Carotenoid assembly regulates quinone diffusion and the Roseiflexus castenholzii reaction center-light harvesting complex architecture

Cryo-EM structure of a minimal reaction center-light-harvesting complex from the phototrophic bacterium Chloroflexus aurantiacus

Photosynthetic organisms have developed various light-harvesting antenna systems to capture light and transfer energy to reaction centers (RCs). Simultaneous utilization of the integral membrane light-harvesting antenna (LH complex) and the extrinsic antenna (chlorosomes) makes the phototrophic bacterium Chloroflexus (Cfx.) aurantiacus an ideal model for studying filamentous anoxygenic phototrophs (FAPs). Here, we determined the structure of a minimal RC-LH photocomplex from Cfx. aurantiacus J-10-fl (CaRC-LH) at 3.05-Å resolution. The CaRC-LH binds only to seven LH subunits, which form a crescent-shaped antenna surrounding the movable menaquinone-10 (QB) binding site of CaRC. In this complex with minimal LH units, an extra antenna is required to ensure sufficient light-gathering, providing a clear explanation for the presence of chlorosomes in Cfx. aurantiacus. More importantly, the semicircle of the antenna represents a novel RC-LH assembly pattern. Our structure provides a basis for understanding the existence of chlorosomes in Cfx. aurantiacus and the possible assembly pattern of RC-LH.

Structure of ATP synthase from an early photosynthetic bacterium Chloroflexus aurantiacus

F-type ATP synthase (F1FO) catalyzes proton motive force-driven ATP synthesis in mitochondria, chloroplasts, and bacteria. Different from the mitochondrial and bacterial enzymes, F1FO from photosynthetic organisms have evolved diverse structural and mechanistic details to adapt to the light-dependent reactions. Although complete structure of chloroplast F1FO has been reported, no high-resolution structure of an F1FO from photosynthetic bacteria has been available. Here, we report cryo-EM structures of an intact and functionally competent F1FO from Chloroflexus aurantiacus (CaF1FO), a filamentous anoxygenic phototrophic bacterium from the earliest branch of photosynthetic organisms. The structures of CaF1FO in its ADP-free and ADP-bound forms for three rotational states reveal a previously unrecognized architecture of ATP synthases. A pair of peripheral stalks connect to the CaF1 head through a dimer of δ-subunits, and associate with two membrane-embedded a-subunits that are asymmetrically positioned outside and clamp CaFO's c10-ring. The two a-subunits constitute two proton inlets on the periplasmic side and two proton outlets on the cytoplasmic side, endowing CaF1FO with unique proton translocation pathways that allow more protons being translocated relative to single a-subunit F1FO. Our findings deepen understanding of the architecture and proton translocation mechanisms of F1FO synthases and suggest innovative strategies for modulating their activities by altering the number of a-subunit.

The Thermal-Stable LH1-RC Complex of a Hot Spring Purple Bacterium Powers Photosynthesis with Extremely Low-Energy Near-Infrared Light

Blastochloris (Blc.) tepida is a hot spring purple nonsulfur phototrophic bacterium that contains bacteriochlorophyll (BChl) b. Here, we present a 2.21 Å cryo-EM structure of the thermostable light-harvesting 1-reaction center (LH1-RC) complex from Blc. tepida. The LH1 ring comprises 16 circularly arranged αβγ-subunits plus one αβ-subunit that surround the RC complex composed of C-, H-, L-, and M-subunits. In a comparative study, the Blc. tepida LH1-RC showed numerous electrostatic and hydrophobic interactions both within the LH1 complex itself and between the LH1 and the RC complexes that are absent from the LH1-RC complex of its mesophilic counterpart, Blc. viridis. These additional interactions result in a tightly packed LH1-RC architecture with a reduced accessible surface area per volume that enhances the thermal stability of the Blc. tepida complex and allows the light reactions of photosynthesis to proceed at hot spring temperatures. Moreover, based on high-resolution structural information combined with spectroscopic evidence, the unique photosynthetic property of the Blc. tepida LH1-RC─absorption of energy-poor near-infrared light beyond 1000 nm─can be attributed to strong hydrogen-bonding interactions between the C3-acetyl C═O of the LH1 BChl b and two LH1 α-Trp residues, structural rigidity of the LH1, and the enhanced exciton coupling of the LH1 BChls of this thermophile.

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

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.

Light-Quality-Adapted Carotenoid Photoprotection in the Photosystem of Roseiflexus castenholzii

The photosystem of filamentous anoxygenic phototroph Roseiflexus (Rfl.) castenholzii comprises a light-harvesting (LH) complex encircling a reaction center (RC), which intensely absorbs blue-green light by carotenoid (Car) and near-infrared light by bacteriochlorophyll (BChl). To explore the influence of light quality (color) on the photosynthetic activity, we compared the pigment compositions and triplet excitation dynamics of the LH-RCs from Rfl. castenholzii was adapted to blue-green light (bg-LH-RC) and to near-infrared light (nir-LH-RC). Both LH-RCs bind γ-carotene derivatives; however, compared to that of nir-LH-RC (12%), bg-LH-RC contains substantially higher keto-γ-carotene content (43%) and shows considerably faster BChl-to-Car triplet excitation transfer (10.9 ns vs 15.0 ns). For bg-LH-RC, but not nir-LH-RC, selective photoexcitation of Car and the 800 nm-absorbing BChl led to Car-to-Car triplet transfer and BChl-Car singlet fission reactions, respectively. The unique excitation dynamics of bg-LH-RC enhances its photoprotection, which is crucial for the survival of aquatic anoxygenic phototrophs from photooxidative stress.