Thermophilic Purple Sulfur Bacterium Research Articles

Calcium and the ecology of photosynthesis in purple sulfur bacteria.

The ecological success of purple sulfur bacteria (PSB) is linked to their ability to collect near-infrared solar energy by membrane-integrated, pigment-protein photocomplexes. These include a Core complex containing both light-harvesting 1 (LH1) and reaction centre (RC) components (called the LH1-RC photocomplex) present in all PSB and a peripheral light-harvesting complex present in most but not all PSB. In research to explain the unusual absorption properties of the thermophilic purple sulfur bacterium Thermochromatium tepidum, Ca2+ was discovered bound to LH1 polypeptides in its LH1-RC; further work showed that calcium controls both the thermostability and unusual spectrum of the Core complex. Since then, Ca2+ has been found in the LH1-RC photocomplexes of several other PSB, including mesophilic species, but not in the LH1-RC of purple non-sulfur bacteria. Here we focus on four species of PSB-two thermophilic and two mesophilic-and describe how Ca2+ is integrated into and affects their photosynthetic machinery and why this previously overlooked divalent metal is a key nutrient for their ecological success.

Identification of metal-sensitive structural changes in the Ca2+-binding photocomplex from Thermochromatium tepidum by isotope-edited vibrational spectroscopy.

Calcium ions play a dual role in expanding the spectral diversity and structural stability of photocomplexes from several Ca2+-requiring purple sulfur phototrophic bacteria. Here, metal-sensitive structural changes in the isotopically labeled light-harvesting 1 reaction center (LH1-RC) complexes from the thermophilic purple sulfur bacterium Thermochromatium (Tch.) tepidum were investigated by perfusion-induced attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy. The ATR-FTIR difference spectra induced by exchanges between native Ca2+ and exogenous Ba2+ exhibited interconvertible structural and/or conformational changes in the metal binding sites at the LH1 C-terminal region. Most of the characteristic Ba2+/Ca2+ difference bands were detected even when only Ca ions were removed from the LH1-RC complexes, strongly indicating the pivotal roles of Ca2+ in maintaining the LH1-RC structure of Tch. tepidum. Upon 15N-, 13C- or 2H-labeling, the LH1-RC complexes exhibited characteristic 15N/14N-, 13C/12C-, or 2H/1H-isotopic shifts for the Ba2+/Ca2+ difference bands. Some of the 15N/14N or 13C/12C bands were also sensitive to further 2H-labelings. Given the band frequencies and their isotopic shifts along with the structural information of the Tch. tepidum LH1-RC complexes, metal-sensitive FTIR bands were tentatively identified to the vibrational modes of the polypeptide main chains and side chains comprising the metal binding sites. Furthermore, important new IR marker bands highly sensitive to the LH1 BChl a conformation in the Ca2+-bound states were revealed based on both ATR-FTIR and near-infrared Raman analyses. The present approach provides valuable insights concerning the dynamic equilibrium between the Ca2+- and Ba2+-bound states statically resolved by x-ray crystallography.

Complete genome of the thermophilic purple sulfur Bacterium Thermochromatium tepidum compared to Allochromatium vinosum and other Chromatiaceae.

The complete genome sequence of the thermophilic purple sulfur bacterium Thermochromatium tepidum strain MCT (DSM 3771T) is described and contrasted with that of its mesophilic relative Allochromatium vinosum strain D (DSM 180T) and other Chromatiaceae. The Tch. tepidum genome is a single circular chromosome of 2,958,290 base pairs with no plasmids and is substantially smaller than the genome of Alc. vinosum. The Tch. tepidum genome encodes two forms of RuBisCO and contains nifHDK and several other genes encoding a molybdenum nitrogenase but lacks a gene encoding a protein that assembles the Fe-S cluster required to form a functional nitrogenase molybdenum-iron cofactor, leaving the phototroph phenotypically Nif-. Tch. tepidum contains genes necessary for oxidizing sulfide to sulfate as photosynthetic electron donor but is genetically unequipped to either oxidize thiosulfate as an electron donor or carry out assimilative sulfate reduction, both of which are physiological hallmarks of Alc. vinosum. Also unlike Alc. vinosum, Tch. tepidum is obligately phototrophic and unable to grow chemotrophically in darkness by respiration. Several genes present in the Alc. vinosum genome that are absent from the genome of Tch. tepidum likely contribute to the major physiological differences observed between these related purple sulfur bacteria that inhabit distinct ecological niches.

Photosynthetic Growth and Energy Conversion in an Engineered Phototroph Containing Thermochromatium tepidum Light-Harvesting Complex 1 and the Rhodobacter sphaeroides Reaction Center Complex.

Light-harvesting complex 1 (LH1) of the thermophilic purple sulfur bacterium Thermochromatium tepidum can be expressed in the purple non-sulfur bacterium Rhodobacter sphaeroides and forms a functional RC-LH1 complex with the native Rba. sphaeroides reaction center (Nagashima, K. V. P., et al. Proc. Natl. Acad. Sci. U. S. A. 2017, 114, 10906-10911). Although there is a large uphill energy gap between Tch. tepidum LH1 and the Rba. sphaeroides RC in this chimeric complex, it has been shown that light energy can be transferred, consistent with that seen in the native Rba. sphaeroides RC-LH1 complex. In this study, the contribution of this chimeric complex to growth and photosynthetic energy conversion in the hybrid organism was quantified. The mutant synthesizing this chimeric complex was grown phototrophically under 940 nm light-emitting diode (LED) light preferentially absorbed by Tch. tepidum LH1 and showed faster growth at low intensities of this wavelength than both a mutant strain of Rba. sphaeroides lacking LH2 and a mutant lacking all light-harvesting complexes. When grown with 850 nm LED light, the strain containing the native Rba. sphaeroides LH1-RC grew faster than the chimeric strain. Electron transfer from the RC to the membrane-integrated cytochrome bc1 complex was also estimated by flash-induced absorption changes in heme b. The rate of ubiquinone transport through the LH1 ring structure in the chimeric strain was virtually the same as that in native Rba. sphaeroides. We conclude that Tch. tepidum LH1 can perform the physiological functions of native LH1 in Rba. sphaeroides.

Properties and structure of a low-potential, penta-heme cytochrome c552 from a thermophilic purple sulfur photosynthetic bacterium Thermochromatium tepidum.

The thermophilic purple sulfur bacterium Thermochromatium tepidum possesses four main water-soluble redox proteins involved in the electron transfer behavior. Crystal structures have been reported for three of them: a high potential iron-sulfur protein, cytochrome c', and one of two low-potential cytochrome c552 (which is a flavocytochrome c) have been determined. In this study, we purified another low-potential cytochrome c552 (LPC), determined its N-terminal amino acid sequence and the whole gene sequence, characterized it with absorption and electron paramagnetic spectroscopy, and solved its high-resolution crystal structure. This novel cytochrome was found to contain five c-type hemes. The overall fold of LPC consists of two distinct domains, one is the five heme-containing domain and the other one is an Ig-like domain. This provides a representative example for the structures of multiheme cytochromes containing an odd number of hemes, although the structures of multiheme cytochromes with an even number of hemes are frequently seen in the PDB database. Comparison of the sequence and structure of LPC with other proteins in the databases revealed several characteristic features which may be important for its functioning. Based on the results obtained, we discuss the possible intracellular function of this LPC in Tch. tepidum.

C-terminal cleavage of the LH1 α-polypeptide in the Sr2+-cultured Thermochromatium tepidum.

The light-harvesting 1 reaction center (LH1-RC) complex in the thermophilic purple sulfur bacterium Thermochromatium (Tch.) tepidum binds Ca ions as cofactors, and Ca-binding is largely involved in its characteristic Q y absorption at 915nm and enhanced thermostability. Ca2+ can be biosynthetically replaced by Sr2+ in growing cultures of Tch. tepidum. However, the resulting Sr2+-substituted LH1-RC complexes in such cells do not display the absorption maximum and thermostability of those from Ca2+-grown cells, signaling that inherent structural differences exist in the LH1 complexes between the Ca2+- and Sr2+-cultured cells. In this study, we examined the effects of the biosynthetic Sr2+-substitution and limited proteolysis on the spectral properties and thermostability of the Tch. tepidum LH1-RC complex. Preferential truncation of two consecutive, positively charged Lys residues at the C-terminus of the LH1 α-polypeptide was observed for the Sr2+-cultured cells. A proportion of the truncated LH1 α-polypeptide increased during repeated subculturing in the Sr2+-substituted medium. This result suggests that the truncation is a biochemical adaptation to reduce the electrostatic interactions and/or steric repulsion at the C-terminus when Sr2+ substitutes for Ca2+ in the LH1 complex. Limited proteolysis of the native Ca2+-LH1 complex with lysyl protease revealed selective truncations at the Lys residues in both C- and N-terminal extensions of the α- and β-polypeptides. The spectral properties and thermostability of the partially digested native LH1-RC complexes were similar to those of the biosynthetically Sr2+-substituted LH1-RC complexes in their Ca2+-bound forms. Based on these findings, we propose that the C-terminal domain of the LH1 α-polypeptide plays important roles in retaining proper structure and function of the LH1-RC complex in Tch. tepidum.

Effects of Calcium Ions on the Thermostability and Spectroscopic Properties of the LH1-RC Complex from a New Thermophilic Purple Bacterium Allochromatium tepidum.

The light harvesting-reaction center (LH1-RC) complex from a new thermophilic purple sulfur bacterium Allochromatium (Alc.) tepidum was isolated and characterized by spectroscopic and thermodynamic analyses. The purified Alc. tepidum LH1-RC complex showed a high thermostability comparable to that of another thermophilic purple sulfur bacterium Thermochromatium tepidum, and spectroscopic characteristics similar to those of a mesophilic bacterium Alc. vinosum. Approximately 4-5 Ca2+ per LH1-RC were detected by inductively coupled plasma atomic emission spectroscopy and isothermal titration calorimetry. Upon removal of Ca2+, the denaturing temperature of the Alc. tepidum LH1-RC complex dropped accompanied by a blue-shift of the LH1 Qy absorption band. The effect of Ca2+ was also observed in the resonance Raman shift of the C3-acetyl νC═O band of bacteriochlorophyll-a, indicating changes in the hydrogen-bonding interactions between the pigment and LH1 polypeptides. Thermodynamic parameters for the Ca2+-binding to the Alc. tepidum LH1-RC complex indicated that this reaction is predominantly driven by the largely favorable electrostatic interactions that counteract the unfavorable negative entropy change. Our data support a hypothesis that Alc. tepidum may be a transitional organism between mesophilic and thermophilic purple bacteria and that Ca2+ is one of the major keys to the thermostability of LH1-RC complexes in purple bacteria.

Characterization of the quinones in purple sulfur bacterium Thermochromatium tepidum

Quinone distributions in the thermophilic purple sulfur bacterium Thermochromatium tepidum have been investigated at different levels of the photosynthetic apparatus. Here we show that, on average, the intracytoplasmic membrane contains 18 ubiquinones (UQ) and 4 menaquinones (MQ) per reaction center (RC). About one-third of the quinones are retained in the light-harvesting–reaction center core complex (LH1–RC) with a similar ratio of UQ to MQ. The numbers of quinones essentially remains unchanged during crystallization of the LH1–RC. There are 1–2 UQ and 1 MQ associated with the RC-only complex in the purified solution sample. Our results suggest that a large proportion of the quinones are confined to the core complex and at least five UQs remain invisible in the current LH1–RC crystal structure.

The roles of C-terminal residues on the thermal stability and local heme environment of cytochrome c' from the thermophilic purple sulfur bacterium Thermochromatium tepidum.

A soluble cytochrome (Cyt) c' from thermophilic purple sulfur photosynthetic bacterium Thermochromatium (Tch.) tepidum exhibits marked thermal tolerance compared with that from the closely related mesophilic counterpart Allochromatium vinosum. Here, we focused on the difference in the C-terminal region of the two Cyts c' and examined the effects of D131 and R129 mutations on the thermal stability and local heme environment of Cyt c' by differential scanning calorimetry (DSC) and resonance Raman (RR) spectroscopy. In the oxidized forms, D131K and D131G mutants exhibited denaturing temperatures significantly lower than that of the recombinant control Cyt c'. In contrast, R129K and R129A mutants denatured at nearly identical temperatures with the control Cyt c', indicating that the C-terminal D131 is an important residue maintaining the enhanced thermal stability of Tch. tepidum Cyt c'. The control Cyt c' and all of the mutants increased their thermal stability upon the reduction. Interestingly, D131K exhibited narrow DSC curves and unusual thermodynamic parameters in both redox states. The RR spectra of the control Cyt c' exhibited characteristic bands at 1,635 and 1,625cm(-1), ascribed to intermediate spin (IS) and high spin (HS) states, respectively. The IS/HS distribution was differently affected by the D131 and R129 mutations and pH changes. Furthermore, R129 mutants suggested the lowering of their redox potentials. These results strongly indicate that the D131 and R129 residues play significant roles in maintaining the thermal stability and modulating the local heme environment of Tch. tepidum Cyt c'.

All-atom structures and calcium binding sites of the bacterial photosynthetic LH1-RC core complex from Thermochromatium tepidum.

Computationally derived structures of the photosynthetic core complex composed of the light-harvesting (LH) system LH1 and the reaction center (RC) from a thermophilic purple sulfur bacterium Thermochromatium tepidum are reported providing first models of the LH1 system at atomic resolution. We used the known primary structure of α and β polypeptides from this particular LH1 complex and the related bacterial LH templates to design the LH1 torus composed of 16 αβ subunits trapping bacteriochlorophyll (BChl-a) dimers and carotenoid molecules. The macromolecule of RC was placed in the center of the ring and the LH1-RC complex was inserted inside the lipid bilayer to simulate the membrane environment. Since thermal stability of the LH1-RC complex is linked to Ca(2+) binding by the complex, location of trapping sites of calcium ions in the LH1 polypeptides is examined by using molecular dynamics simulations of the entire system solvated in water with CaCl2 molecules in the system. The newly predicted Ca(2+) trapping sites can be responsible for attractive interaction of neighboring αβ subunits of LH1 with relevance to stability of the calcium-bound LH1-RC complex.

Structure Analysis and Comparative Characterization of the Cytochrome c′ and Flavocytochrome c from Thermophilic Purple Photosynthetic Bacterium Thermochromatium tepidum

The thermodynamic and spectroscopic properties of two soluble electron transport proteins, cytochrome (Cyt) c' and flavocytochrome c, isolated from thermophilic purple sulfur bacterium Thermochromatium (Tch.) tepidum were examined and compared with those of the corresponding proteins from a closely related mesophilic bacterium Allochromatium (Alc.) vinosum. These proteins share sequence identities of 82% for the cytochromes c' and 86% for the flavocytochromes c. Crystal structures of the two proteins have been determined at high resolutions. Differential scanning calorimetry and denaturing experiments show that both proteins from Tch. tepidum are thermally and structurally much more stable than their mesophilic counterparts. The denaturation temperature of Tch. tepidum Cyt c' was 22 °C higher than that of Alc. vinosum Cyt c', and the midpoints of denaturation using guanidine hydrochloride were 2.0 and 1.2 M for the Tch. tepidum and Alc. vinosum flavocytochromes c, respectively. The enhanced stabilities can be interpreted on the basis of the structural and sequence information obtained in this study: increased number of hydrogen bonds formed between main chain nitrogen and oxygen atoms, more compact structures and reduced number of glycine residues. Many residues with large side chains in Alc. vinosum Cyt c' are substituted by alanines in Tch. tepidum Cyt c'. Both proteins from Tch. tepidum exhibit high structural similarities to their counterparts from Alc. vinosum, and the different residues between the corresponding proteins are mainly located on the surface and exposed to the solvent. Water molecules are found in the heme vicinity of Tch. tepidum Cyt c' and form hydrogen bonds with the heme ligand and C-terminal charged residues. Similar bound waters are also found in the vicinity of one heme group in the diheme subunit of Tch. tepidum flavocytochrome c. Electron density map of the Tch. tepidum flavocytochrome c clearly revealed the presence of disulfur atoms positioned between two cysteine residues at the active site near the FAD prosthetic group. The result strongly suggests that flavocytochrome c is involved in the sulfide oxidation in vivo. Detailed discussion is given on the relationships between the crystal structures and the spectroscopic properties observed for these proteins.

3PT233 好熱性紅色硫黄細菌Thermochromatium tepidum由来cytochrome c'における耐熱化メカニズムの検討(日本生物物理学会第50回年会(2012年度))

3PT233 好熱性紅色硫黄細菌Thermochromatium tepidum由来cytochrome c'における耐熱化メカニズムの検討(日本生物物理学会第50回年会(2012年度))

Effects of Aggregation on the Excitation Dynamics of LH2 from Thermochromatium tepidum in Aqueous Phase and in Chromatophores

We carried out femtosecond magic-angle and polarized pump-probe spectroscopies for the light-harvesting complex 2 (LH2) from Thermochromatium (Tch.) tepidum in aqueous phase and in chromatophores. To examine the effects of LH2 aggregation on the dynamics of excitation energy transfer, dominant monodispersed and aggregated LH2s were prepared by controlling the surfactant concentrations. The aqueous preparations solubilized with different concentrations of n-dodecyl-β-D-maltoside (DDM) show similar visible-to-near-infrared absorption spectra, but distinctively different aggregation states, as revealed by using dynamic light scattering. The B800 → B850 intra-LH2 energy transfer time was determined to be 1.3 ps for isolated LH2, which, upon aggregation in aqueous phase or clustering in chromatophores, shortened to 1.1 or 0.9 ps, respectively. The light-harvesting complex 1 (LH1) of this thermophilic purple sulfur bacterium contains bacteriochlorophyll a absorbing at 915 nm (B915), and the LH2(B850) → LH1(B915) intercomplex transfer time in chromatophores was found to be 6.6 ps. For chromatophores, a depolarization time of 21 ps was derived from the anisotropy kinetics of B850*, which is attributed to the migration of B850* excitation before being trapped by LH1. In addition, the B850* annihilation is accelerated upon LH2 aggregation in aqueous phase, but it is much less severe upon LH2 clustering in the intracytoplasmic membrane. These results are helpful in understanding the light-harvesting function of a bacterial photosynthetic membrane incorporating different types of antenna complexes.

Gene sequencing and characterization of the light-harvesting complex 2 from thermophilic purple sulfur bacterium Thermochromatium tepidum

In this study, gene sequences coding for the light-harvesting (LH) 2 polypeptides from a thermophilic purple sulfur bacterium Thermochromatium tepidum are reported and characterization of the LH2 complex is described. Three sets of pucBA genes have been identified, and the gene products have been analyzed by electrophoresis and reversed-phase chromatography. The result shows that all of the genes are expressed but the distribution of the expression is not uniform. The gene products undergo post-translational modification, where two of the β-polypeptides appear to be N-terminally methylated. Absorption spectrum of the purified LH2 complex exhibits Q (y) transitions at 800 and 854nm in dodecyl β-maltopyranoside solution, and the circular dichroism spectrum shows a "molischianum"-like characteristic. No spectral change was observed for the LH2 when the bacterium was cultured under different conditions of light intensity. In lauryl dimethylamine N-oxide (LDAO) solution, significant changes in the absorption spectrum were observed. The B850 peak decreased and blue-shifted with increasing the LDAO concentration, whereas the B800 intensity increased without change in the peak position. The spectral changes can be partially or almost completely reversed by addition of metal ions, and the divalent cations seem to be more effective. The results indicate that ionic interactions may exist between LH2, detergent molecules and metal ions. Possible mechanisms involved in the detergent- and cation-induced spectral changes are discussed.

Examination of the putative Ca2+-binding site in the light-harvesting complex 1 of thermophilic purple sulfur bacterium Thermochromatium tepidum

The core light-harvesting complex (LH1) of purple sulfur photosynthetic bacterium Thermochromatium tepidum exhibits an unusual absorption maximum at 915nm for the Q (y) transition, and is highly stable when copurified with reaction center (RC) in a LH1-RC complex form. In previous studies, we demonstrated that the calcium ions are involved in both the large red shift and the enhanced thermal stability, and possible Ca(2+)-binding sites were proposed. In this study, we further examine the putative binding sites in the LH1 polypeptides using purified chromatophores. Incubation of the chromatophores in the presence of EDTA revealed no substantial change in the absorption maximum of LH1 Q (y) transition, whereas further addition of detergents to the chromatophores-EDTA solution resulted in a blue-shift for the LH1 Q (y) peak with the final position at 892nm. The change of the LH1 Q (y) peak to shorter wavelengths was relatively slow compared to that of the purified LH1-RC complex. The blue-shifted LH1 Q (y) transition in chromatophores can be restored to its original position by addition of Ca(2+) ions. The results suggest that the Ca(2+)-binding site is exposed on the inner surface of chromatophores, corresponding to the C-terminal region of LH1. An Asp-rich fragment in the LH1 α-polypeptide is considered to form a crucial part of the binding network. The slow response of LH1 Q (y) transition upon exposure to EDTA is discussed in terms of the membrane environment in the chromatophores.

2P288 生合成的にSr置換された好熱性紅色硫黄細菌Thermochromatium tepidumにおける色素蛋白質複合体の特性評価(光生物-光合成,第48回日本生物物理学会年会)

2P288 生合成的にSr置換された好熱性紅色硫黄細菌Thermochromatium tepidumにおける色素蛋白質複合体の特性評価(光生物-光合成,第48回日本生物物理学会年会)

Calcium Ions Are Involved in the Unusual Red Shift of the Light-harvesting 1 Qy Transition of the Core Complex in Thermophilic Purple Sulfur Bacterium Thermochromatium tepidum

Thermophilic purple sulfur bacterium, Thermochromatium tepidum, can grow at temperatures up to 58 degrees C and exhibits an unusual Qy absorption at 915 nm for the core light-harvesting complex (LH1), an approximately 35-nm red shift from those of its mesophilic counterparts. We demonstrate in this study, using a highly purified LH1-reaction center complex, that the LH1 Qy transition is strongly dependent on metal cations and Ca2+ is involved in the unusual red shift. Removal of the Ca2+ resulted in formation of a species with the LH1 Qy absorption at 880 nm, and addition of the Ca2+ to the 880-nm species recovered the native 915-nm form. Interchange between the two forms is fully reversible. Based on spectroscopic and isothermal titration calorimetry analyses, the Ca2+ binding to the LH1 complex was estimated to occur in a stoichiometric ratio of Ca2+/alphabeta-subunit = 1:1 and the binding constant was in 10(5) m(-1) order of magnitude, which is comparable with those for EF-hand Ca2+-binding proteins. Despite the high affinity, conformational changes in the LH1 complex upon Ca2+ binding were small and occurred slowly, with a typical time constant of approximately 6 min. Replacement of the Ca2+ with other metal cations caused blue shifts of the Qy bands depending on the property of the cations, indicating that the binding site is highly selective. Based on the amino acid sequences of the LH1 complex, possible Ca2+-binding sites are proposed that consist of several acidic amino acid residues near the membrane interfaces of the C-terminal region of the alpha-polypeptide and the N-terminal region of the beta-polypeptide.

Purification, characterization and crystallization of the core complex from thermophilic purple sulfur bacterium Thermochromatium tepidum

A light-harvesting-reaction center (LH1-RC) core complex has been highly purified from a thermophilic purple sulfur bacterium, Thermochromatium tepidum. The bacteriochlorophyll (BChl) a molecules in the LH1 exhibit a Qy transition at 914 nm, more than 25 nm red-shift from those of its mesophilic counterparts. The LH1-RC complex was isolated in a monomeric form as confirmed by sucrose density gradient centrifugation, blue native PAGE and size-exclusion chromatography. Four subunits (L, M, H and a tetraheme cytochrome) in RC and two polypeptides (α and β) in LH1 were identified. Spirilloxanthin was determined to be the predominant carotenoid in the core complex. The purified core complex was highly stable, no significant change in the LH1 Qy transition was observed over 10 days of incubation at room temperature in dark. Circular dichroism spectrum of the LH1 complex was characterized by low intensity and nonconservative spectral shape, implying a high symmetry of the large LH1 ring and interaction between the BChl a and carotenoid molecules. A dimeric feature of the BChl a molecules in LH1 was revealed by magnetic circular dichroism spectrum. Crystals of the core complex were obtained which diffracted X-rays to about 10 Å.

Structural basis of bacterial photosynthetic reaction centers.

The photosynthetic reaction center (RC) is the first membrane protein whose three-dimensional structure was revealed at the atomic level by X-ray crystallograph more than fifteen years ago. Structural information about RC made a great contribution to the understanding of the reaction mechanism of the complicated membrane protein complex. High-resolution structures of RCs from three photosynthetic bacteria are now available, namely, those from two mesophilic purple non-sulfur bacteria, Blastochloris viridis and Rhodobacter sphaeroides, and that from a thermophilic purple sulfur bacterium, Thermochromatium tepidum. In addition, a variety of structural studies, mainly by X-ray crystallography, are still being performed to give more detailed insight into the reaction mechanism of this membrane protein. This review deals with structural studies of bacterial RC complexes, and a discussion about the electron transfer reaction between RCs and electron donors is the main focus out of several topics addressed by these structural studies. The structural data from three RCs and their electron donors provided reliable models for molecular recognition in the primary step of bacterial photosynthesis.

Structure of the H subunit of the photosynthetic reaction center from the thermophilic purple sulfur bacterium, Thermochromatium tepidum Implications for the specific binding of the lipid molecule to the membrane protein complex.

The photosynthetic reaction center (RC) is a transmembrane protein complex that catalyzes light-driven electron transport across the photosynthetic membrane. The complete amino-acid sequence of the H subunit of the RC from a thermophilic purple sulfur bacterium, Thermochromatium tepidum, has been determined for the first time among purple sulfur bacteria. The H subunit consists of 259 amino acids and has a molecular mass of 28 187. The deduced amino-acid sequences of this H subunit showed a significant (40%) degree of identity with those from mesophilic purple nonsulfur bacteria. The determined primary structure of the H subunit was compared with the structures of mesophilic B. viridis and R. sphaeroides based on the three-dimensional structure of the H subunit from T. tepidum, which has been recently determined by X-ray crystallography. One lipid molecule was found in the crystal structure of the T. tepidum RC, and the head group of the lipid appears to be stabilized by the electrostatic interactions with the conserved basic residues in the H subunit. The above comparison has suggested the existence of a lipid-binding site on the molecular surface at which a lipid molecule can interact with the RC in a specific manner.