DataSheet_1_Energy transfer from phycobilisomes to photosystem I at 77 K.pdf

Phycobilisomes serve as a light-harvesting antenna of both photosystem I (PSI) and II (PSII) in cyanobacteria, yet direct energy transfer from phycobilisomes to PSI is not well documented. Here we recorded picosecond time-resolved fluorescence at wavelengths of 605–760 nm in isolated photosystem I (PSI), phycobilisomes and intact cells of a PSII-deficient mutant of Synechocystis sp. PCC 6803 at 77 K to study excitation energy transfer and trapping. By means of a simultaneous target analysis of the kinetics of isolated complexes and whole cells, the pathways and dynamics of energy transfer in vitro and in vivo were established. We establish that the timescale of the slowest equilibration between different terminal emitters in the phycobilisome is ≈800 ps. It was estimated that the terminal emitter in about 40% of the phycobilisomes transfers its energy with a rate constant of 42 ns−1 to PSI. This energy transfer rate is higher than the rates of equilibration within the phycobilisome – between the rods and the core or between the core cylinders – and is evidence for the existence of specific phycobilisome-PSI interactions. The rest of the phycobilisomes remain unconnected or slowly transferring energy to PSI.

蓝藻中的藻红蛋白体既是光合系统I（PSI）又是光合系统II（PSII）的光能捕获天线，然而，从藻红蛋白体到PSI的直接能量传递尚未得到充分记录。在本研究中，我们对来自Synechocystis sp. PCC 6803的PSII缺陷型突变体的分离光合系统I（PSI）、藻红蛋白体和完整细胞在77K下的605-760 nm波长范围内进行了皮秒时间分辨荧光记录，以研究激发能的转移和捕获。通过同时对分离复合物和全细胞的动力学进行目标分析，我们确立了体外和体内能量转移的途径与动力学。研究发现，藻红蛋白体中不同终端发射体之间最慢的平衡时间尺度约为800 ps。估算结果显示，约40%的终端发射体以42 ns−1的速率将能量传递给PSI。这一能量转移速率高于藻红蛋白体内部的平衡速率——即在棒状结构和核心之间，或核心圆柱体之间——这为藻红蛋白体-PSI特异相互作用的存在提供了证据。其余的藻红蛋白体保持未连接状态或缓慢地将能量传递给PSI。