The membrane-associated CpcG2-phycobilisome in Synechocystis: a new photosystem I antenna.

The phycobilisome (PBS) is a supramolecular antenna complex required for photosynthesis in cyanobacteria and bilin-containing red algae. While the basic architecture of PBS is widely conserved, the phycobiliproteins, core structure and linker polypeptides, show significant diversity across different species. By contrast, we recently reported that the unicellular cyanobacterium Synechocystis sp. PCC 6803 possesses two types of PBSs that differ in their interconnecting "rod-core linker" proteins (CpcG1 and CpcG2). CpcG1-PBS was found to be equivalent to conventional PBS, whereas CpcG2-PBS retains phycocyanin rods but is devoid of the central core. This study describes the functional analysis of CpcG1-PBS and CpcG2-PBS. Specific energy transfer from PBS to photosystems that was estimated for cells and thylakoid membranes based on low-temperature fluorescence showed that CpcG2-PBS transfers light energy preferentially to photosystem I (PSI) compared to CpcG1-PBS, although they are able to transfer to both photosystems. The preferential energy transfer was also supported by the increased photosystem stoichiometry (PSI/PSII) in the cpcG2 disruptant. The cpcG2 disruptant consistently showed retarded growth under weak PSII light, in which excitation of PSI is limited. Isolation of thylakoid membranes with high salt showed that CpcG2-PBS is tightly associated with the membrane, while CpcG1-PBS is partly released. CpcG2 is characterized by its C-terminal hydrophobic segment, which may anchor CpcG2-PBS to the thylakoid membrane or PSI complex. Further sequence analysis revealed that CpcG2-like proteins containing a C-terminal hydrophobic segment are widely distributed in many cyanobacteria.