Structural Study of the Light-Harvesting Complex LH2 from the Purple Sulfur Bacteria Ectothiorhodospira haloalkaliphila by Cryoelectronic Microscopy

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–.