ИСТИНА

As high-intensity solar radiation can lead to extensive damaging of photosynthetic apparatus, cyanobacteria have developed a mechanism of non-photochemical quenching (NPQ) of phycobilisome fluorescence. In Synechocystis this role is carried by the Orange Carothenoid Protein (OCP), which reacts to high-intensity light by a series of conformational changes, allowing it to bind to the phycobilisome and reduce the flow of energy into the photosystems. In order to study the mechanism of energy migration a number or of simplified systems was investigated, namely the phycobilisomes of Synechosystis PCC6803 mutants CK and ΔPSI/PSII, devoid of allophycocyanin and both photosystems accordingly. Using a single photon counting technique we have registered fluorescence decay spectra with picosecond time resolution. The studies were performed in a wide range of temperatures - from 4 to 300 K. Simultaneous light adaptation and fluorescence decay kinetics acquisition was conducted in a flow cell at room temperature. The OCP induced NPQ has been shown to be of exclusively dynamic under photon flux densities up to 1000 µE. The analysis of the fluorescence quenching dynamics indicates that the site of NPQ is the core of phycobilisome. A significant rise of energy transfer rate from allophycocyanin to terminal emitters at low temperatures has been shown, which we attribute to decreased rate of reverse energy transfer. The activation of OCP not only results fluorescence quenching, but affects the rates of energy transfer as shown by decay associated spectra analysis. It was shown that the ability of OCP to quench the fluorescence is strongly temperature dependent.