Abstact

Aerobic anoxygenic phototrophic bacteria (AAPB) are widely distributed in nature and they are important members of the marine phototrophic community. However, a structural and functional understanding of the AAPB photosynthetic apparatus is still lacking. Here, we present cryo-EM structures of the LH1-RC (core) and LH2 (peripheral) photocomplexes from the model aerobic phototroph Erythrobacter (Ery.) sanguineus. The LH1 αβ-heterodimers bind the carotenoids bacteriorubixanthinal and caloxanthin-pigments that are absent from anaerobic anoxygenic phototrophs-to form a closed ring structure. Ery. sanguineus LH1-RC contains a lipid-anchored polypeptide unrelated to any of the auxiliary proteins identified in the core complexes of purple bacteria so far. The Ery. sanguineus LH2 complex shows unique absorption characteristics, with its Qy transition being blue-shifted to 814 nm. This work provides structural insights into the unusual photosynthetic properties of AAPB and points to new avenues to further explore their biology.