Abstract
Far-red light photoacclimation exhibited by some cyanobacteria allows these organisms to use the far-red region of the solar spectrum (700–800 nm) for photosynthesis. Part of this process includes the replacement of six photosystem I (PSI) subunits with isoforms that confer the binding of chlorophyll (Chl) f molecules that absorb far-red light (FRL). However, the exact sites at which Chl f molecules are bound are still challenging to determine. To aid in the identification of Chl f-binding sites, we solved the cryo-EM structure of PSI from far-red light-acclimated cells of the cyanobacterium Synechococcus sp. PCC 7335. We identified six sites that bind Chl f with high specificity and three additional sites that are likely to bind Chl f at lower specificity. All of these binding sites are in the core-antenna regions of PSI, and Chl f was not observed among the electron transfer cofactors. This structural analysis also reveals both conserved and nonconserved Chl f-binding sites, the latter of which exemplify the diversity in FRL-PSI among species. We found that the FRL–PSI structure also contains a bound soluble ferredoxin, PetF1, at low occupancy, which suggests that ferredoxin binds less transiently than expected according to the canonical view of ferredoxin-binding to facilitate electron transfer. We suggest that this may result from structural changes in FRL-PSI that occur specifically during FRL photoacclimation.
Highlights
Photosystem I (PSI) is a plastocyanin:ferredoxin photooxidoreductase that is essential for the light reactions of oxygenic photosynthesis [1, 2]
When grown in white light (WL), all Chls found in PSI are Chl a, but when grown in far-red light (FRL), 10% of the Chls are replaced by Chl f in the PSI complexes of Far-red light photoacclimation (FaRLiP)-capable cyanobacteria [11]
The FRLacclimated PSI (FRL-PSI) structural model from H. hongdechloris lacked PsaF2, PsaJ2, and Fd which are present in low occupancy in Synechococcus 7335 FRL-PSI
Summary
FRL-PSI was isolated from FRL-acclimated Synechococcus 7335 cells and plunge-frozen for cryo-EM, as described in Experimental procedures. In T. elongatus [3] and Fischerella 7521 [14, 15] that have the psaX gene (which is lacking in Leptolyngbya-like strains such as Synechococcus 7335 and H. hongdechloris), the Chl B40 binding site corresponds to that for the stromal half of the single transmembrane helix of subunit PsaX found in those PSI structures. The structure-based view of sequence similarity resembles that observed previously when the same procedure was carried out for Fischerella 7521 FRL-PSI [14] In both organisms, especially low sequence similarity is present: (A) in PsaA2, near the monomer-monomer interface, (B) near the center of the trimer where PsaL2 and PsaI2 are located, and (C) near the peripheral PsaF2 and PsaJ2 subunits, especially on the lumenal side. We measured the distances between the Fe-S clusters (Fig. 4C) and found that they most closely resemble the Fd from the T. elongatus PSI-Fd X-ray crystal structure that exhibited the shortest FB to [2Fe-2S] distance [41] (Table S4)
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