Direct Energy Transfer from Allophycocyanin-Free Rod-Type CpcL-Phycobilisome to Photosystem I

Phycobilisomes (PBSs) are photosynthetic antenna megacomplexes comprised of pigment-binding proteins (cores and rods) joined with linker proteins. A rod-type PBS that does not have a core is connected to photosystem I (PSI) by a pigment-free CpcL linker protein, which induces a red-shift of the absorption band of phycocyanobilin (PCB) in the rod (red-PCB). Herein, the isolated supercomplex of the rod-type PBS and the PSI tetramer from Anabaena sp. PCC 7120 were probed by picosecond laser spectroscopy at 77 K and by decay-associated spectral analysis to show that red-PCB mediates the fast (time constant = 90 ps) and efficient (efficiency = 95%) transfer of excitation energy from PCB in rod to chlorophyll a (Chl a) in PSI. According to the Forster energy transfer mechanism, this high efficiency corresponds to a 4-nm distance between red-PCB and Chl a, suggesting that {beta}-84 PCB in rod acts as red-PCB. TOC GRAPHIC O_FIG_DISPLAY_L [Figure 1] M_FIG_DISPLAY C_FIG_DISPLAY