title: Attachment of phycobilisomes in an antenna-photosystem I supercomplex of cyanobacteria.
abstract: Oxygenic photosynthesis is driven by photosystems I and II (PSI and PSII, respectively). Both have specific antenna complexes and the phycobilisome (PBS) is the major antenna protein complex in cyanobacteria, typically consisting of a core from which several rod-like subcomplexes protrude. PBS preferentially transfers light energy to PSII, whereas a PSI-specific antenna has not been identified. The cyanobacterium Anabaena sp. PCC 7120 has rod-core linker genes (cpcG1-cpcG2-cpcG3-cpcG4). Their products, except CpcG3, have been detected in the conventional PBS. Here we report the isolation of a supercomplex that comprises a PSI tetramer and a second, unique type of a PBS, specific to PSI. This rod-shaped PBS includes phycocyanin (PC) and CpcG3 (hereafter renamed "CpcL"), but no allophycocyanin or CpcGs. Fluorescence excitation showed efficient energy transfer from PBS to PSI. The supercomplex was analyzed by electron microscopy and single-particle averaging. In the supercomplex, one to three rod-shaped CpcL-PBSs associate to a tetrameric PSI complex. They are mostly composed of two hexameric PC units and bind at the periphery of PSI, at the interfaces of two monomers. Structural modeling indicates, based on 2D projection maps, how the PsaI, PsaL, and PsaM subunits link PSI monomers into dimers and into a rhombically shaped tetramer or "pseudotetramer." The 3D model further shows where PBSs associate with the large subunits PsaA and PsaB of PSI. It is proposed that the alternative form of CpcL-PBS is functional in harvesting energy in a wide number of cyanobacteria, partially to facilitate the involvement of PSI in nitrogen fixation.
authors.affiliation: Department of Life Sciences (Biology), Graduate School of Arts and Sciences, University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan.
authors.forename: Mai
authors.initials: M
authors.lastname: Watanabe
authors.forename: Dmitry A
authors.initials: DA
authors.lastname: Semchonok
authors.forename: Mariam T
authors.initials: MT
authors.lastname: Webber-Birungi
authors.forename: Shigeki
authors.initials: S
authors.lastname: Ehira
authors.forename: Kumiko
authors.initials: K
authors.lastname: Kondo
authors.forename: Rei
authors.initials: R
authors.lastname: Narikawa
authors.forename: Masayuki
authors.initials: M
authors.lastname: Ohmori
authors.forename: Egbert J
authors.initials: EJ
authors.lastname: Boekema
authors.forename: Masahiko
authors.initials: M
authors.lastname: Ikeuchi
mesh_terms: D017033:Anabaena / Q000378:metabolism*; D002458:Cell Fractionation; D016000:Cluster Analysis; D015151:Immunoblotting; D008854:Microscopy, Electron; D008958:Models, Molecular; D045331:Photosystem I Protein Complex / Q000378:metabolism*; D045524:Phycobilisomes / Q000378:metabolism*; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence
chemical_list: D045331:Photosystem I Protein Complex; D045524:Phycobilisomes