Abstract
Generally, photosystem (PS) I complex functions as a trimer in cyanobacteria and as a monomer associated with light-harvesting chlorophyll complex in higher plants. Recently, we demonstrated a filamentous N2-fixing cyanobacterium Anabaena sp. PCC 7120 has tetramer and dimer of PSI but no trimer at all (Watanabe et al. 2010 PCP in press). Here, we improved the protocol and successfully isolated from Anabaena a supercomplex consisting of the PSI tetramer and phycocyanin. Normally, phycobiliproteins are assembled via linker polypeptides into rod and core complexes of phycobilisome (PBS), which preferentially transfers light energy to PSII. On the other hand, the Anabaena supercomplex lacked the core subcomplex of PBS based on the spectroscopic and polypeptide analyses. However, a rod-core linker polypeptide CpcG3 was identified in the supercomplex. The Anabaena genome encodes 4 copies of cpcG, while only three except CpcG3 have been detected in the conversional PBS. Since CpcG3 has a hydrophobic C-terminus, it may directly connect the rod subcomplexes with the PSI. We will also discuss the possible universal role of the antenna-PSI supercomplex in cyanobacterial photosynthesis.
