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 flashinduced 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.
All Science Journal Classification (ASJC) codes
- Molecular Biology
- Cell Biology