Isolation and characterization of cytoplasmic membranes and chlorosomes from the green bacterium Chloroflexus aurantiacus

Light and oxygen regulation of the synthesis of bacteriochlorophylls a and c in Chloroflexus aurantiacus

Control of the synthesis of bacteriochlorophylls (Bchls) a and c by light and oxygen was studied in Chloroflexus aurantiacus grown in batch or chemostat culture with serine as the growth-limiting substrate. For comparison, inhibition by gabaculine of the formation of selected tetrapyrroles was studied. The inhibitory effect of gabaculine decreased in the following order of tetrapyrrole formation: coproporphyrin greater than Bchl c greater than Bchl a. Not only did addition of 5-aminolevulinate (ALA) reverse the inhibition by gabaculine, it also caused an increase in Bchl c content when the cultures grew at high concentrations of ALA. Inhibition of Bchl a, Bchl c, and coproporphyrin formation by oxygen was similar to inhibition by gabaculine. Addition of ALA to aerated cultures led to significant accumulation of coproporphyrin. These results suggest that oxygen inhibits tetrapyrrole formation at a site before ALA formation. Control by light was studied with chemostat cultures transferred from 5 klx to 25 klx. This resulted in only a transient increase of the protein level of the culture, while specific contents of Bchls c and a and the ratio Bchl c/Bchl a decreased to lower steady states. However, the specific content of coproporphyrin increased. Addition of ALA to chemostat cultures adapted to 50 klx increased specific coproporphyrin and Bchl c contents by factors of about 20 and 4, respectively, while the specific Bchl a content was only slightly increased and protein levels were unaffected. Increasing the serine concentration caused an initial increase in the specific Bchl c content, which returned to the original value as soon as the protein content had attained its maximal level. These results suggest that light does not control ALA formation as strictly as oxygen and that competition of biomass formation and tetrapyrrole synthesis for common precursors may be influenced by light.

The functional role of protein in the organization of bacteriochlorophyll c in chlorosomes of Chloroflexus aurantiacus

The preparation of five different fractions containing bacteriochlorophyll (Bchl) c and their absorption and circular dichroic properties have been described. The fractions investigated were purified chlorosomes, proteolytically modified chlorosomes, chlorosomes treated with lithium dodecyl sulfate (LDS) which were subsequently subjected to size-exclusion chromatography, in vitro Bchl c aggregates and, additionally, the so-called GEF chlorosomes [prepared according to Griebenow and Holzwarth (1989) Biochim. Biophys. Acta 973, 235-240]. Proteolysis of chlorosomes caused a 35-40% decrease in absorption intensity, a 6-8 nm blue shift of the 740-nm peak and, in particular, a drastic increase of rotational strength as revealed by CD spectroscopy. Although oligomeric Bchl c aggregates and LDS-treated chlorosomes had absorption characteristics similar to Bchl c in vivo, the data clearly indicated that protein, perhaps the chlorosome-specific Mr-3700 polypeptide, was involved in the organization of Bchl c in chlorosomes from C. aurantiacus. Furthermore, the results showed that the LDS-treated chlorosome fraction was most likely comprised of a micellar complex of Bchl c with LDS which represented an entity entirely different from chlorosomes.

Isolation and protein chemical characterization of the B806-866 antenna complex of the green thermophilic bacterium Chloroflexus aurantiacus

The B806-866 antenna complex was isolated from cytoplasmic membranes of the green thermophilic bacterium Chloroflexus aurantiacus. The membranes were treated with 7 M urea at 50 degrees C, the B806-866 antenna complex was solubilized with a mixture of Noni-fjdet P-40 (octylphenoxypolyethoxyethanol (Sigma)) and sodium dodecylsulphate (2:1) and isolated by sucrose density gradient centrifugation. This antenna complex was characterized by reversed-phase chromatography (fast polypeptide and polynucleotide liquid chromatography), amino acid and sequence analyses. The B806-866 antenna of Chloroflexus aurantiacus consists of two polypeptides: the B806-866-alpha and B806-866-beta polypeptides in an apparent stoichiometric ratio of 1:1, which may be equivalent to the structural elementary unit found in the antenna systems of many species of Rhodospirillaceae.

Gene encoding the 5.7-kilodalton chlorosome protein of Chloroflexus aurantiacus: regulated message levels and a predicted carboxy-terminal protein extension

The major light-harvesting pigment of the green filamentous bacterium Chloroflexus aurantiacus is bacteriochlorophyll (Bchl) c, localized in chlorosomes attached to the inner surface of the cytoplasmic membrane. Chlorosomes consist of four polypeptides and associated pigments and lipids. Previous studies of the inducible assembly of the photosynthetic apparatus had indicated that the major chlorosomal polypeptides are present as high-molecular-weight aggregates before the appearance of mature chlorosomes, and a mechanism for posttranslational processing of a polyprotein had been proposed. We have isolated the gene (csmA) encoding the 5.7-kilodalton chlorosomal polypeptide from C. aurantiacus in order to determine whether this protein is synthesized as part of a polyprotein. Analysis of the nucleotide sequence of csmA indicates that the gene is not large enough to encode more than one known chlorosome polypeptide. Transcriptional analysis indicates that csmA is transcribed as a small message whose abundance is regulated in response to oxygen, so that no csmA message is detectable in cells grown aerobically in the dark. Comparison of the sequence predicted by csmA with the peptide sequence of the Bchl c binding protein purified from chlorosomes indicates that this protein is synthesized with a carboxy-terminal extension of 27 amino acids. We discuss possible roles for this carboxy-terminal extension in the assembly of chlorosomes.

Selective solubilization of chlorosome proteins in Chloroflexus aurantiacus

Proteins were solubilized selectively from chlorosomes of Chloroflexus aurantiacus by electrophoretic gel filtration according to Griebenow et al. Whereas the 11 kDa and 18 kDa proteins were extracted almost completely, the remaining modified chlorosomes contained high amounts of pigment and c-protein. It was concluded that the c-protein in contradiction to the publication by Griebenow et al. is indeed localized in the interior of Chloroflexus chlorosomes.

Complementation of a reaction center-deficient Rhodobacter sphaeroides pufLMX deletion strain in trans with pufBALM does not restore the photosynthesis-positive phenotype

The puf operon in Rhodobacter sphaeroides is composed of the genes for the photosynthetic reaction center L and M subunits, light-harvesting antenna complex I, and one other open reading frame termed pufX. Complementation of a reaction center-deficient, photosynthetically incompetent pufLMX deletion strain in trans with a fragment containing the entire puf operon, including pufX and an additional 1,100 base pairs of DNA downstream of pufX, restored the reaction center and the photosynthesis-positive phenotype. Complementation of the same strain with pufBALM restores the reaction center to the level seen with the entire puf operon but not the photosynthesis-positive phenotype. Northern (RNA) blot analysis revealed that oxygen regulated transcription was not blocked in the absence of pufX and the downstream region. Spectroscopic and protein analyses indicated that the pigment-binding protein complexes, including the reaction center, were expressed and showed normal absorption characteristics. A 20% reduction in the amount of light-harvesting antenna complex II and a corresponding increase in the amount of light-harvesting antenna complex I were observed in the deletion strain harboring the plasmid with the puf insert lacking the pufX gene and the downstream region compared with those complemented with the entire puf operon and an additional downstream 1,100 base pairs.

In vitro kinetics of reduction of cytochrome c554 isolated from the reaction center of the green phototrophic bacterium, Chloroflexus aurantiacus

The photochemical reaction center in the green bacterium Chloroflexus aurantiacus is similar to that found in purple phototrophic bacteria and interacts with a multiheme membrane-bound cytochrome. We have examined the kinetics of reduction of the pure solubilized reaction center cytochrome by laser flash photolysis of solutions containing lumiflavin or FMN. Reduction by lumiflavin semiquinone followed single exponential kinetics and the observed rate constant (kobs) was linearly dependent on protein concentration (k = 1.8 X 10(7) M-1s-1 heme-1). This result suggests either that the four hemes have similar reduction rate constants which cannot be resolved or that there are large differences in rate constant and only the most reactive heme (or hemes) was observed under these conditions. To determine the relative reactivities of the four hemes, we varied the extent of heme reduction at a single total protein concentration. As the hemes were progressively reduced by steady-state illumination prior to laser flash photolysis, kobs for the reaction with fully reduced lumiflavin decreased nonlinearly. Second-order rate constants for the four hemes were assigned by nonlinear least-squares analysis of kobs vs oxidized heme concentration data. The second-order rate constants obtained in this way for the highest and lowest potential hemes differed by a factor of about 20, which is larger than expected for c-type cytochromes based on redox potential alone (a factor of about 3 would be expected). This is interpreted as being due to differences in steric accessibility. Relative to the highest potential heme, which is as reactive as a typical c-type cytochrome, we estimated a steric effect of approximately twofold for heme 2, and steric effects of approximately fivefold for hemes 3 and 4. Using fully reduced FMN as reductant of oxidized cytochrome, ionic strength effects indicate a minus-minus interaction, with approximately a -2 charge near the site of reduction of the highest potential heme.

The primary structure of the Chloroflexus aurantiacus reaction-center polypeptides

The complete nucleotide sequence of two Chloroflexus aurantiacus reaction-center genes has been obtained. The amino acid sequence deduced from the first gene showed 40% similarity to the L subunit of the Rhodobacter sphaeroides reaction center. This L subunit was 310 amino acids long and had an approximate molecular mass of 35 kDa. The second gene began 17 bases downstream from the first gene. The amino acid sequence deduced from it (307 amino acids; 34950 Da) was 42% similar to the M subunit of the Rhodobacter sphaeroides reaction center. 20% of the deduced primary structure were confirmed through automated Edman degradation of cyanogen bromide peptide fragments or N-chlorosuccinimide peptide fragments isolated from the purified reaction-center complex or from the individual subunits. The peptides were isolated by preparative gel electrophoresis combined with molecular sieve chromatography in the presence of a mixture of formic acid, acetonitrile, 2-propanol and water. This method appeared to be applicable to the isolation of other hydrophobic proteins and their peptides.

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.

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

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

The photochemical reaction center of Chloroflexus aurantiacus is composed of two structurally similar polypeptides

A method has been devised which allowed the isolation of highly purified reaction center from the thermophilic green bacterium, Chloroflexus aurantiacus. The procedure consisted of three chromatography steps. The final step was fast protein liquid chromatography on Mono Q in the presence of nonanoyl-N-methylglucamide (Mega-9). The purified reaction center complex was photochemically active and had an A280/A813 of 1.4 or less. Under non-denaturing conditions, a pigmented protein band having a Mr of 52,000-55,000 was observed in sodium dodecyl sulfate gels. When the isolated complex was heat-dissociated in the presence of sodium dodecyl sulfate, just two polypeptides having very similar Mr (24,000 and 24,500) were observed. Two protein bands were also observed in two-dimensional isoelectric focusing/sodium-dodecyl-sulfate polyacrylamide gel electrophoresis; the PI values of the two polypeptides were 6.5 and 6.7. Partial peptide mapping of the two isolated subunits, using both enzymatic and chemical cleavage techniques, yielded almost identical patterns which indicated a high degree of sequence homology between the two polypeptides. The N-terminal amino acid sequences of the two polypeptides were identical and did not exhibit any homology to reaction center subunits of purple sulfur bacteria. The Chloroflexus reaction center is believed to be composed of one molecule of each polypeptide, the photoactive bacteriochlorophyll a dimer and, as accessory pigments, an additional bacteriochlorophyll a and three bacteriopheophytins. Hence, it appears to be the smallest photochemically active reaction center isolated to date.

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.

A new bacteriochlorophyll a-protein complex associated with chlorosomes of green sulfur bacteria

Chlorosomes were prepared from Chlorobium limicola f. thiosulfatophilum by sucrose density gradient centrifugation. Cells broken in the presence of 2 M NaSCN yielded three chlorosome fractions in the gradient: low density (no sucrose), medium density (approx. 18% sucrose), and high density (approx. 26% sucrose). All fractions were stable at any chlorosome concentration. Cells broken in the absence of 2 M NaSCN also yielded three fractions, but only the high-density fraction contained stable chlorosomes. The medium-density chlorosomes were stable only when highly concentrated. Upon dilution, bacteriochlorophyll (BChl) c was degraded to bacteriopheophytin c and concomitantly a band at 794 nm (BChl a) was revealed. Two 794-nm fractions were observed with the same densities as low- and medium-density chlorosomes. The protein composition of the 794-nm fractions was similar to that of the stable chlorosome fractions. All showed a 4-5 kDa (Mr) protein as a major component, but no trace of the 40-kDa protein characteristic of the water-soluble BChl a-protein of green sulfur bacteria. BChl a was present in all types of chlorosomes, in stable chlorosomes the BChl c/BChl a ratio was approx. 90. A special BChl a-protein (794 nm) inside the chlorosome is postulated to mediate energy transfer from BChl c to the water-soluble BChl a-protein in the baseplate.

Pigment organization and energy transfer in the green photosynthetic bacterium Chloroflexus aurantiacus : I. The cytoplasmic membrane

We have studied the pigment arrangement in purified cytoplasmic membranes of the thermophilic green bacterium Chloroflexus aurantiacus. The membranes contain 30-35 antenna bacteriochlorophyll a molecules per reaction center; these are organized in the B808-866 light-harvesting complex, together with carotenoids in a 2:1 molar ratio. Measurements of linear dichroism in a pressed polyacrylamide gel permitted the accurate determination of the orientation of the optical transition dipole moments with respect to the membrane plane. Combination of linear dichroism and low temperature fluorescence polarization data shows that the Qy transitions of the BChl 866 molecules all lie almost perfectly parallel to the membrane plane, but have no preferred orientation within the plane. The BChl 808 Qy transitions make an average angle of about 44° with this plane. This demonstrates that there are clear structural differences between the B808-866 complex of C. aurantiacus and the B800-850 complex of purple bacteria. Excitation energy transfer from carotenoid to BChl a proceeds with about 40% efficiency, while the efficiency of energy transfer from BChl 808 to BChl 866 approaches 100%. From the minimal energy transfer rate between the two spectral forms of BChl a, obtained by analysis of low temperature fluorescence emission spectra, a maximal distance between BChl 808 and BChl 866 of 23 Å was derived.

Pigment organization and energy transfer in the green photosynthetic bacterium Chloroflexus aurantiacus : II. The chlorosome

The transfer of excitation energy and the pigment arrangement in isolated chlorosomes of the thermophilic green bacterium Chloroflexus aurantiacus were studied by means of absorption, fluorescence and linear dichroism spectroscopy, both at room temperature and at 4 K. The low temperature absorption spectrum shows bands of the main antenna pigments BChl c and carotenoid, in addition to which bands of BChl a are present at 798 and 613 nm. Fluorescence measurements showed that excitation energy from BChl c and carotenoid is transferred to BChl a, which presumably functions as an intermediate in energy transfer from the chlorosome to the cytoplasmic membrane. Measurements of fluorescence polarization and the use of two different orientation techniques for linear dichroism experiments enabled us to determine the orientation of several transition dipole moments with respect to each other and to the three principal axes of the chlorosome. The Qy transition of BChl a is oriented almost perfectly perpendicular to the long axis of the chlorosome. The Qy transition of BChl c and the γ-carotene transition dipole are almost parallel to each other. They make an angle of about 40° with the long axis and of about 70° with the short axis of the chlorosome; the angle between these transitions and the BChl a Qy transition is close to the magic angle (55°).

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.

Primary photochemistry in the facultative green photosynthetic bacterium Chloroflexus aurantiacus

The mechanism of primary photochemistry has been investigated in purified cytoplasmic membranes and isolated reaction centers of Chloroflexus aurantiacus. Redox titrations on the cytoplasmic membranes indicate that the midpoint redox potential of P870, the primary electron donor bacteriochlorophyll, is +362 mV. An early electron acceptor, presumably menaquinone has Em 8.1 = -50 mV, and a tightly bound photooxidizable cytochrome c554 has Em 8.1 = +245 mV. The isolated reaction center has a bacteriochlorophyll to bacteriopheophytin ratio of 0.94:1. A two-quinone acceptor system is present, and is inhibited by o-phenanthroline. Picosecond transient absorption and kinetic measurements indicate the bacteriopheophytin and bacteriochlorophyll form an earlier electron acceptor complex.