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.

蓝藻中的藻蓝素体充当光合系统I（PSI）和II（PSII）的光能捕获天线，然而藻蓝素体与PSI之间的直接能量转移尚未得到充分记录。在本研究中，我们记录了来自Synechocystis sp. PCC 6803的PSII缺陷突变体中分离的光合系统I（PSI）、藻蓝素体及完整细胞的皮秒时间分辨荧光光谱，波长范围为605-760 nm，在77 K下研究激发能的转移与捕获。通过同时分析分离复合物和整个细胞的动力学，我们建立了体外及体内的能量转移途径和动力学。我们确定藻蓝素体中不同末端发射体之间最慢的平衡时间尺度约为800 ps。估计大约40%的藻蓝素体末端发射体以42 ns−1的速率将能量转移到PSI。这种能量转移速率高于藻蓝素体内部（棒状结构与其核心之间，或核心圆柱体之间）的平衡速率，这为藻蓝素体-PSI特异性相互作用的存证。其余的藻蓝素体则保持未连接或缓慢将能量转移到PSI。