article · PLANT PHYSIOLOGY
The monomeric photosystem I-light-harvesting antenna complex I (PSI-LHCI) supercomplex from the extremophilic red alga Cyanidioschyzon merolae acts as an evolutionary intermediate between cyanobacteria and higher plants. This supercomplex demonstrates robustness under diverse extreme conditions through three primary mechanisms. First, photoprotective zeaxanthin accumulates in the reaction centre and antenna. Second, structural remodelling adjusts the effective light absorption cross section. Third, dynamic readjustments occur in the stoichiometry of two PSI-LHCI isomers alongside oligomeric changes and the dissociation of the PsaK core subunit. Under low-light treatments, the largest supercomplex binds up to eight Lhcr antenna subunits arranged in two rows on the PsaF/PsaJ side. Furthermore, tests revealed no evidence of functional coupling between phycobilisomes and the purified supercomplex under various light regimes.
Discovering how extremophile algae manage extreme conditions reveals how photosynthetic machinery evolved between primitive bacteria and complex plants. Identifying these fundamental photoprotective mechanisms enhances the understanding of how biological systems maintain energy harvesting and stability during environmental stress.
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The monomeric photosystem I-light-harvesting antenna complex I (PSI-LHCI) supercomplex from the extremophilic red alga <i>Cyanidioschyzon merolae</i> represents an intermediate evolutionary link between the cyanobacterial PSI reaction center and its green algal/higher plant counterpart. We show that the <i>C. merolae</i> PSI-LHCI supercomplex is characterized by robustness in various extreme conditions. By a combination of biochemical, spectroscopic, mass spectrometry, and electron microscopy/single particle analyses, we dissected three molecular mechanisms underlying the inherent robustness of the <i>C. merolae</i> PSI-LHCI supercomplex: (1) the accumulation of photoprotective zeaxanthin in the LHCI antenna and the PSI reaction center; (2) structural remodeling of the LHCI antenna and adjustment of the effective absorption cross section; and (3) dynamic readjustment of the stoichiometry of the two PSI-LHCI isomers and changes in the oligomeric state of the PSI-LHCI supercomplex, accompanied by dissociation of the PsaK core subunit. We show that the largest low light-treated <i>C. merolae</i> PSI-LHCI supercomplex can bind up to eight Lhcr antenna subunits, which are organized as two rows on the PsaF/PsaJ side of the core complex. Under our experimental conditions, we found no evidence of functional coupling of the phycobilisomes with the PSI-LHCI supercomplex purified from various light conditions, suggesting that the putative association of this antenna with the PSI supercomplex is absent or may be lost during the purification procedure.
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DOI: 10.1104/pp.17.01022
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