title: Characterization and evolution of tetrameric photosystem I from the thermophilic cyanobacterium Chroococcidiopsis sp TS-821.
abstract: Photosystem I (PSI) is a reaction center associated with oxygenic photosynthesis. Unlike the monomeric reaction centers in green and purple bacteria, PSI forms trimeric complexes in most cyanobacteria with a 3-fold rotational symmetry that is primarily stabilized via adjacent PsaL subunits; however, in plants/algae, PSI is monomeric. In this study, we discovered a tetrameric form of PSI in the thermophilic cyanobacterium Chroococcidiopsis sp TS-821 (TS-821). In TS-821, PSI forms tetrameric and dimeric species. We investigated these species by Blue Native PAGE, Suc density gradient centrifugation, 77K fluorescence, circular dichroism, and single-particle analysis. Transmission electron microscopy analysis of native membranes confirms the presence of the tetrameric PSI structure prior to detergent solubilization. To investigate why TS-821 forms tetramers instead of trimers, we cloned and analyzed its psaL gene. Interestingly, this gene product contains a short insert between the second and third predicted transmembrane helices. Phylogenetic analysis based on PsaL protein sequences shows that TS-821 is closely related to heterocyst-forming cyanobacteria, some of which also have a tetrameric form of PSI. These results are discussed in light of chloroplast evolution, and we propose that PSI evolved stepwise from a trimeric form to tetrameric oligomer en route to becoming monomeric in plants/algae.
authors.affiliation: Department of Biochemistry, Cellular, and Molecular Biology, University of Tenessee, Knoxville, Tenessee 37996.
authors.forename: Meng
authors.initials: M
authors.lastname: Li
authors.forename: Dmitry A
authors.initials: DA
authors.lastname: Semchonok
authors.forename: Egbert J
authors.initials: EJ
authors.lastname: Boekema
authors.forename: Barry D
authors.initials: BD
authors.lastname: Bruce
mesh_terms: D000595:Amino Acid Sequence; D001704:Biopolymers / Q000378:metabolism*; D002942:Circular Dichroism; D000458:Cyanobacteria / Q000378:metabolism*; D046529:Microscopy, Electron, Transmission; D008969:Molecular Sequence Data; D063347:Native Polyacrylamide Gel Electrophoresis; D045331:Photosystem I Protein Complex / Q000737:chemistry / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid
chemical_list: D001704:Biopolymers; D045331:Photosystem I Protein Complex