Cryo‐EM Analysis of a Tri‐Heme Cytochrome‐Associated RC‐LH1 Complex from the Marine Photoheterotrophic Bacterium <i>Dinoroseobacter Shibae</i>

The reaction center-light harvesting 1 (RC-LH1) complex converts solar energy into electrical energy, driving the initiation of photosynthesis. The authors present a cryo-electron microscopy structure of the RC-LH1 isolated from a marine photoheterotrophic bacterium Dinoroseobacter shibae. The RC comprises four subunits, including a three-heme cytochrome (Cyt) c protein, and is surrounded by a closed LH ring composed of 17 pairs of antenna subunits. Notably, a novel subunit with an N-terminal "helix-turn-helix" motif embedded in the gap between the RC and the LH ring is identified. The purified RC-LH1 complex exhibits high stability in solutions containing Mg²⁺ or Ca²⁺. The periplasmic Cyt c₂ is predicted to bind at the junction between the Cyt subunit and the membrane plane, enabling electron transfer from Cyt c₂ to the proximal heme of the tri-heme Cyt, and subsequently to the special pair of bacteriochlorophylls. These findings provide structural insights into the efficient energy and electron transfer processes within a distinct type of RC-LH1, and shed light on evolutionary adaptations of photosynthesis.