Examination of the putative Ca<Superscript>2+</Superscript>-binding site in the light-harvesting complex 1 of thermophilic purple sulfur bacterium <Emphasis Type="Italic">Thermochromatium</Emphasis><Emphasis Type="Italic">tepidum</Emphasis>

The core light-harvesting complex (LH1) of purple sulfur photosynthetic bacterium Thermochromatium tepidum exhibits an unusual absorption maximum at 915 nm 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 Ca²⁺-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 892 nm. 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 Ca²⁺ ions. The results suggest that the Ca²⁺-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.     

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 Q_y 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 Q_y 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.     

Femtosecond spectroscopy of native and carotenoidless purple-bacterial LH2 clarifies functions of carotenoids

EET between the two circular bacteriochlorophyll compartments B800 and B850 in native (containing the carotenoid rhodopin) and carotenoidless LH2 isolated from the photosynthetic purple sulfur bacterium Allochromatium minutissimum was investigated by femtosecond time-resolved transient absorption spectroscopy. Both samples were excited with 120-fs laser pulses at 800 nm, and spectral evolution was followed in the 720-955 nm range at different delay times. No dependence of transient absorption in the B800 band on the presence of the carotenoid rhodopin was found. Together with the likewise virtually unchanged absorption spectra in the bacteriochlorophyll Q_y region, these observations suggest that absence of rhodopin does not significantly alter the structure of the pigment-protein complex including interactions between bacteriochlorophylls. Apparently, rhodopin does also not accelerate B800 to B850 EET in LH2, contrary to what has been suggested previously. Moreover, “carotenoid-catalyzed internal conversion” can also be excluded for the bacteriochlorophylls in LH2 of A. minutissimum. Together with previous results obtained with two-photon fluorescence excitation spectroscopy, it can also be concluded that there is neither EET from rhodopin to B800 nor (back-)EET from B800 to rhodopin.     

Electrostatic charge controls the lowest LH1 Qy transition energy in the triply extremophilic purple phototrophic bacterium,Halorhodospira halochloris

Halorhodospira (Hlr.) halochloris is a unique phototrophic purple bacterium because it is a triple extremophile—the organism is thermophilic, alkalophilic, and halophilic. The most striking photosynthetic feature of Hlr. halochloris is that the bacteriochlorophyll (BChl) b-containing core light-harvesting (LH1) complex surrounding its reaction center (RC) exhibits its LH1 Q_y absorption maximum at 1016 nm, which is the lowest transition energy among phototrophic organisms. Here we report that this extraordinarily red-shifted LH1 Q_y band of Hlr. halochloris exhibits interconvertible spectral shifts depending on the electrostatic charge distribution around the BChl b molecules. The 1016 nm band of the Hlr. halochloris LH1-RC complex was blue-shifted to 958 nm upon desalting or pH decrease but returned to its original position when supplemented with salts or pH increase. Resonance Raman analysis demonstrated that these interconvertible spectral shifts are not associated with the strength of hydrogen-bonding interactions between BChl b and LH1 polypeptides. Furthermore, circular dichroism signals for the LH1 Q_y transition of Hlr. halochloris appeared with a positive sign (as in BChl b-containing Blastochloris species) and opposite those of BChl a-containing purple bacteria, possibly due to a combined effect of slight differences in the transition dipole moments between BChl a and BChl b and in the interactions between adjacent BChls in their assembled state. Based on these findings and LH1 amino acid sequences, it is proposed that Hlr. halochloris evolved its unique and tunable light-harvesting system with electrostatic charges in order to carry out photosynthesis and thrive in its punishing hypersaline and alkaline habitat.     

Antenna organization in the purple sulfur bacteria Chromatium tepidum and Chromatium vinosum

Structural aspects of the core antenna in the purple sulfur bacteria Chromatium tepidum and Chromatium vinosum were studied by means of fluorescence emission and singlet-singlet annihilation measurements. In both species the number of bacteriochlorophylls of the core antenna between which energy transfer can occur corresponds to one core-reaction center complex only. From measurements of variable fluorescence we conclude that in C. tepidum excitation energy can be transferred back from the core antenna (B920) to the peripheral B800–850 complex in spite of the relatively large energy gap, and on basis of annihilation measurements a model of separate core-reaction center units accompanied by their own peripheral antenna is suggested. C. vinosum contains besides a core antenna, B890, two peripheral antennae, B800–820 and B800–850. Energy transfer was found to occur from the core to B800–850, but not to B800–820, and it was concluded that in C. vinosum each core-reaction center complex has its own complement of B800–850. The results reported here are compared to those obtained earlier with various strains and species of purple non-sulfur bacteria.Abbreviations BChl- bacteriochlorophyll - B800–820 and B800–850- antenna complexes with Q_y-band absorption maxima near 800 nm and 820 or 850 nm, respectively - B890 and B920- antenna complexes with Q_y-band absorption maxima near 890 and 920 nm, respectively - LH1- light harvesting 1 or core antenna - LH2- light harvesting 2 or peripheral antenna     

Quinone transport in the closed light-harvesting 1 reaction center complex from the thermophilic purple bacterium Thermochromatium tepidum

Redox-active quinones play essential roles in efficient light energy conversion in type-II reaction centers of purple phototrophic bacteria. In the light-harvesting 1 reaction center (LH1-RC) complex of purple bacteria, Q_B is converted to Q_BH₂ upon light-induced reduction and Q_BH₂ is transported to the quinone pool in the membrane through the LH1 ring. In the purple bacterium Rhodobacter sphaeroides, the C-shaped LH1 ring contains a gap for quinone transport. In contrast, the thermophilic purple bacterium Thermochromatium (Tch.) tepidum has a closed O-shaped LH1 ring that lacks a gap, and hence the mechanism of photosynthetic quinone transport is unclear. Here we detected light-induced Fourier transform infrared (FTIR) signals responsible for changes of Q_B and its binding site that accompany photosynthetic quinone reduction in Tch. tepidum and characterized Q_B and Q_BH₂ marker bands based on their ¹⁵N- and ¹³C-isotopic shifts. Quinone exchanges were monitored using reconstituted photosynthetic membranes comprised of solubilized photosynthetic proteins, membrane lipids, and exogenous ubiquinone (UQ) molecules. In combination with ¹³C-labeling of the LH1-RC and replacement of native UQ₈ by ubiquinones of different tail lengths, we demonstrated that quinone exchanges occur efficiently within the hydrophobic environment of the lipid membrane and depend on the side chain length of UQ. These results strongly indicate that unlike the process in Rba. sphaeroides, quinone transport in Tch. tepidum occurs through the size-restricted hydrophobic channels in the closed LH1 ring and are consistent with structural studies that have revealed narrow hydrophobic channels in the Tch. tepidum LH1 transmembrane region.     

The rate of Qx→Qy relaxation in bacteriochlorophylls of reaction centers from Rhodobacter sphaeroides determined by kinetics of the ultrafast carotenoid bandshift

Transient absorption changes induced by excitation of isolated reaction centers (RCs) from Rhodobacter sphaeroides with 600 nm laser pulses of 20 fs (full width at half maximum) were monitored in the wavelength region of 420–560 nm. The spectral features of the spectrum obtained are characteristic for an electrochromic band shift of the single carotenoid (Car) molecule spheroidene, which is an integral constituent of these RCs. This effect is assigned to an electrochromic bandshift of Car due to the local electric field of the dipole moment formed by electronic excitation of bacteriochlorophyll (BChl) molecule(s) in the neighborhood of Car. Based on the known distances between the pigments, the monomeric BChl (B_B) in the inactive B-branch is inferred to dominate this effect. The excitation of B_B at 600 nm leads to a transition into the S₂ state (Q_x band), which is followed by rapid internal conversion to the S₁ state (Q_y band), thus leading to a change of strength and orientation of the dipole moment, i.e., of the electric field acting on the Car molecule. Therefore, the time course of the electrochromic bandshift reflects the rate of the internal conversion from S₂ to S₁ of B_B. The evaluation of the kinetics leads to a value of 30 fs for this relaxation process. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.     

Stabilization of charge separation and cardiolipin confinement in antenna-reaction center complexes purified from Rhodobacter sphaeroides

The reaction center-light harvesting complex 1 (RC-LH1) purified from the photosynthetic bacterium Rhodobacter sphaeroides has been studied with respect to the kinetics of charge recombination and to the phospholipid and ubiquinone (UQ) complements tightly associated with it. In the antenna-RC complexes, at 6.5 < pH < 9.0, P⁺Q_B⁻ recombines with a pH independent average rate constant <k> more than three times smaller than that measured in LH1-deprived RCs. At increasing pH values, for which <k> increases, the deceleration observed in RC-LH1 complexes is reduced, vanishing at pH > 11.0. In both systems kinetics are described by a continuous rate distribution, which broadens at pH > 9.5, revealing a strong kinetic heterogeneity, more pronounced in the RC-LH1 complex. In the presence of the antenna the Q_AQ_B⁻ state is stabilized by about 40 meV at 6.5 < pH < 9.0, while it is destabilized at pH > 11. The phospholipid/RC and UQ/RC ratios have been compared in chromatophore membranes, in RC-LH1 complexes and in the isolated peripheral antenna (LH2). The UQ concentration in the lipid phase of the RC-LH1 complexes is about one order of magnitude larger than the average concentration in chromatophores and in LH2 complexes. Following detergent washing RC-LH1 complexes retain 80-90 phospholipid and 10-15 ubiquinone molecules per monomer. The fractional composition of the lipid domain tightly bound to the RC-LH1 (determined by TLC and ³¹P-NMR) differs markedly from that of chromatophores and of the peripheral antenna. The content of cardiolipin, close to 10% weight in chromatophores and LH2 complexes, becomes dominant in the RC-LH1 complexes. We propose that the quinone and cardiolipin confinement observed in core complexes reflects the in vivo heterogeneous distributions of these components. Stabilization of the charge separated state in the RC-LH1 complexes is tentatively ascribed to local electrostatic perturbations due to cardiolipin.     

Picosecond fluorescence of intact and dissolved PSI-LHCI crystals

Over the past several years, many crystal structures of photosynthetic pigment-protein complexes have been determined, and these have been used extensively to model spectroscopic results obtained on the same proteins in solution. However, the crystal structure is not necessarily identical to the structure of the protein in solution. Here, we studied picosecond fluorescence of photosystem I light-harvesting complex I (PSI-LHCI), a multisubunit pigment-protein complex that catalyzes the first steps of photosynthesis. The ultrafast fluorescence of PSI-LHCI crystals is identical to that of dissolved crystals, but differs considerably from most kinetics presented in the literature. In contrast to most studies, the data presented here can be modeled quantitatively with only two compartments: PSI core and LHCI. This yields the rate of charge separation from an equilibrated core (22.5 ± 2.5 ps) and rates of excitation energy transfer from LHCI to core (k_LC) and vice versa (k_CL). The ratio between these rates, R = k_CL/k_LC, appears to be wavelength-dependent and scales with the ratio of the absorption spectra of LHCI and core, indicating the validity of a detailed balance relation between both compartments. k_LC depends slightly but nonsystematically on detection wavelength, averaging (9.4 ± 4.9 ps)⁻¹. R ranges from 0.5 (<690 nm) to ∼1.3 above 720 nm.     

Two-photon excitation spectrum of fluorescence of the light-harvesting complex B800–850 from Allochromatium minutissimum within 1200–1500 (600–750) nm spectral range is not carotenoid mediated

Two types of peripheral light-harvesting complexes LH2 (B800–850) from photosynthetic purple bacterium Allochromatium minutissimum were studied. First type containing carotenoids was prepared from wild type cells. The other one was obtained from carotenoid depleted cells grown with diphenylamine. We have shown that under laser femtosecond excitation within absorption 1200–1500 nm wavelength range the two-photon excitation of LH2 complexes takes place. This can be observed as fluorescence of bacteriochlorophyll (BChl) spectral form B850 (BChl molecules of circular aggregate with strong exciton interaction in 850 nm spectral domain). LH2 fluorescence excitation spectra under two-photon excitation are the same for carotenoid-containing and carotenoidless preparations. In both cases the broad band with peak near 1350 (675) nm (FWHM ～ 240 (120) nm) was found. It is concluded that the broad band with peak near 1350 (675) nm in two-photon excitation spectra of LH2 complexes from Allochromatium minutissimum cannot be interpreted as two-photon excitation band of the optically forbidden S₀ → S₁ transition of carotenoids (rhodopin). Possible nature of this band is discussed.     

Excitation energy transfer in chlorosomes of Chlorobium phaeobacteroides strain CL1401: the role of carotenoids

The role of carotenoids in chlorosomes of the green sulfur bacterium Chlorobium phaeobacteroides, containing bacteriochlorophyll (BChl) e and the carotenoid (Car) isorenieratene as main pigments, was studied by steady-state fluorescence excitation, picosecond single-photon timing and femtosecond transient absorption (TA) spectroscopy. In order to obtain information about energy transfer from Cars in this photosynthetic light-harvesting antenna with high spectral overlap between Cars and BChls, Car-depleted chlorosomes, obtained by inhibition of Car biosynthesis by 2-hydroxybiphenyl, were employed in a comparative study with control chlorosomes. Excitation spectra measured at room temperature give an efficiency of 60–70% for the excitation energy transfer from Cars to BChls in control chlorosomes. Femtosecond TA measurements enabled an identification of the excited state absorption band of Cars and the lifetime of their S₁ state was determined to be 10 ps. Based on this lifetime, we concluded that the involvement of this state in energy transfer is unlikely. Furthermore, evidence was obtained for the presence of an ultrafast (>100 fs) energy transfer process from the S₂ state of Cars to BChls in control chlorosomes. Using two time-resolved techniques, we further found that the absence of Cars leads to overall slower decay kinetics probed within the Q_y band of BChl e aggregates, and that two time constants are generally required to describe energy transfer from aggregated BChl e to baseplate BChl a.This revised version was published online in October 2005 with corrections to the Cover Date.     

Temperature and Carbon Assimilation Regulate the Chlorosome Biogenesis in Green Sulfur Bacteria

Green photosynthetic bacteria adjust the structure and functionality of the chlorosome—the light-absorbing antenna complex—in response to environmental stress factors. The chlorosome is a natural self-assembled aggregate of bacteriochlorophyll (BChl) molecules. In this study, we report the regulation of the biogenesis of the Chlorobaculum tepidum chlorosome by carbon assimilation in conjunction with temperature changes. Our studies indicate that the carbon source and thermal stress culture of C. tepidum grows slower and incorporates fewer BChl c in the chlorosome. Compared with the chlorosome from other cultural conditions we investigated, the chlorosome from the carbon source and thermal stress culture displays (a) smaller cross-sectional radius and overall size, (b) simplified BChl c homologs with smaller side chains, (c) blue-shifted Q_y absorption maxima, and (d) a sigmoid-shaped circular dichroism spectra. Using a theoretical model, we analyze how the observed spectral modifications can be associated with structural changes of BChl aggregates inside the chlorosome. Our report suggests a mechanism of metabolic regulation for chlorosome biogenesis.     

Pigment organization in the photosynthetic apparatus of Roseiflexus castenholzii

The light-harvesting-reaction center (LHRC) complex from the chlorosome-lacking filamentous anoxygenic phototroph (FAP), Roseiflexus castenholzii (R. castenholzii) was purified and characterized for overall pigment organization. The LHRC is a single complex that is comprised of light harvesting (LH) and reaction center (RC) polypeptides as well as an attached c-type cytochrome. The dominant carotenoid found in the LHRC is keto-γ-carotene, which transfers excitation to the long wavelength antenna band with 35% efficiency. Linear dichroism and fluorescence polarization measurements indicate that the long wavelength antenna pigments absorbing around 880 nm are perpendicular to the membrane plane, with the corresponding Q_y transition dipoles in the plane of the membrane. The antenna pigments absorbing around 800 nm, as well as the bound carotenoid, are oriented at a large angle with respect to the membrane. The antenna pigments spectroscopically resemble the well-studied LH2 complex from purple bacteria, however the close association with the RC makes the light harvesting component of this complex functionally more like LH1.     

The two light-harvesting membrane chromoproteins of Thermochromatium tepidum expose distinct robustness against temperature and pressure

An increased robustness against high temperature and the much red-shifted near-infrared absorption spectrum of excitons in the LH1-RC core pigment-protein complex from the thermophilic photosynthetic purple sulfur bacterium Thermochromatium tepidum has recently attracted much interest. In the present work, thermal and hydrostatic pressure stability of the peripheral LH2 and core LH1-RC complexes from this bacterium were in parallel investigated by various optical spectroscopy techniques applied over a wide spectral range from far-ultraviolet to near-infrared. In contrast to expectations, very distinct robustness of the complexes was established, while the sturdiness of LH2 surpassed that of LH1-RC both with respect to temperatures between 288 and 360 K, and pressures between 1 bar and 14 kbar. Subtle structural variances related to the hydrogen bond network are likely responsible for the extra stability of LH2.     

Probing binding site of bacteriochlorophyll a and carotenoid in the reconstituted LH1 complex from Rhodospirillum rubrum S1 by Stark spectroscopy

Stark spectroscopy is a powerful technique to investigate the electrostatic interactions between pigments as well as between the pigments and the proteins in photosynthetic pigment–protein complexes. In this study, Stark spectroscopy has been used to determine two nonlinear optical parameters (polarizability change Tr(Δα) and static dipole-moment change |Δμ| upon photoexcitation) of isolated and of reconstituted LH1 complexes from the purple photosynthetic bacterium, Rhodospirillum (Rs.) rubrum. The integral LH1 complex was prepared from Rs. rubrum S1, while the reconstituted complex was assembled by addition of purified carotenoid (all-trans-spirilloxanthin) to the monomeric subunit of LH1 from Rs. rubrum S1. The reconstituted LH1 complex has its Q_y absorption maximum at 878 nm. This is shifted to the blue by 3 nm in comparison to the isolated LH1 complex. The energy transfer efficiency from carotenoid to bacteriochlorophyll a (BChl a), which was determined by fluorescence excitation spectroscopy of the reconstituted LH1 complex, is increased to 40%, while the efficiency in the isolated LH1 complex is only 28%. Based on the differences in the values of Tr(Δα) and |Δμ|, between these two preparations, we can calculate the change in the electric field around the BChl a molecules in the two situations to be E _Δ ≈ 3.4 × 10⁵ [V/cm]. This change can explain the 3 nm wavelength shift of the Q_y absorption band in the reconstituted LH1 complex.     

Excitation energy transfer kinetics and efficiency in phototrophic green sulfur bacteria

A series of spectroscopic measurements were performed on membrane fractions and detergent-solubilized complexes from the green sulfur bacterium (GSB) Chlorobaculum (Cba.) tepidum. The excitation migration through the entire GSB photosynthetic apparatus cannot be observed upon excitation of membranes in the chlorosome region at 77?K. In order to observe energy transfer from the Fenna-Matthews-Olson (FMO) protein to the reaction center (RC), FMO was directly excited at ~800?nm in transient absorption experiments. However, interpretation of the results is complicated by the spectral overlap between FMO and the RC. The availability of the Y16F FMO mutant, whose absorption spectrum is drastically different from that of the WT, has enabled the selection of spectral regions where either only FMO or the RC contributes. The application of a directed kinetic modeling approach, or target analysis, revealed the various decay and energy transfer pathways within the pigment-protein complexes. The calculated FMO-to-RC excitation energy transfer efficiencies are approximately 25% and 48% for the Y16F and WT samples, respectively.     

D1 protein variants in Photosystem II from Thermosynechococcus elongatus studied by low temperature optical spectroscopy

In Photosystem II (PSII) from Thermosynechococcus elongatus, high-light intensity growth conditions induce the preferential expression of the psbA₃ gene over the psbA₁ gene. These genes encode for the D1 protein variants labeled D1:3 and D1:1, respectively. We have compared steady state absorption and photo-induced difference spectra at < 10 K of PSII containing either D1:1 or D1:3. The following differences were observed. (i) The pheophytin Q_x band was red-shifted in D1:3 (547.3 nm) compared to D1:1 (544.3 nm). (ii) The electrochromism on the Pheo_D1 Q_x band induced by Q_A⁻ (the C550 shift) was more asymmetric in D1:3. (iii) The two variants differed in their responses to excitation with far red (704 nm) light. When green light was used there was little difference between the two variants. With far red light the stable (t_1/2 > 50 ms) Q_A⁻ yield was ∼ 95% in D1:3, and ∼ 60% in D1:1, relative to green light excitation. (iv) For the D1:1 variant, the quantum efficiency of photo-induced oxidation of side-pathway donors was lower. These effects can be correlated with amino acid changes between the two D1 variants. The effects on the pheophytin Q_x band can be attributed to the hydrogen bond from Glu130 in D1:3 to the 13¹-keto of Pheo_D1, which is absent for Gln130 in D1:1. The reduced yield with red light in the D1:1 variant could be associated with either the Glu130Gln change, and/or the four changes near the binding site of P_D1, in particular Ser153Ala. Photo-induced Q_A⁻ formation with far red light is assigned to the direct optical excitation of a weakly absorbing charge transfer state of the reaction centre. We suggest that this state is blue-shifted in the D1:1 variant. A reduced efficiency for the oxidation of side-pathway donors in the D1:1 variant could be explained by a variation in the location and/or redox potential of P⁺.     

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 854?nm 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.     

Temperature and Carbon Assimilation Regulate the Chlorosome Biogenesis in Green Sulfur Bacteria

Green photosynthetic bacteria adjust the structure and functionality of the chlorosome—the light-absorbing antenna complex—in response to environmental stress factors. The chlorosome is a natural self-assembled aggregate of bacteriochlorophyll (BChl) molecules. In this study, we report the regulation of the biogenesis of the Chlorobaculum tepidum chlorosome by carbon assimilation in conjunction with temperature changes. Our studies indicate that the carbon source and thermal stress culture of C. tepidum grows slower and incorporates fewer BChl c in the chlorosome. Compared with the chlorosome from other cultural conditions we investigated, the chlorosome from the carbon source and thermal stress culture displays (a) smaller cross-sectional radius and overall size, (b) simplified BChl c homologs with smaller side chains, (c) blue-shifted Q_y absorption maxima, and (d) a sigmoid-shaped circular dichroism spectra. Using a theoretical model, we analyze how the observed spectral modifications can be associated with structural changes of BChl aggregates inside the chlorosome. Our report suggests a mechanism of metabolic regulation for chlorosome biogenesis.     

Energy transfer in purple bacterial photosynthetic units from cells grown in various light intensities

Three photosynthetic membranes, called intra-cytoplasmic membranes (ICMs), from wild-type and the ?pucBA_abce mutant of the purple phototrophic bacterium Rps. palustris were investigated using optical spectroscopy. The ICMs contain identical light-harvesting complex 1–reaction centers (LH1–RC) but have various spectral forms of light-harvesting complex 2 (LH2). Spectroscopic studies involving steady-state absorption, fluorescence, and femtosecond time-resolved absorption at room temperature and at 77 K focused on inter-protein excitation energy transfer. The studies investigated how energy transfer is affected by altered spectral features of the LH2 complexes as those develop under growth at different light conditions. The study shows that LH1 → LH2 excitation energy transfer is strongly affected if the LH2 complex alters its spectroscopic signature. The LH1 → LH2 excitation energy transfer rate modeled with the Förster mechanism and kinetic simulations of transient absorption of the ICMs demonstrated that the transfer rate will be 2–3 times larger for ICMs accumulating LH2 complexes with the classical B800–850 spectral signature (grown in high light) compared to the ICMs from the same strain grown in low light. For the ICMs from the ?pucBA_abce mutant, in which the B850 band of the LH2 complex is blue-shifted and almost degenerate with the B800 band, the LH1 → LH2 excitation energy transfer was not observed nor predicted by calculations.