Spectroscopic characterization of PS I core complexes from thermophilic Synechococcus sp. Identical reoxidation kinetics of A^-₁ before and after removal of theiron-sulfur-clusters F_A and F_B

Monomeric and trimeric PS I complexes missing the three stromal subunits E,C and D (termed PS I core complexes) were prepared from the thermophilic cyanobacterium Synechococcus sp. by incubation with urea. The subunits E,C and D are sequentially removed. In the monomeric PS I the subunit C is removed with a half life of approx. 5 min. This is about eight times faster than in the trimeric PS I complex. In parallel with the removal of the F _{A B} containing subunit C the reduction kinetics of P700⁺ changed from a half life of about 25 ms to about 750 μs. The partner of P700⁺ in the 750 μs charge recombination was identified to be F_X by the difference spectrum of this phase. There are some minor differences in the spectra of trimeric and monomeric PS I core complexes. At 77K the forward electron transfer from A ^-₁ to F_X is blocked in the major fraction of the PS I core complexes and P700⁺A^-₁ recombines with a half life of about 220 μs. In the remaining fraction P700⁺F_X^- is formed and decays with a half life of approx. 10 ms at 77 K. The kinetics of the forward electron transfer from A^-₁ to the iron-sulfur-clusters was measured in the native PS I and the corresponding core complexes. The reoxidation kinetics of A^-₁ are identical in both cases (t _{1 2} = 180 ns). We conclude that Fx is an obligatory intermediate in the normal forward electron transfer.