Bacteria with the simplest system for solar energy absorption and conversion use various types of light-harvesting complexes for these purposes. Light-harvesting complex 2 (LH2), an important component of the bacterial photosynthetic apparatus, has been structurally well characterized among purple non-sulfur bacteria. In contrast, so far only one high-resolution LH2 structure from sulfur bacteria is known. Here, we report the near-atomic resolution cryoelectron microscopy (cryo-EM) structure of the LH2 complex from the purple sulfur bacterium Ectothiorhodospira haloalkaliphila, which allowed us to determine the predominant polypeptide composition of this complex and the identification of the most probable type of its carotenoid. Comparison of our structure with the only known LH2 complex from a sulfur bacterium revealed severe differences in the overall ring-like organization. Expanding the architectural universe of bacterial light-harvesting complexes, our results demonstrate that, as observed for non-sulfur bacteria, the LH2 complexes of sulfur bacteria may also exhibit various types of spatial organization.

中文翻译：

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来自硫紫细菌嗜盐碱体外硫菌 LH2 的光捕获复合物 LH2 的近原子冷冻电镜结构


具有最简单的太阳能吸收和转换系统的细菌为此目的使用各种类型的光收集复合物。光捕获复合物 2 （LH2） 是细菌光合装置的重要组成部分，在紫色非硫细菌中已在结构上得到很好的表征。相比之下，到目前为止，只有一种来自硫细菌的高分辨率 LH2 结构是已知的。在这里，我们报告了来自紫色硫细菌嗜盐碱体外硫菌的 LH2 复合物的近原子分辨率冷冻电子显微镜 （cryo-EM） 结构，这使我们能够确定该复合物的主要多肽组成并鉴定其类胡萝卜素的最可能类型。将我们的结构与来自硫细菌的唯一已知 LH2 复合物进行比较，揭示了整体环状组织的严重差异。扩展了细菌光捕获复合物的结构范围，我们的结果表明，正如在非硫细菌中观察到的那样，硫细菌的 LH2 复合物也可能表现出各种类型的空间组织。