KINETICS OF EXCITATION-ENERGY TRANSFER IN THE CYANOBACTERIAL PHYCOBILISOME-PHOTOSYSTEM-II COMPLEX

Fluorescence decays on a Picosecond timescale were recorded at various excitation and emission wavelengths for intact cells of the cyanobacterium Synechococcus 6301. Fluorescence decays were measured by single photon timing and resolved into exponential components by global data analysis. Exponential fluorescence decay components were assigned to PS II, PS I and components of the phycobilisome according to their excitation spectra as determined from a time-resolved excitation-emission matrix and according to the effects of PS II reaction centre closure on the lifetimes and relative amplitudes. We have used the experimentally observed decay components from PS II and the phycobilisome core to estimate the effective rate constants for energy transfer within the phycobilisome/PS II complex and for charge separation at PS II. We show that the rate-limiting step for energy migration in the phycobilisome/PS II complex is a relatively slow energy transfer step from the phycobilisome terminal emitters to the PS II core complex. These results also support the idea that adaptation of cells to light-state 2 decreases the extent of energy transfer from phycobilisomes to PS II.