PATH OF ELECTRON-TRANSFER IN PHOTOSYSTEM-1 - DIRECT EVIDENCE OF FORWARD ELECTRON-TRANSFER FROM A(1) TO FE-SX

Pulsed EPR spectroscopy and selective removal of the iron-sulfur centers in photosystem 1 have been used to study forward electron transfer from the secondary electron acceptor A(1). At cryogenic temperatures where forward electron transfer is inhibited, we have observed a g = 2.003 electron spin-echo signal presenting a characteristic phase shift. This out-of-phase signal is attributed to the electron spin-polarized pair P-700(+)/A(1)(-), if decays with t(1/e) = 23 mu s, reflecting the recombination reaction. At room temperature the out-of-phase signal is also observed, but it decays with t(1/e), = 200 ns in untreated photosystem 1, due to forward electron transfer from A(1)(-) to one of the iron-sulfur centers. This rate is unchanged in Fe-S-A/B-depleted PS1 but is lost when the iron-sulfur center Fe-Sx is removed. In the preparations depleted of all iron-sulfur centers the out-of-phase signal decays with t(1/e) = 1.3 mu s, reflecting either the back reaction or the decay of polarization. These results demonstrate that the electron transfer pathway in photosystem 1 is P-700 --> A(1) --> Fe-S-X --> Fe-S-A/B.