The genomes of some purple photosynthetic bacteria contain a multigene puc family encoding a series of α- and β-polypeptides that together form a heterogeneous antenna of light-harvesting 2 (LH2) complexes. To unravel this complexity, we generated four sets of puc deletion mutants in Rhodopseudomonas palustris, each encoding a single type of pucBA gene pair and enabling the purification of complexes designated as PucA-LH2, PucB-LH2, PucD-LH2, and PucE-LH2. The structures of all four purified LH2 complexes were determined by cryogenic electron microscopy (cryo-EM) at resolutions ranging from 2.7 to 3.6 Å. Uniquely, each of these complexes contains a hitherto unknown polypeptide, γ, that forms an extended undulating ribbon that lies in the plane of the membrane and that encloses six of the nine LH2 αβ-subunits. The γ-subunit, which is located near to the cytoplasmic side of the complex, breaks the C9 symmetry of the LH2 complex and binds six extra bacteriochlorophylls (BChls) that enhance the 800-nm absorption of each complex. The structures show that all four complexes have two complete rings of BChls, conferring absorption bands centered at 800 and 850 nm on the PucA-LH2, PucB-LH2, and PucE-LH2 complexes, but, unusually, the PucD-LH2 antenna has only a single strong near-infared (NIR) absorption peak at 803 nm. Comparison of the cryo-EM structures of these LH2 complexes reveals altered patterns of hydrogen bonds between LH2 αβ-side chains and the bacteriochlorin rings, further emphasizing the major role that H bonds play in spectral tuning of bacterial antenna complexes.

Cambridge Institute for Medical Research University of Cambridge Cambridge CB2 0XY United Kingdom

Laboratory of Anoxygenic Phototrophs Institute of Microbiology Czech Academy of Sciences Třeboň 37981 Czechia

Materials and Structure Analysis Thermofisher Scientific Eindhoven 5651 GG The Netherlands

School of Biosciences The University of Sheffield Sheffield S10 2TN United Kingdom

School of Molecular Biosciences Glasgow University Glasgow G12 8QQ United Kingdom

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Two solutions for efficient light-harvesting in phototrophic Gemmatimonadota

The Impact of Carotenoid Energy Levels on the Exciton Dynamics and Singlet-Triplet Annihilation in Isolated Bacterial Light-Harvesting 2 Complexes

Cryo-EM structures of light-harvesting 2 complexes from Rhodopseudomonas palustris reveal the molecular origin of absorption tuning