TETRAMERIC PHOTOSYSTEM I: FROM INITIAL DISCOVERY AND CHARACTERIZATION IN CHROOCOCCIDIOPSIS SP. TS-821 TO EXPLORATION OF ITS DISTRIBUTION AND UNDERSTANDING OF ITS SIGNIFICANCE IN CYANOBACTERIA

Photosystem I (PSI) forms trimeric complexes in most characterized cyanobacteria. We had reported the tetrameric form of PSI in the unicellular cyanobacterium, Chroococcidiopsis sp. TS-821 (TS-821). Using Cryo-EM, a 3D model of the PSI tetramer structure at 11.5 [Angstrom] resolution was obtained and a 2D map within the membrane plane of at 6.1 [Angstrom]. In contrast to the three-fold symmetry in trimeric PSI crystal structure from T. elongatus, two different inter-monomer interactions involving PsaLs are found in the PSI tetramer. Phylogenetic analysis based on PsaL protein sequences shows that TS-821 is closely related to heterocyst-forming cyanobacteria. Additionally, this tetrameric form of PSI was closely correlated with a different genomic organization of the psa genes. Our results show that most heterocyst-forming cyanobacteria and close relatives have tetrameric or dimeric PSI, as distinguished from other cyanobacteria possessing mainly trimeric PSI. These cyanobacteria showed differential stability to detergent solubilization and ability to form tetrameric PSI. To investigate physiological influences on the formation of a tetrameric PSI, different environmental factors have been investigated. Our data show that nitrogen source, temperature, and salinity have little effect on the presence of tetrameric PSI. However, we observe that high light intensity tends to promote formation of the tetrameric PSI in at least three different cyanobacteria (TS-821, PCC 7414, and PCC 7428). Conversely, growth under low light promotes an increase in the trimer for strains that contain both oligomeric forms. In addition, preliminary spectral analysis indicates that the high light induced tetramer contains additional carotenoids. The evolutionary and physiological implications of multiple oligomeric forms of PSI will be discussed as well.