Structural insights into LH2 complexes formed by a purple sulfur bacterium with inhibited carotenoid biosynthesis

The LH2 complex is essential for light harvesting in many photosynthetic bacteria. To elucidate the specific structural role of carotenoids, we analyzed LH2 complexes from Ectothiorhodospira haloalkaliphila with inhibited carotenoid biosynthesis. This approach allowed us to study complexes incorporating the colorless carotenoid phytoene instead of the native, colored pigments. A 1.92 Å cryo-EM reconstruction revealed that phytoene fully substitutes for the native carotenoids while maintaining the octameric symmetry of the complex and the precise arrangement of bacteriochlorophylls. These results demonstrate that the architectural function of carotenoids in LH2 complexes is maintained even when their light-absorption capability is altered, providing new mechanistic insight into the structural basis of pigment-protein interactions in photosynthetic antenna complexes.