SPECTROSCOPIC CHARACTERIZATION OF PS-I CORE COMPLEXES FROM THERMOPHILIC SYNECHOCOCCUS SP - IDENTICAL REOXIDATION KINETICS OF A(1)(-) BEFORE AND AFTER REMOVAL OF THE IRON-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 mu s. The partner of P700(+) in the 750 mu s 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 mu s. 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 F-X is an obligatory intermediate in the normal forward electron transfer.