Cryo-EM structure of photosystem II supercomplex from a green microalga with extreme phototolerance

Abstract

Abstract Photosystem II (PSII) is essential for energy conversion during oxygenic photosynthesis in plants and algae. Chlorella ohadii , one of the fastest multiplying green algae, thrives under the harsh desert sun but lacks the standard PSII photoprotective mechanisms involving LhcSR/PsbS proteins or protein phosphorylation. Here, we present the cryo-EM structure of the PSII supercomplex from C. ohadii at 2.9 Å resolution, which is used to determine whether the exceptional resistance to desert conditions has a structural basis in PSII. The structure reveals a distinct PsbO isoform and additional subunits, PsbR and PsbY, which enhance core complex stability through extensive interactions. Furthermore, the trimeric light-harvesting complexes (LHCII) are bound to the PSII core by specific light-harvesting proteins whose down-regulation in response to high-light conditions implies a reduction in the number of bound LHCII trimers. These structural modifications, together with the high accumulation of specific polyamines in the thylakoid membrane, play a key role in maintaining PSII stability and photoprotection, allowing C. ohadii to survive in extreme conditions.