Assembly of photosystem III. 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 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.Biochemistry & Molecular BiologySCI(E)51ARTICLE2320355-2036627