neutral, radical cation and radical anion states are presented. These nqcs are compared with experimental values obtained by ESEEM spectroscopy on several different radical ions.

The Effect of Bacteriochlorophyll g Oxidation on Energy and Electron Transfer in Reaction Centers from Heliobacterium modesticaldum

The heliobacteria are a family of strictly anaerobic, Gram-positive, photoheterotrophs in the Firmicutes. They make use of a homodimeric type I reaction center (RC) that contains ∼20 antenna bacteriochlorophyll (BChl) g molecules, a special pair of BChl g' molecules (P800), two 8(1)-OH-Chl aF molecules (A0), a [4Fe-4S] iron-sulfur cluster (FX), and a carotenoid (4,4'-diaponeurosporene). It is known that in the presence of light and oxygen BChl g is converted to a species with an absorption spectrum identical to that of Chl a. Here, we show that main product of the conversion is 8(1)-OH-Chl aF. Smaller amounts of two other oxidized Chl aF species are also produced. In the presence of light and oxygen, the kinetics of the conversion are monophasic and temperature dependent, with an activation energy of 66 ± 2 kJ mol(-1). In the presence of oxygen in the dark, the conversion occurs in two temperature-dependent kinetic phases: a slow phase followed by a fast phase with an activation energy of 53 ± 1 kJ mol(-1). The loss of BChl g' occurs at the same rate as the loss of Bchl g; hence, the special pair converts at the same rate as the antenna Chl's. However, the loss of P800 photooxidiation and flavodoxin reduction is not linear with the loss of BChl g. In anaerobic RCs, the charge recombination between P800(+) and FX(-) at 80 K is monophasic with a lifetime of 4.2 ms, but after exposure to oxygen, an additional phase with a lifetime of 0.3 ms is observed. Transient EPR data show that the line width of P800(+) increases as BChl g is converted to Chl aF and the rate of electron transfer from A0 to FX, as estimated from the net polarization generated by singlet-triplet mixing during the lifetime of P800(+)A0(-), is unchanged. The transient EPR data also show that conversion of the BChl g results in increased formation of triplet states of both BChl g and Chl aF. The nonlinear loss of P800 photooxidiation and flavodoxin reduction, the biphasic backreaction kinetics, and the increased EPR line width of P800(+) are all consistent with a model in which the BChl g'/BChl g' and BChl g'/Chl aF' special pairs are functional but the Chl aF'/Chl aF' special pair is not.

Engineered biosynthesis of bacteriochlorophyll b in Rhodobacter sphaeroides

Bacteriochlorophyll b has the most red-shifted absorbance maximum of all naturally occurring photopigments. It has a characteristic ethylidene group at the C8 position in place of the more common ethyl group, the product of a C8-vinyl reductase, which is carried by the majority of chlorophylls and bacteriochlorophylls used in photosynthesis. The subsequent and first step exclusive to bacteriochlorophyll biosynthesis, the reduction of the C7 = C8 bond, is catalyzed by chlorophyllide oxidoreductase. It has been demonstrated that the enzyme from bacteriochlorophyll a-utilizing bacteria can catalyze the formation of compounds carrying an ethyl group at C8 from both ethyl- and vinyl-carrying substrates, indicating a surprising additional C8-vinyl reductase function, while the enzyme from organisms producing BChl b could only catalyze C7 = C8 reduction with a vinyl substrate, but this product carried an ethylidene group at the C8 position. We have replaced the native chlorophyllide oxidoreductase-encoding genes of Rhodobacter sphaeroides with those from Blastochloris viridis, but the switch from bacteriochlorophyll a to b biosynthesis is only detected when the native conventional C8-vinyl reductase is absent. We propose a non-enzymatic mechanism for ethylidene group formation based on the absence of cellular C8-vinyl reductase activity.

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We engineer the production of a foreign photopigment in Rhodobacter sphaeroides.

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Native COR-encoding genes are replaced with those from Blastochloris viridis.

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Bacteriochlorophyll b is produced upon additional deletion of conventional 8VR.

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We propose that loss of 8VR activity by COR leads to ethylidene bond formation.

Photoreduction of zinc 8-vinylated chlorophyll derivative to bacteriochlorophyll-b/g analog possessing an 8-ethylidene group

When a pyridine solution of zinc methyl 8-vinyl-mesopyropheophorbide-a was irradiated with visible light in the presence of ethanol, ascorbic acid and diazabicylo[2.2.2]octane under nitrogen at room temperature, zinc (7R/S,8E)-8-ethylidene-bacteriochlorin was obtained via 1,4-hydrogenation. The 1,4-photoreduction is similar to the enzymatic reduction of 8-vinyl-chlorophyllides to (E)-8-ethylidene-bacteriochlorins in anoxygenic photosynthetic bacteria producing bacteriochlorophylls-b/g. The resulting zinc 8-ethylidene-bacteriochlorin was readily isomerized to the chemically more stable 8-ethyl-chlorin by further illumination. As a by-product, zinc 8-vinyl-7,8-cis-bacteriochlorin was slightly formed by photoinduced 1,2-hydrogenation of zinc 8-vinyl-chlorin.

Expression and characterization of the diheme cytochrome c subunit of the cytochrome bc complex in Heliobacterium modesticaldum

Heliobacterium modesticaldum is a Gram-positive, anaerobic, anoxygenic photoheterotrophic bacterium. Its cytochrome bc complex (Rieske/cyt b complex) has some similarities to cytochrome b(6)f complexes from cyanobacteria and chloroplasts, and also shares some characteristics of typical bacterial cytochrome bc(1) complexes. One of the unique factors of the heliobacterial cytochrome bc complex is the presence of a diheme cytochrome c instead of the monoheme cytochrome f in the cytochrome b(6)f complex or the monoheme cytochrome c(1) in the bc(1) complex. To understand the structure and function of this diheme cytochrome c protein, we expressed the N-terminal transmembrane-helix-truncated soluble H. modesticaldum diheme cytochrome c in Escherichia coli. This 25kDa recombinant protein possesses two c-type hemes, confirmed by mass spectrometry and a variety of biochemical techniques. Sequence analysis of the H. modesticaldum diheme cytochrome c indicates that it may have originated from gene duplication and subsequent gene fusion, as in cytochrome c(4) proteins. The recombinant protein exhibits a single redox midpoint potential of +71mV versus NHE, which indicates that the two hemes have very similar protein environments.

Heliobacterial Rieske/cytb complex

Data on structure and function of the Rieske/cytb complex from Heliobacteria are scarce. They indicate that the complex is related to the b (6) f complex in agreement with the phylogenetic position of the organism. It is composed of a diheme cytochrome c, and a Rieske iron-sulfur protein, together with transmembrane cytochrome b (6) and subunit IV. Additional small subunits may be part of the complex. The cofactor content comprises heme c (i), first discovered in the Q(i) binding pocket of b (6) f complexes. The redox midpoint potentials are more negative than in b (6) f complex in agreement with the lower redox midpoint potentials (by about 150 mV) of its reaction partners, menaquinone, and cytochrome c (553). The enzyme is implicated in cyclic electron transfer around the RCI. Functional studies are favored by the absence of antennae and the simple photosynthetic reaction chain but are hampered by the high oxygen sensitivity of the organism, its chlorophyll, and lipids.

An overview on chlorophylls and quinones in the photosystem I-type reaction centers

Minor but key chlorophylls (Chls) and quinones in photosystem (PS) I-type reaction centers (RCs) are overviewed in regard to their molecular structures. In the PS I-type RCs, the prime-type chlorophylls, namely, bacteriochlorophyll (BChl) a' in green sulfur bacteria, BChl g' in heliobacteria, Chl a' in Chl a-type PS I, and Chl d' in Chl d-type PS I, function as the special pairs, either as homodimers, (BChl a')(2) and (BChl g')(2) in anoxygenic organisms, or heterodimers, Chl a/a' and Chl d/d' in oxygenic photosynthesis. Conversions of BChl g to Chl a and Chl a to Chl d take place spontaneously under mild condition in vitro. The primary electron acceptors, A (0), are Chl a-derivatives even in anoxygenic PS I-type RCs. The secondary electron acceptors are naphthoquinones, whereas the side chains may have been modified after the birth of cyanobacteria, leading to succession from menaquinone to phylloquinone in oxygenic PS I.

Energy metabolism of Heliobacterium modesticaldum during phototrophic and chemotrophic growth

Background

Heliobacterium modesticaldum is a gram-positive nitrogen-fixing phototrophic bacterium that can grow either photoheterotrophically or chemotrophically but not photoautotrophically. Surprisingly, this organism is lacking only one gene for the complete reverse tricarboxylic acid (rTCA) cycle required for autotrophic carbon fixation. Along with the genomic information reported recently, we use multiple experimental approaches in this report to address questions regarding energy metabolic pathways in darkness, CO₂ fixation, sugar assimilation and acetate metabolism.

Results

We present the first experimental evidence that D-ribose, D-fructose and D-glucose can be photoassimilated by H. modesticaldum as sole carbon sources in newly developed defined growth medium. Also, we confirm two non-autotrophic CO₂-fixation pathways utilized by H. modesticaldum: reactions catalyzed by pyruvate:ferredoxin oxidoreductase and phosphoenolpyruvate carboxykinase, and report acetate excretion during phototrophic and chemotrophic growth. Further, genes responsible for pyruvate fermentation, which provides reducing power for nitrogen assimilation, carbon metabolism and hydrogen production, are either active or up-regulated during chemotrophic growth. The discovery of ferredoxin-NADP⁺ oxidoreductase (FNR) activity in cell extracts provides the reducing power required for carbon and nitrogen metabolisms. Moreover, we show that photosynthetic pigments are produced by H. modesticaldum during the chemotrophic growth, and demonstrate that H. modesticaldum performs nitrogen fixation during both phototrophic and chemotrophic growth.

Conclusion

Collectively, this report represents the first comprehensive studies for energy metabolism in heliobacteria, which have the simplest known photosynthetic machinery among the entire photosynthetic organisms. Additionally, our studies provide new and essential insights, as well as broaden current knowledge, on the energy metabolism of the thermophilic phototrophic bacterium H. modesticaldum during phototrophic and chemotrophic growth.

Chlorophyll Biosynthesis in Bacteria: The Origins of Structural and Functional Diversity

The use of photochemical reaction centers to convert light energy into chemical energy, chlorophototrophy, occurs in organisms belonging to only five eubacterial phyla: Cyanobacteria, Proteobacteria, Chlorobi, Chloroflexi, and Firmicutes. All chlorophototrophs synthesize two types of pigments: (a) chlorophylls and bacteriochlorophylls, which function in both light harvesting and uniquely in photochemistry; and (b) carotenoids, which function primarily as photoprotective pigments but can also participate in light harvesting. Although hundreds of carotenoids have been identified, only 12 types of chlorophylls (Chl a, b, d; divinyl-Chl a and b; and 8(1)-hydroxy-Chl a) and bacteriochlorophylls (BChl a, b, c, d, e, and g) are currently known to occur in bacteria. This review summarizes recent progress in the identification of genes and enzymes in the biosynthetic pathways leading to Chls and BChls, the essential tetrapyrrole cofactors of photosynthesis, and addresses the mechanisms for generating functional diversity for solar energy capture and conversion in chlorophototrophs.

Heliobacterial photosynthesis

Heliobacteria contain Type I reaction centers (RCs) and a homodimeric core, but unlike green sulfur bacteria, they do not contain an extended antenna system. Given their simplicity, the heliobacterial RC (HbRC) should be ideal for the study of a prototypical homodimeric RC. However, there exist enormous gaps in our knowledge, particularly with regard to the nature of the secondary and tertiary electron acceptors. To paraphrase S. Neerken and J. Amesz (2001 Biochim Biophys Acta 1507:278-290): with the sole exception of primary charge separation, little progress has been made in recent years on the HbRC, either with respect to the polypeptide composition, or the nature of the electron acceptor chain, or the kinetics of forward and backward electron transfer. This situation, however, has changed. First, the low molecular mass polypeptide that contains the terminal FA and FB iron-sulfur clusters has been identified. The change in the lifetime of the flash-induced kinetics from 75 ms to 15 ms on its removal shows that the former arises from the P798+ [FA/FB]- recombination, and the latter from P798+ FX- recombination. Second, FX has been identified in HbRC cores by EPR and Mössbauer spectroscopy, and shown to be a [4Fe-4S]1+,2+ cluster with a ground spin state of S=3/2. Since all of the iron in HbRC cores is in the FX cluster, a ratio of approximately 22 Bchl g/P798 could be calculated from chemical assays of non-heme iron and Bchl g. Third, the N-terminal amino acid sequence of the FA/FB-containing polypeptide led to the identification and cloning of its gene. The expressed protein can be rebound to isolated HbRC cores, thereby regaining both the 75 ms kinetic phase resulting from P798+ [FA/FB]- recombination and the light-induced EPR resonances of FA- and FB-. The gene was named 'pshB' and the protein 'PshB' in keeping with the accepted nomenclature for Type I RCs. This article reviews the current state of knowledge on the structure and function of the HbRC.

De Novo Synthesis of Stable Tetrahydroporphyrinic Macrocycles: Bacteriochlorins and a Tetradehydrocorrin

[structures: see text] Bacteriochlorins (tetrahydroporphyrins) are attractive for diverse photochemical applications owing to their strong absorption in the near-infrared spectral region, as exemplified by the bacterial photosynthetic pigment bacteriochlorophyll a, yet often are labile toward dehydrogenation to give the chlorin. Tetradehydrocorrins (ring-contracted tetrahydroporphyrins) are attractive for studies of catalysis analogous to that of vitamin B12. An eight-step synthesis toward such tetrahydroporphyrinic macrocycles begins with p-tolualdehyde and proceeds to a dihydrodipyrrin-acetal (1) bearing a geminal dimethyl group and a p-tolyl substituent. Self-condensation of 1 in CH3CN containing BF3 x OEt2 at room temperature afforded a readily separable mixture of two free base bacteriochlorins and a free base B,D-tetradehydrocorrin. Each bacteriochlorin contains two geminal dimethyl groups to lock-in the bacteriochlorin hydrogenation level, p-tolyl substituents at opposing (2,12) beta-positions, and the absence (H-BC) or presence (MeO-BC) of a methoxy group at the 5- (meso) position. The B,D-tetradehydrocorrin (TDC) lies equidistant between the hydrogenation levels of corrin and corrole, is enantiomeric, and contains two geminal dimethyl groups, 2,12-di-p-tolyl substituents, and an acetal group at the pyrroline-pyrrole junction. Examination of the effect of the concentrations of 1 (2.5-50 mM) and BF3 x OEt2 (10-500 mM) revealed a different response surface for each of H-BC, MeO-BC, and TDC, enabling relatively selective preparation of a given macrocycle. The highest isolated yield of each was 49, 30, and 66%, respectively. The macrocycles are stable to routine handling in light and air. The bacteriochlorins display characteristic spectral features; for example, H-BC exhibits near-IR absorption (lambda(Qy) = 737 nm, epsilon(Qy) = 130,000 M(-1) cm(-1)) and emission (lambda(em) = 744 nm, phi(f) = 0.14). In summary, this simple entry to stable bacteriochlorins and tetradehydrocorrins should facilitate a wide variety of applications.

Quest for minor but key chlorophyll molecules in photosynthetic reaction centers - unusual pigment composition in the reaction centers of the chlorophyll d-dominated cyanobacterium Acaryochloris marina

A short overview, based on our own findings, is given of the minor pigments that function as key components in photosynthesis. Recently, we found the presence of chlorophyll a, chlorophyll d' and pheophytin a as minor pigments in the chlorophyll d-dominated cyanobacterium Acaryochloris marina.

The antenna reaction center complex of heliobacteria: composition, energy conversion and electron transfer

A survey is given of various aspects of the photosynthetic processes in heliobacteria. The review mainly refers to results obtained since 1995, which had not been covered earlier. It first discusses the antenna organization and pigmentation. The pigments of heliobacteria include some unusual species: bacteriochlorophyll (BChl) g, the main pigment, 8(1) hydroxy chlorophyll a, which acts as primary electron acceptor, and 4,4'-diaponeurosporene, a carotenoid with 30 carbon atoms. Energy conversion within the antenna is very fast: at room temperature thermal equilibrium among the approx. 35 BChls g of the antenna is largely completed within a few ps. This is then followed by primary charge separation, involving a dimer of BChl g (P798) as donor, but recent evidence indicates that excitation of the acceptor pigment 8(1) hydroxy chlorophyll a gives rise to an alternative primary reaction not involving excited P798. The final section of the review concerns secondary electron transfer, an area that is relatively poorly known in heliobacteria.

Molecular evidence that the capacity for endosporulation is universal among phototrophic heliobacteria

Although enrichment cultures for anoxygenic phototrophic heliobacteria commonly contain sporulating cells, once strains of heliobacteria are obtained in pure culture, they all but cease to sporulate. In fact, some species of heliobacteria have never been observed to sporulate. Thus, despite their phylogenetic connection to endospore-forming bacteria, the question of sporulation capacity in heliobacteria remains open. We have investigated this problem using PCR and Southern hybridization as tools and show here that all recognized species of heliobacteria tested, as well as several unclassified strains, contain homologs to the ssp genes of Clostridium and Bacillus species, genes that encode key sporulation-specific proteins. It can therefore be concluded that as a group, heliobacteria are likely all to be endospore-forming bacteria in agreement with their phylogenetic placement within the 'low GC' Gram-positive bacteria.

The primary electron acceptor of green sulfur bacteria, bacteriochlorophyll 663, is chlorophyll a esterified with Delta2,6-phytadienol

The primary electron acceptor of green sulfur bacteria, bacteriochlorophyll (BChl) 663, was isolated at high purity by an improved purification procedure from a crude reaction center complex, and the molecular structure was determined by fast atom bombardment mass spectroscopy (FAB-mass), (1)H- and (13)C-NMR spectrometry, double quantum filtered correlation spectroscopy (DQF-COSY), heteronuclear multiple-quantum coherence (HMQC) and heteronuclear multiple-bond correlation (HMBC) spectral measurements. BChl 663 was 2.0 mass units smaller than plant Chl a. The NMR spectra showed that the macrocycle was identical to that of Chl a. In the esterifying alcohol, a singlet P7(1) signal was observed at the high-field side of the singlet P3(1) signal in BChl 663, while a doublet peak of P7(1) overlapped that of P11(1) in Chl a. A signal of P7-proton, seen in Chl a, was lacking, and the P6-proton appeared as a triplet signal near the triplet P2-proton signal in BChl 663. These results indicate the presence in BChl 663 of a C=C double bond between P6 and P7 in addition to that between P2 and P3. The structure of BChl 663 was hence concluded to be Chl a esterified with 2,6-phytadienol instead of phytol. In addition to BChl 663, two molecules of the 13(2)-epimer of BChl a, BChl a', were found to be present per reaction center, which may constitute the primary electron donor.

Discovery of the heliobacteria

The first photosynthetic bacterium obtained in pure culture wasRhodospirillum rubrum, isolated by Erwin Esmarch in 1887. The organism appeared to be an aerobic heterotroph, and Esmarch was unaware of its photosynthetic capability. The overall general characteristics of a number of major species of photosynthetic bacteria were described by Molisch and van Niel before 1945. Subsequently, our knowledge of the anoxygenic phototrophs increased greatly through the systematic study of numerous new species isolated from enrichment cultures in which capacity for anaerobic (and anoxygenic) growth with light as the energy source was a primary selective factor. A further refinement of the enrichment technique required ability to use N2 as the sole source of nitrogen for growth under anaerobic photosynthetic conditions, and this led to the isolation of additional new species, including the heliobacteria. The first recognition of the heliobacteria was facilitated by serendipity, which was a significant factor in a number of other researches on photosynthetic bacteria (Gest 1992).

A microbiologist's odyssey: Bacterial viruses to photosynthetic bacteria

Perspective can be defined as the relationships or relative importance of facts or matters from any special point of view. Thus, my Personal perspective reflects the threads I followed in a 50-year journey of research in the complex tapestry of bioenergetics and various aspects of microbial metabolism. An early interest in biochemical and microbial evolution led to the fertile hunting grounds of anoxygenic photosynthetic bacteria. Viewed as a physiological class, these organisms show remarkable metabolic versatility in that certain individual species are capable of using all the known major types of energy conversion (photosynthetic, respiratory, and fermentative) to support growth. Since such anoxyphototrophs are readily amenable to molecular genetic/biological manipulation, it can be expected that they will eventually provide important clues for unraveling the evolutionary relationships of the several kinds of energy conversion. I gradually came to believe that understanding the evolution of phototrophs would require detailed knowledge not only of how light is converted to chemical energy, but also of a) pathways of monomer production from extracellular sources of carbon and nitrogen and b) mechanisms cells use for integrating ATP regeneration with the energy-requiring biosyntheses of biological macromolecules. Serendipic observation of photoproduction of H2 from organic compounds by Rhodospirillum rubrum in 1949 led to discovery of N2 fixation by anoxyphototrophs, and this capacity was later exploited for the isolation of hitherto unknown species of photosynthetic prokaryotes, including the heliobacteria. Recent studies on the reaction centers of the heliobacteria suggest the possibility that these bacteria are descendents of early phototrophs that gave rise to oxygenic photosynthetic organisms.

The derivation of the formyl-group oxygen of chlorophyll b in higher plants from molecular oxygen. Achievement of high enrichment of the 7-formyl-group oxygen from 18O2 in greening maize leaves

The mechanism of formation of the formyl group of chlorophyll b has long been obscure but, in this paper, the origin of the 7-formyl-group oxygen of chlorophyll b in higher plants was determined by greening etiolated maize leaves, excised from dark-grown plants, by illumination under white light in the presence of either H2(18)O or 18O2 and examining the newly synthesized chlorophylls by mass spectroscopy. To minimize the possible loss of 18O label from the 7-formyl substituent by reversible formation of chlorophyll b-7(1)-gem-diol (hydrate) with unlabelled water in the cell, the formyl group was reduced to a hydroxymethyl group during extraction with methanol containing NaBH4: chlorophyll a remained unchanged during this rapid reductive extraction process. Mass spectra of chlorophyll a and [7-hydroxymethyl]-chlorophyll b extracted from leaves greened in the presence of either H2(18)O or 18O2 revealed that 18O was incorporated only from molecular oxygen but into both chlorophylls: the mass spectra were consistent with molecular oxygen providing an oxygen atom not only for incorporation into the 7-formyl group of chlorophyll b but also for the well-documented incorporation into the 13(1)-oxo group of both chlorophylls a and b [see Walker, C. J., Mansfield, K. E., Smith, K. M. & Castelfranco, P. A. (1989) Biochem. J. 257, 599-602]. The incorporation of isotope led to as much as 77% enrichment of the 13(1)-oxo group of chlorophyll a: assuming identical incorporation into the 13(1) oxygen of chlorophyll b, then enrichment of the 7-formyl oxygen was as much as 93%. Isotope dilution by re-incorporation of photosynthetically produced oxygen from unlabelled water was negligible as shown by a greening experiment in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. The high enrichment using 18O2, and the absence of labelling by H2(18)O, unequivocally demonstrates that molecular oxygen is the sole precursor of the 7-formyl oxygen of chlorophyll b in higher plants and strongly suggests a single pathway for the formation of the chlorophyll b formyl group involving the participation of an oxygenase-type enzyme.

Protein sequences and redox titrations indicate that the electron acceptors in reaction centers from heliobacteria are similar to Photosystem I

Photosynthetic reaction centers isolated from Heliobacillus mobilis exhibit a single major protein on SDS-PAGE of 47 000 Mr. Attempts to sequence the reaction center polypeptide indicated that the N-terminus is blocked. After enzymatic and chemical cleavage, four peptide fragments were sequenced from the Heliobacillus mobilis apoprotein. Only one of these sequences showed significant specific similarity to any of the protein and deduced protein sequences in the GenBank data base. This fragment is identical with 56% of the residues, including both cysteines, found in the highly conserved region that is proposed to bind iron-sulfur center FX in the Photosystem I reaction center peptide that is the psaB gene product. The similarity to the psaA gene product in this region is 48%.Redox titrations of laser-flash-induced photobleaching with millisecond decay kinetics on isolated reaction centers from Heliobacterium gestii indicate a midpoint potential of -414 mV with n=2 titration behavior. In membranes, the behavior is intermediate between n=1 and n=2, and the apparent midpoint potential is -444 mV. This is compared to the behavior in Photosystem I, where the intermediate electron acceptor A1, thought to be a phylloquinone molecule, has been proposed to undergo a double reduction at low redox potentials in the presence of viologen redox mediators.These results strongly suggest that the acceptor side electron transfer system in reaction centers from heliobacteria is indeed analogous to that found in Photosystem I. The sequence similarities indicate that the divergence of the heliobacteria from the Photosystem I line occurred before the gene duplication and subsequent divergence that lead to the heterodimeric protein core of the Photosystem I reaction center.

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.

Probing the bacteriochlorophyll binding site by reconstitution of the light-harvesting complex of Rhodospirillum rubrum with bacteriochlorophyll a analogues

Structural features of bacteriochlorophyll (BChl) a that are required for binding to the light-harvesting proteins of Rhodospirillum rubrum were determined by testing for reconstitution of the B873 or B820 (structural subunit of B873) light-harvesting complexes with BChl a analogues. The results indicate that the binding site is very specific; of the analogues tested, only derivatives of BChl a with ethyl, phytyl, and geranylgeranyl esterifying alcohols and BChl b (phytyl) successfully reconstituted to form B820- and B873-type complexes. BChl analogues lacking magnesium, the C-3 acetyl group, or the C-13(2) carbomethoxy group did not reconstitute to form B820 or B873. Also unreactive were 13(2)-hydroxyBChl a and 3-acetylchlorophyll a. Competition experiments showed that several of these nonreconstituting analogues significantly slowed BChl a binding to form B820 and blocked BChl a-B873 formation, indicating that the analogues may competitively bind to the protein even though they do not form red-shifted complexes. With the R. rubrum polypeptides, BChl b formed complexes that were further red-shifted than those of BChl a; however, the energies of the red shifts, binding behavior, and circular dichroism (CD) spectra were similar. B873 complexes reconstituted with the geranylgeranyl BChl a derivative, which contains the native esterifying alcohol for R. rubrum, showed in-vivo-like CD features, but the phytyl and ethyl B873 complexes showed inverted CD features in the near infrared. The B820 complex with the ethyl derivative was about 30-fold less stable than the two longer esterifying alcohol derivatives, but all formed stable B873 complexes.

Isolation of a photoactive photosynthetic reaction center-core antenna complex from Heliobacillus mobilis

A photoactive reaction center-core antenna complex was isolated from the photosynthetic bacterium Heliobacillus mobilis by extraction of membranes with Deriphat 160c followed by differential centrifugation and sucrose density gradient ultracentrifugation. The purified complex contained a Mr 47,000 polypeptide(s) that bound both the primary donor (P800) and approximately 24 antenna bacteriochlorophylls g. Time-resolved fluorescence emission spectroscopy indicated that the antenna bacteriochlorophylls g are active in energy transfer to P800, exhibiting a decay time of 25 ps. The complex contained 1.4 menaquinones, 9 Fe, and 3 labile S2- per P800. The complex was photoactive with an exponential decay time of 14 ms for P800+ yet showed no EPR-detectable Fe-S center signal in the g less than or equal to 2.0 region, either by chemical reduction to -600 mV or by illumination of reduced samples. The complex is similar to photosystem I of oxygen-evolving photosynthetic systems in that both the primary donor and a core antenna are bound to the same pigment-protein complex.

Primary photosynthetic processes in Heliobacterium chlorum at 15K

Absorbance changes induced by 25-ps laser flashes were measured in membranes of Heliobacterium chlorum at 15 K. Absorbance difference spectra, measured at various times after the flash showed negative bands in the Qy region at 812, 793 and 665 nm. The first of these bands was attributed to the formation of excited singlet states of a long-wavelength form of antenna bacteriochlorophyll g (BChl g 808). Absorbance changes of shorter wavelength absorbing antenna BChls g were at least an order of magnitude smaller, indicating rapid excitation energy transfer (i.e. within the time resolution of the apparatus) from these BChls to BChl g 808. Excited BChl g 808 showed a bi-exponential decay with time constants of 50 and 200 ps. The bands at 793 and 665 nm may be attributed to the primary charge separation and reflect the photooxidation of the primary electron donor P-798 and photoreduction of a primary electron acceptor absorbing near 670 nm, presumably a BChl c or Chl a-like pigment. The bleaching of this pigment reversed with a time constant of 300 ps at 15 K and of 800 ps at 300 K. This indicates that electron transfer from the primary to the secondary electron acceptor is approximately 2.5 times faster at 15 K than at room temperature.