Near-atomic cryo-EM structure of the light-harvesting complex LH2 from the sulfur purple bacterium Ectothiorhodospira haloalkaliphila.

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.