Regulation of excitation energy transfer in organisms containing phycobilins.

The mechanism of excitation energy redistribution (state transition) in organisms containing phycobilins is reviewed. Recent measurements using time-resolved fluorescence spectroscopy in the picosecond range confirm that the state transition in cyanobacteria and red algae is controlled by changes in the kinetics of energy transfer from PS 2 to PS 1 (spillover) rather than by physical dislocation of the phycobilisome and reassociation between the two photosystems (mobile antenna model). Contrary to the analogous situation in higher plants, there is no compelling evidence for the involvement of a protein phosphorylation event in the rapid time range of the state transition, but a variety of data indicate that a membrane conformational change occurs that might change the relative distance between, and/or orientation of the two photosystems within the thylakoid. The state transition is most probably initiated by the redox state of the intersystem electron transport chain, and the conversion to state 1 is driven by coupled PS1 cyclic electron transport. The cryptomonads also undergo wavelength dependent changes in excitation energy distribution by a mechanism very similar to that observed in the red algae and cyanobacteria. However, the changes in energy distribution in this group are most likely related to a photoprotection mechanism for PS2 rather than to a state transition.