| Popis: |
The rubA gene was insertionally inactivated in Synechococcus sp. PCC 7002, and the properties of photosystem I complexes were characterized spectroscopically. X-band EPR spectroscopy at low temperature shows that the three terminal iron-sulfur clusters, FX, FA, and FB, are missing in whole cells, thylakoids, and photosystem (PS) I complexes of the rubA mutant. The flash-induced decay kinetics of both P700+ in the visible and A1 − in the near-UV show that charge recombination occurs between P700+ and A1 − in both thylakoids and PS I complexes. The spin-polarized EPR signal at room temperature from PS I complexes also indicates that forward electron transfer does not occur beyond A1. In agreement, the spin-polarized X-band EPR spectrum of P700+ A1 − at low temperature shows that an electron cycle between A1 − and P700+ occurs in a much larger fraction of PS I complexes than in the wild-type, wherein a relatively large fraction of the electrons promoted are irreversibly transferred to [FA/FB]. The electron spin polarization pattern shows that the orientation of phylloquinone in the PS I complexes is identical to that of the wild type, and out-of-phase, spin-echo modulation spectroscopy shows the same P700+ to A1 − center-to-center distance in photosystem I complexes of wild type and the rubA mutant. In contrast to the loss of FX, FB, and FA, the Rieske iron-sulfur protein and the non-heme iron in photosystem II are intact. It is proposed that rubredoxin is specifically required for the assembly of the FX iron-sulfur cluster but that FX is not required for the biosynthesis of trimeric P700-A1 cores. Since the PsaC protein requires the presence of FX for binding, the absence of FA and FB may be an indirect result of the absence of FX. |
