Assembly of photosystem III.: Rubredoxin is required for the in vivo assembly of Fx 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 photosystein (PS) I complexes of the rubA mutant. The flashinduced decay kinetics of both P700(+) in the visible and A(1)(-) in the near-UV show that charge recombination occurs between P700degrees 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 A, In agreement, the spin-polarized X-band EPR spectrum of P700(+) A(1)(-) at low temperature shows that an electron cycle between A(1)(-) and P700degrees 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 P700degrees to A(1)(-) 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 11 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-A(1) cores. Since the PsaC protein requires the presence of Fx for binding, the absence of F-A and F-B, may be an indirect result of the absence of F-X.