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Publication Abstract from PubMed

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-Q(y) 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 A resolution. The LH1 complex forms a tricyclic ring structure composed of 16 alphabetagamma-polypeptides and one alphabeta-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-alpha3beta3 and this was positioned within the complex. The potential quinone channel should be the space between the LH1-alpha3beta3 that accommodates the single lycopene but does not contain a gamma-polypeptide, B800 and B820. Our results provide a structural explanation for the unusual Q(y) 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.

Structural insights into the unusual core photocomplex from a triply extremophilic purple bacterium, Halorhodospira halochloris.,Qi CH, Wang GL, Wang FF, Wang J, Wang XP, Zou MJ, Ma F, Madigan MT, Kimura Y, Wang-Otomo ZY, Yu LJ J Integr Plant Biol. 2024 Feb 27. doi: 10.1111/jipb.13628. PMID:38411333^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.