Department of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic.
J Bacteriol. 2013 Apr;195(8):1727-34. doi: 10.1128/JB.02052-12. Epub 2013 Feb 8.
Chlorosomes are large light-harvesting complexes found in three phyla of anoxygenic photosynthetic bacteria. Chlorosomes are primarily composed of self-assembling pigment aggregates. In addition to the main pigment, bacteriochlorophyll c, d, or e, chlorosomes also contain variable amounts of carotenoids. Here, we use X-ray scattering and electron cryomicroscopy, complemented with absorption spectroscopy and pigment analysis, to compare the morphologies, structures, and pigment compositions of chlorosomes from Chloroflexus aurantiacus grown under two different light conditions and Chlorobaculum tepidum. High-purity chlorosomes from C. aurantiacus contain about 20% more carotenoid per bacteriochlorophyll c molecule when grown under low light than when grown under high light. This accentuates the light-harvesting function of carotenoids, in addition to their photoprotective role. The low-light chlorosomes are thicker due to the overall greater content of pigments and contain domains of lamellar aggregates. Experiments where carotenoids were selectively extracted from intact chlorosomes using hexane proved that they are located in the interlamellar space, as observed previously for species belonging to the phylum Chlorobi. A fraction of the carotenoids are localized in the baseplate, where they are bound differently and cannot be removed by hexane. In C. tepidum, carotenoids cannot be extracted by hexane even from the chlorosome interior. The chemical structure of the pigments in C. tepidum may lead to π-π interactions between carotenoids and bacteriochlorophylls, preventing carotenoid extraction. The results provide information about the nature of interactions between bacteriochlorophylls and carotenoids in the protein-free environment of the chlorosome interior.
藻胆体是在三种厌氧光合细菌门中发现的大型光捕获复合物。藻胆体主要由自组装的色素聚集体组成。除了主要色素细菌叶绿素 c、d 或 e 外，藻胆体还含有不同数量的类胡萝卜素。在这里，我们使用 X 射线散射和电子 cryomicroscopy，辅以吸收光谱和色素分析，来比较在两种不同光照条件下生长的 Chloroflexus aurantiacus 和 Chlorobaculum tepidum 的藻胆体的形态、结构和色素组成。从低光照下生长的 C. aurantiacus 中获得的高纯度藻胆体，每个细菌叶绿素 c 分子中类胡萝卜素的含量比在高光下生长时多 20%。这除了发挥其光保护作用外，还突出了类胡萝卜素的光捕获功能。由于色素含量总体上较高，低光下的藻胆体更厚，并包含层状聚集体的域。使用己烷从完整的藻胆体中选择性提取类胡萝卜素的实验证明，它们位于层间空间中，这与先前观察到的属于 Chlorobi 门的物种相同。一部分类胡萝卜素定位于基板中，在那里它们以不同的方式结合，并且不能用己烷去除。在 C. tepidum 中，即使从藻胆体内部，己烷也不能提取类胡萝卜素。C. tepidum 中色素的化学结构可能导致类胡萝卜素和细菌叶绿素之间的π-π相互作用，从而阻止类胡萝卜素的提取。这些结果提供了有关在藻胆体内部无蛋白质环境中细菌叶绿素和类胡萝卜素之间相互作用性质的信息。
