Diversity Among Cyanobacterial Photosystem I Oligomers

Abstract

Unraveling the oligomeric states of the photosystem I complex is essential to understanding the evolution and native mechanisms of photosynthesis. The molecular composition and functions of this complex are highly conserved among cyanobacteria, algae, and plants; however, its structure varies considerably between species. In cyanobacteria, the photosystem I complex is a trimer in most species, but monomer, dimer and tetramer arrangements with full physiological function have recently been characterized. Higher order oligomers have also been identified in some heterocyst-forming cyanobacteria and their close unicellular relatives. Given technological progress in cryo-electron microscope single particle technology, structures of PSI dimers, tetramers and some heterogeneous supercomplexes have been resolved into near atomic resolution. Recent developments in photosystem I oligomer studies have largely enriched theories on the structure and function of these photosystems.

Keywords: cyanobacteria (blue-green algae); oligomers states; photosystem I; structure; supercomplexes organization.

FIGURE 1

Top (A) and side (B) views of subunits composition in monomeric photosystem I complex from thermophilic cyanobacterium Thermosynechococcus elongatus (PDB: 1JB0). There are nine transmembrane proteins (PsaA, PsaB, PsaF, PsaJ, PsaK, PsaL, PsaM, PsaI, and PsaX) in which most of cofactors are non-charged bonded to perform light harvesting, light-to-electron energy conversion, and excitation transport. And three out membrane proteins (PsaC, PsaD, and PsaE) are located in stromal side and responsible for the electron transfer from PSI complex to the soluble acceptors.

FIGURE 2

PsaL structure comparison among monomeric, dimeric and trimeric states of PSI complex. (A) Structure differences of PsaL between PSI monomer and dimer; (B) structure differences between of PsaL between PSI monomer and trimer; (C) structure differences of PsaL between PSI dimer and trimer. The losing fragments and extended loop regions were marked by red and purple rectangles, respectively. PSI monomer (PDB code: 6LU1) and trimer (PDB code: 1JB0) from Thermosynechococcus elongatus and PSI tetramer (PDB code: 6K61) from Anabaena sp. PCC 7120 were source molecules from which PsaL structures were extracted.

FIGURE 3

Photosystem I diversities in cyanobacteria. (A) Model of PSI dimer in Anabaena sp. PCC 7120 (PDB code: 6K61). (B) Model of PSI trimer in Thermosynechococcus elongatus (PDB code:1JB0). (C) Model of PSI tetramer in Anabaena sp. PCC 7120 (PDB code: 6TCL). (D) Predicted 3D map of PSI hexamer architecture. The interface involves subunits of PsaA, PsaB, Psak, PsaI, PsaM, and PsaL were colored and labeled separately. (E) Cryo-EM single particle density map of two face-to-face associated PSI tetramer complex. (F) Predicted 3D map of PSI octamer architecture. It should be realized that the models revealed in Figures 3A–C,E are experimental resolved and reported in the literature, while models revealed in Figures 3D,F are speculated according to the acknowledged structure basis of PSI dimer, trimer and tetramer.