PHOTOELECTRIC CHARACTERIZATION OF FORWARD ELECTRON-TRANSFER TO IRON-SULFUR CENTERS IN PHOTOSYSTEM-I

The photoelectric response of oriented PS I membranes from the cyanoba cterium Synechocystis 6803 has been investigated in the nanosecond tim e range. Besides an unresolved rapidly rising phase, there is a furthe r positive electrogenic phase with a rise time constant of 220 +/- 20 ns, The amplitude of the 220-ns phase is 66 +/- 10% that of the subnan osecond phase. The fast phase contains two kinetic components faster t han 100 ps, which have recently been resolved and attributed to primac y charge separation (P(+)A(o)(-) formation) and subsequent electron tr ansfer to A(1), respectively (Hecks, B., Wulf, K., Breton, J., Leibl, W., and Trissl, H.-W, (1994) Biochemistry 33, 8619-8624), The 220-ns p hase is lost under conditions where iron-sulfur centers F-A, F-B, and F-X are prereduced, and its kinetics match the reoxidation kinetics of A(1)(-) as verified by absorbance change measurements at 380 nm. Ther efore, this electrogenic phase is attributed to electron transfer to t he iron-sulfur centers that function as further electron accepters in the PS I reaction center. Gradual removal of F-A and F-B by urea treat ment reveals that the amplitude of the 220-ns phase is linearly correl ated with the fraction of F-A,F-B present. However, complete removal o f F-A,F-B does not lead to a complete loss of the nanosecond phase but reduces its amplitude by more than a factor of 2 to yield an amplitud e of 25-30% relative to the initial picosecond rise, with only a sligh t change in kinetics. The residual amplitude is further reduced when a large fraction of F-X is removed. These results show that electron tr ansfer from A(1) to F-X as well as from A(1) to F-A,F-B is electrogeni c. They support a model where F-X is an intermediate electron acceptor between A(1) and F-A,F-B and suggest that the reduction of F-X is the rate-limiting process for the reduction of F-A,F-B The relative ampli tudes of the electrogenic phases allow an estimation of the dielectric ally weighted transmembrane distances between the electron carriers in PS I. A(1) is found to be almost centrally positioned between A(o) an d F-X. This structural information is compared to the structure emergi ng from X-ray crystallography.