Assembly of photosystem I: II. Rubredoxin is required for the in vivo assembly of F_x in Synechococcus sp. PCC 7002 as shown by optical and EPR spectroscopy

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, F_x, F_A, and F_B, are missing in whole cells, thylakoids, and photosystem (PS) I complexes of the rubA mutant. The flashinduced decay kinetics of both P700⁺ in the visible and A₁^- in the near-UV show that charge recombination occurs between P700⁺ and A^1- 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 [F_A/F_B]. 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 F_x, F_B, and F_A, 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 F_x iron-sulfur cluster but that F_x is not required for the biosynthesis of trimeric P700-A₁ cores. Since the PsaC protein requires the presence of F_x for binding, the absence of F_A and F_B may be an indirect result of the absence of F_x.