Structural insights into the unusual core photocomplex from a triply extremophilic purple bacterium, Halorhodospira halochloris

Abstract

Halorhodospira (Hlr.) halochloris is a triply extremophilic phototrophic purple sulfur bacterium, as it is thermophilic, alkaliphilic, and extremely halophilic. The light‐harvesting‐reaction center (LH1–RC) core complex of this bacterium displays an LH1‐Qy transition at 1,016 nm, which is the lowest‐energy wavelength absorption among all known phototrophs. Here we report the cryo‐EM structure of the LH1–RC at 2.42 Å resolution. The LH1 complex forms a tricyclic ring structure composed of 16 αβγ‐polypeptides and one αβ‐heterodimer around the RC. From the cryo‐EM density map, two previously unrecognized integral membrane proteins, referred to as protein G and protein Q, were identified. Both of these proteins are single transmembrane‐spanning helices located between the LH1 ring and the RC L‐subunit and are absent from the LH1–RC complexes of all other purple bacteria of which the structures have been determined so far. Besides bacteriochlorophyll b molecules (B1020) located on the periplasmic side of the Hlr. halochloris membrane, there are also two arrays of bacteriochlorophyll b molecules (B800 and B820) located on the cytoplasmic side. Only a single copy of a carotenoid (lycopene) was resolved in the Hlr. halochloris LH1–α3β3 and this was positioned within the complex. The potential quinone channel should be the space between the LH1–α3β3 that accommodates the single lycopene but does not contain a γ‐polypeptide, B800 and B820. Our results provide a structural explanation for the unusual Qy red shift and carotenoid absorption in the Hlr. halochloris spectrum and reveal new insights into photosynthetic mechanisms employed by a species that thrives under the harshest conditions of any phototrophic microorganism known.

Energy Transfer and Exciton Relaxation in B880-B800-RC Complex through Two-Dimensional Electronic Spectroscopy

“…The LH1, LH2s and a spectroscopic variant of LH2, B800-B820, have been extensively studied by various ultrafast spectroscopy techniques. − , Nevertheless, more LHs with discrepant structures have been revealed recently. − Among them, Roseiflexus ( Rfl .) castenholzii , Roseospirillum parvum and Halorhodospira halochloris exhibit distinctive characters that the LH2s are absent while the LH1s with unusual “LH2-like” structure have to accomplish the functions of both LH2 and LH1.…”