ENERGY-TRANSFER AND TRAPPING IN PHOTOSYSTEM-I REACTION CENTERS FROM CYANOBACTERIA

A mutant strain of the cyanobacterium Synechocystis 6803, TolE4B, was constructed by genetic deletion of the protein that links phycobilisom es to thylakoid membranes and of the CP43 and CP47 proteins of photosy stem II (PSII), leaving the photosystem I (PSI) center as the sole chr omophore in the photosynthetic membranes. Both intact membrane and det ergent-isolated samples of PSI were characterized by time-resolved and steady-state fluorescence methods. A decay component of approximate t o 25 ps dominates (99% of the amplitude) the fluorescence of the membr ane sample. This result indicates that an intermediate lifetime is not associated with the intact membrane preparation and the charge separa tion in PSI is irreversible. The decay time of the detergent-isolated sample is similar. The 600-nm excited steady-state fluorescence spectr um displays a red fluorescence peak at approximate to 703 nm at room t emperature. The 450-nm excited steady-state fluorescence spectrum is d ominated by a single peak around 700 nm without 680-nm ''bulk'' fluore scence. The experimental results were compared with several computer s imulations. Assuming an antenna size of 130 chlorophyll molecules, an apparent charge separation time of approximate to 1 ps is estimated. A lternatively, the kinetics could be modeled on the basis of a two-doma in antenna for PSI, consistent with the available structural data, eac h containing approximate to 65 chlorophyll a molecules. If excitation can migrate freely within each domain and communication between domain s occurs only close to the reaction center, a charge separation time o f 3-4 ps is obtained instead.