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The primary processes of photosynthesis for purple photosynthesising bacteria occur in light-harvesting (LH) complexes. The LH2 complex contains polypeptides; bacteriochlorophyll; and, in most cases, carotenoids. There are three known spatial structures of LH2 complexes from purple nonsulfur bacteria; however, high-resolution structures have not been established for purple sulfur bacteria. The results of the structural study of two light-harvesting complexes LH2 from purple sulfur bacteria Ectothiorhodospira haloalkaliphila by cryoelectronic microscopy are reported. Images of carotenoid-containing (LH2+) and carotenoidless (LH2–) variants of the complex, demonstrating a characteristic architecture of the objects of this type, are obtained. A 3D reconstruction of LH2+ is performed with a resolution of 4.5 Å; it coincides with the previously established crystal structure. The presence of particles of different morphology is shown for LH2–. [ABSTRACT FROM AUTHOR]

The effect of the qualitative and quantitative content of carotenoids on the assembly of light-harvesting complexes isolated from the heat-resistant purple sulfur bacterium Thermochromatium (T.) tepidum was studied. To decrease the content of carotenoids in the photosynthetic membranes and complexes, an inhibitor of carotenonegesis, diphenylamine (DPA), was used. The minimum carotenoid content of 40% was observed in T. tepidum cells grown in the presence of 120 μM DPA. At higher DPA concentrations, the growth of the culture stopped. Photosynthetic membranes and LH2 and LH1-RC complexes were isolated from DPA-treated cells; carotenoid content of these structures was 45 and 55%, respectively. Their spectral characteristics in the near IR region did not depend on the amount of carotenoids in the complexes and were similar to those of the native wild-type complexes. Simultaneously with a general decrease in the content of carotenoids, carotenoids from earlier stages of biosynthesis, such as neurosporene, ζ-carotene, and their hydroxy derivatives, appeared in the DPA-treated complexes. Emergence in DPA-treated complexes of carotenoids with a shorter chain of conjugated double bonds than in the control resulted in an increase in the efficiency of energy transfer from carotenoids to bacteriochlorophyll. [ABSTRACT FROM AUTHOR]

The results of assembling the light-harvesting complexes in the cells of the purple sulfur bacterium Thiorhodospira (T.) sibirica strain Kir-3 while suppressing the biosynthesis of carotenoids with diphenylamine (DPA) were studied. LH2 complexes (B800-850 and B800-830) with different carotenoid composition were isolated from the cells obtained. Maximum inhibition of carotenoid biosynthesis (~90% of the control) was reached at an inhibitor concentration of 53.25 μM (9 mg/L). It was established that changes in the qualitative and quantitative composition of carotenoids do not affect the assembly of B800-830 and B800-850 complexes. It is assumed that, in the population of DPA-LH2 complexes from T. sibirica strain Kir-3, both the carotenoidless complexes and the complexes containing one or two carotenoid molecules can be assembled. These results support the hypothesis that carotenoids are not required for assembling B800-850 and B800-830 complexes. [ABSTRACT FROM AUTHOR]

The direct action of singlet oxygen on the bacteriochlorophyll (BChl) of light-harvesting complexes in the membranes of four species of purple non-sulfur and sulfur photosynthesizing bacteria with and without carotenoids was studied. It was found that BChl in carotenoidless samples is generally more resistant to the action of singlet oxygen compared to the control. It is assumed that carotenoids are not required to protect BChl of bacterial light-harvesting complexes from singlet oxygen, and in the classic work by Griffith et al. [1] the apoptosis process in carotenoidless mutant cells, which involves the destruction of complexes, the appearance of monomeric BChl, and the generation of singlet oxygen caused by BChl, followed by BChl oxidation, was mistakenly attributed to the protective function of carotenoids. [ABSTRACT FROM AUTHOR]

The effect of singlet oxygen was studied in the system of Allochromatium (Alc.) vinosum MSU membranes, rose bengal, and light (547−600nm) with and without a quencher. In the system without a quencher, bleaching of the BChl850 band of the LH2 light-harvesting complex was observed and an absorption peak emerged at 698 nm, which belonged to the oxidized product, 3-acetyl-chlorophyll. The efficiency of five quenchers of singlet oxygen for neutralization of the effect of singlet oxygen on bacteriochlorophyll in the light-harvesting complexes of the membranes Alc. vinosum MSU was studied: sodium ascorbate, histidine, imidazole, trolox, and sodium azide. All of them, with the exception of sodium azide, neutralized successively the effect of singlet oxygen. [ABSTRACT FROM AUTHOR]

Carotenoid mixture enriched by rhodopin and spirilloxanthin was incorporated in LH2 and LH1 complexes from Allochromatium ( Alc.) minutissimum in vitro. The maximum incorporating level was ~95%. Rhodopin (56.4%) and spirilloxanthin (13.8%) were incorporated into the LH1 complex, in contrast to the control complex, which contained primarily spirilloxanthin (66.8%). After incorporating, the LH2 complex contained rhodopin (66.7%) and didehydrorhodopin (14.6%), which was close to their content in the control (67.4 and 20.5%, respectively). Thus, it was shown that carotenoids from the total pool are not selectively incorporated into LH2 and LH1 complexes in vitro in the proportion corresponding to the carotenoid content in the complexes in vivo. [ABSTRACT FROM AUTHOR]

The possibility of embedding the carotenoids of spheroidene-branch biosynthesis (spheroidene and spheroidenone) from non-sulfur bacteria into the diphenylamine antenna complexes (DPA-complexes) from the sulfur bacteria Allochromatium minutissimum and Ectothiorhodospira haloalkaliphila with carotenoid synthesis inhibited by diphenylamine (DPA) was studied for the first time. It was found that spheroidene was embedded into the DPA-complexes from these bacteria at a level of 75-87%, with spheroidene embedding efficiency being 41-68% for the LH1-RC DPA-complexes and 71-89% for the LH2 DPA-complexes. The energy transfer efficiency from carotenoids to bacteriochlorophyll was shown to depend not only on the type of carotenoid but also on the very structure on the antenna complex. [ABSTRACT FROM AUTHOR]