Spectroscopic properties and bacteriochlorophyll c isomer composition of extramembranous light-harvesting complexes in the green sulfur photosynthetic bacterium Chlorobium tepidum and its CT0388-deleted mutant under vitamin B12-limited conditions

Effects of exogenous isoprenoid diphosphates on in vivo attachment to bacteriochlorophyllide c in the green sulfur photosynthetic bacterium Chlorobaculum tepidum

Metabolic substitution of the esterifying chain in bacteriochlorophyll (BChl) c in green photosynthetic bacteria grown by supplementation of exogenous alcohols has attracted attentions to study supramolecular structures and biogenesis of major antenna complexes chlorosomes in these bacteria as well as BChl pigment biosynthesis. Actual substrates in the enzymatic attachment of the esterifying moieties to the precursor of BChl c, namely bacteriochlorophyllide (BChlide) c, in these bacteria are believed to be diphosphate esters of alcoholic substrates, although only intact alcohols have so far been supplemented in the bacterial cultures. We report herein BChl c compositions in the green sulfur photosynthetic bacterium Chlorobaculum tepidum by supplementation with geranyl and geranylgeranyl diphosphates. The supplementation of these diphosphates hardly produced BChl c derivatives esterified with geraniol and geranylgeraniol in Cba. tepidum, whereas these BChl c derivatives were accumulated by supplementation of intact geraniol and geranylgeraniol. The sharp contrast of the incorporation efficiency of the supplemental isoprenoid moieties in BChl c using the isoprenoid diphosphates to that by the isoprenoid alcohols was mainly ascribable to less penetration abilities of the diphosphate substrates into Cba. tepidum cells because of their anionic and polar diphosphate moiety.

Introduction of perfluoroalkyl chain into the esterifying moiety of bacteriochlorophyll c in the green sulfur photosynthetic bacterium Chlorobaculum tepidum by pigment biosynthesis

The green sulfur photosynthetic bacterium Chlorobaculum (Cba.) tepidum was grown in liquid cultures containing perfluoro-1-decanol, 1H,1H,2H,2H-heptadecafluoro-1-decanol [CF3(CF2)7(CH2)2OH] or 1H,1H-nonadecafluoro-1-decanol [CF3(CF2)8CH2OH], to introduce rigid and fluorophilic chains into the esterifying moiety of light-harvesting bacteriochlorophyll (BChl) c. Exogenous 1H,1H,2H,2H-heptadecafluoro-1-decanol was successfully attached to the 17(2)-carboxy group of bacteriochlorophyllide (BChlide) c in vivo: the relative ratio of the unnatural BChl c esterified with this perfluoroalcohol over the total BChl c was 10.3%. Heat treatment of the liquid medium containing 1H,1H,2H,2H-heptadecafluoro-1-decanol with β-cyclodextrin before inoculation increased the relative ratio of the BChl c derivative esterified with this alcohol in the total BChl c in Cba. tepidum. In a while, 1H,1H-nonadecafluoro-1-decanol was not attached to BChlide c in Cba. tepidum, which was grown by its supplementation. These results suggest that the rigidity close to the hydroxy group of the esterifying alcohol is not suitable for the recognition by the BChl c synthase called BchK in Cba. tepidum. The unnatural BChl c esterified with 1H,1H,2H,2H-heptadecafluoro-1-decanol participated in BChl c self-aggregates in chlorosomes.

Theoretical characterization of excitation energy transfer in chlorosome light-harvesting antennae from green sulfur bacteria

We present a theoretical study of excitation dynamics in the chlorosome antenna complex of green photosynthetic bacteria based on a recently proposed model for the molecular assembly. Our model for the excitation energy transfer (EET) throughout the antenna combines a stochastic time propagation of the excitonic wave function with molecular dynamics simulations of the supramolecular structure and electronic structure calculations of the excited states. We characterized the optical properties of the chlorosome with absorption, circular dichroism and fluorescence polarization anisotropy decay spectra. The simulation results for the excitation dynamics reveal a detailed picture of the EET in the chlorosome. Coherent energy transfer is significant only for the first 50 fs after the initial excitation, and the wavelike motion of the exciton is completely damped at 100 fs. Characteristic time constants of incoherent energy transfer, subsequently, vary from 1 ps to several tens of ps. We assign the time scales of the EET to specific physical processes by comparing our results with the data obtained from time-resolved spectroscopy experiments.

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 Qy 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.

Biosynthesis of bacteriochlorophyll c derivatives possessing chlorine and bromine atoms at the terminus of esterifying chains in the green sulfur bacterium Chlorobaculum tepidum

The green sulfur photosynthetic bacterium Chlorobaculum tepidum newly produced BChl c derivatives possessing a chlorine or bromine atom at the terminus of the esterifying chain in the 17-propionate residue by cultivation with exogenous ω-halo-1-alkanols. The relative ratios of BChl c derivatives esterified with 8-chloro-1-octanol and 10-chloro-1-decanol were estimated to be 26.5% and 33.3% by cultivation with these ω-chloro-1-alkanols at the final concentrations of 300 and 150 μM, respectively. In contrast, smaller amounts of unnatural BChls c esterified with ω-bromo-1-alkanols were biosynthesized than those esterified with ω-chloro-1-alkanols: the ratios of BChl c derivatives esterified with 8-bromo-1-octanol and 10-bromo-1-decanol were 11.3% and 12.2% at the concentrations of 300 and 150 μM, respectively. These indicate that ω-chloro-1-alkanols can be incorporated into bacteriochlorophyllide c more than ω-bromo-1-alkanols in the BChl c biosynthetic pathway. The homolog compositions of the novel BChl c derivatives possessing a halogen atom were analogous to those of coexisting natural BChl c esterified with farnesol. These results demonstrate unique properties of BChl c synthase, BchK, which can utilize unnatural substrates containing halogen in the BChl c biosynthesis of Cba. tepidum.

Circular Dichroism Measured on Single Chlorosomal Light-Harvesting Complexes of Green Photosynthetic Bacteria

We report results on circular dichroism (CD) measured on single immobilized chlorosomes of a triple mutant of green sulfur bacterium Chlorobaculum tepidum . The CD signal is measured by monitoring chlorosomal bacteriochlorphyll c fluorescence excited by alternate left and right circularly polarized laser light with a fixed wavelength of 733 nm. The excitation wavelength is close to a maximum of the negative CD signal of a bulk solution of the same chlorosomes. The average CD dissymmetry parameter obtained from an ensemble of individual chlorosomes was gs = -0.025, with an intrinsic standard deviation (due to variations between individual chlorosomes) of 0.006. The dissymmetry value is about 2.5 times larger than that obtained at the same wavelength in the bulk solution. The difference can be satisfactorily explained by taking into account the orientation factor in the single-chlorosome experiments. The observed distribution of the dissymmetry parameter reflects the well-ordered nature of the mutant chlorosomes.