Abstract
Diatom is an important group of marine algae and contributes to around 20% of the global photosynthetic carbon fixation. Photosystem I (PSI) of diatoms is associated with a large number of fucoxanthin-chlorophyll a/c proteins (FCPIs). We report the structure of PSI-FCPI from a diatom Chaetoceros gracilis at 2.38 Å resolution by single-particle cryo-electron microscopy. PSI-FCPI is a monomeric supercomplex consisting of 12 core and 24 antenna subunits (FCPIs), and 326 chlorophylls a, 34 chlorophylls c, 102 fucoxanthins, 35 diadinoxanthins, 18 β-carotenes and some electron transfer cofactors. Two subunits designated PsaR and PsaS were found in the core, whereas several subunits were lost. The large number of pigments constitute a unique and huge network ensuring efficient energy harvesting, transfer and dissipation. These results provide a firm structural basis for unraveling the mechanisms of light-energy harvesting, transfer and quenching in the diatom PSI-FCPI, and also important clues to evolutionary changes of PSI-LHCI.
Highlights
Diatom is an important group of marine algae and contributes to around 20% of the global photosynthetic carbon fixation
Photosystem I (PSI)-fucoxanthin-chlorophyll a/c proteins (FCPIs) supercomplex was purified from a centric diatom C. gracilis (Supplementary Fig. 1a) and its peptide composition, spectroscopic properties, and pigment composition are shown in Supplementary Fig. 1b–e
The overall structure of the C. gracilis PSI-FCPI is an asymmetric heart-shaped monomer when viewed from the stromal side (Fig. 1a), in which the PSI core is composed of 12 subunits, including PsaA/B/C/D/E/F/I/J/ L/M subunits and 2 previously unidentified subunits PsaR and PsaS (Fig. 1 and Supplementary Table 1)
Summary
Diatom is an important group of marine algae and contributes to around 20% of the global photosynthetic carbon fixation. A total of 24 FCPI subunits encircle the diatom PSI core and form 3 layers (Fig. 1a, b, d). Eight FCPI subunits except FCPI-13 in this group are located in the innermost FCPI layer with direct connections to the PSI core (Fig. 1a, b), suggesting that the Lhcr-type proteins are the main peripheral antennas binding tightly to the core and bridging other FCPI subunits with the core.
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