Type I reaction centers (RCs) are multisubunit chlorophyll-protein complexes that function in photosynthetic organisms to convert photons to Gibbs free energy. The unique feature of Type I RCs is the presence of iron-sulfur clusters as electron transfer cofactors. Photosystem I (PS I) of oxygenic phototrophs is the best-studied Type I RC. It is comprised of an interpolypeptide [4Fe-4S] cluster, F(X), that bridges the PsaA and PsaB subunits, and two terminal [4Fe-4S] clusters, F(A) and F(B), that are bound to the PsaC subunit. In this review, we provide an update on the structure and function of the bound iron-sulfur clusters in Type I RCs. The first new development in this area is the identification of F(A) as the cluster proximal to F(X) and the resolution of the electron transfer sequence as F(X)-->F(A)-->F(B)-->soluble ferredoxin. The second new development is the determination of the three-dimensional NMR solution structure of unbound PsaC and localization of the equal- and mixed-valence pairs in F(A)(-) and F(B)(-). We provide a survey of the EPR properties and spectra of the iron-sulfur clusters in Type I RCs of cyanobacteria, green sulfur bacteria, and heliobacteria, and we summarize new information about the kinetics of back-reactions involving the iron-sulfur clusters.

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Vitamin K1 (VK1) is an important cofactor of the electron-transfer chain in photosystem I (PS I), referred to as A1. The special properties of this quinone result from its unique interaction(s) with the protein surrounding. In particular, a single H-bond to neutral A1 was identified previously in the X-ray crystal structure of PS I. During light-induced electron transfer in PS I, A1 is transiently reduced to the radical anion A1*-. In this work, we characterized the electron spin density distribution of A1*- with the aim of understanding the influence of the protein surrounding on it. We studied the light-induced spin-polarized radical pair P700*+A1*- and the photoaccumulated radical anion A1*-, using advanced pulse EPR, ENDOR, and TRIPLE techniques at Q-band (34 GHz). Exchange with fully deuterated quinone in the A1 binding site allowed differentiation between proton hyperfine couplings from the quinone and from the protein surrounding. In addition, DFT calculations on a model of the A1 site were performed and provided proton hyperfine couplings that were in close agreement with the ones determined experimentally. This combined approach allowed the assignment of proton hyperfine coupling tensors to molecular positions, thereby yielding a picture of the spin density distribution in A1*-. Comparison with VK1*- in organic solvents (Epel et al. J. Phys. Chem. B 2006, 110, 11549.) leads to the conclusion that the single H-bond present in both the radical pair P700*+A1*- and the photoaccumulated radical anion A1*- is, indeed, the crucial factor that governs the electronic structure of A1*-.

Chlorosomes of the green sulfur bacterium Chlorobium tepidum have previously been shown to contain at least 10 polypeptides [Chung, S., Frank, G., Zuber, H., and Bryant, D. A. (1994) Photosynth. Res. 41, 261-275]. Based upon the N-terminal amino acid sequences determined for two of these proteins, the corresponding genes were isolated using degenerate oligonucleotide hybridization probes. The csmI and csmJ genes encode proteins of 244 and 225 amino acids, respectively. A third gene, denoted csmX, that predicts a protein of 221 amino acids with strong sequence similarity to CsmI and CsmJ, was found to be encoded immediately upstream from the csmJ gene. All three proteins have strong sequence similarity in their amino-terminal domains to [2Fe-2S] ferredoxins of the adrenodoxin/putidaredoxin subfamily of ferredoxins. CsmI and CsmJ were overproduced in Escherichia coli, and both proteins were shown by EPR spectroscopy to contain iron-sulfur clusters. The g-tensor and relaxation properties are consistent with their assignment as [2Fe-2S] clusters. Isolated chlorosomes were also shown to contain [2Fe-2S] clusters whose properties were similar to those of the recombinant CsmI and CsmJ proteins. Redox titration of isolated chlorosomes showed these clusters to have potentials of about -201 and +92 mV vs SHE. The former potential is similar to that measured by redox titration of the clusters in inclusion bodies of CsmJ. Possible roles for these iron-sulfur proteins in electron transport and light harvesting are discussed.

Photosystem I (PS I) converts the energy of light into chemical energy via transmembrane charge separation. The terminal electron transfer cofactors in PS I are three low-potential [4Fe-4S] clusters named F(X), F(A) and F(B), the last two are bound by the PsaC subunit. We have modelled the F(A) and F(B) binding sites by preparing two apo-peptides (maquettes), sixteen amino acids each. These model peptides incorporate the consensus [4Fe-4S] binding motif along with amino acids from the immediate environment of the iron-sulfur clusters F(A) and F(B). The [4Fe-4S] clusters were successfully incorporated into these model peptides, as shown by optical absorbance, EPR and Mössbauer spectroscopies. The oxidation-reduction potential of the iron-sulfur cluster in the F(A)-maquette is -0.44+/-0.03 V and in the F(B)-maquette is -0.47+/-0.03 V. Both are close to that of F(A) and F(B) in PS I and are considerably more negative than that observed for other [4Fe-4S] model systems described earlier (Gibney, B. R., Mulholland, S. E., Rabanal, F., and Dutton, P. L. Proc. Natl. Acad. Sci. U.S.A. 93 (1996) 15041-15046). Our optical data show that both maquettes can irreversibly bind to PS I complexes, where PsaC-bound F(A) and F(B) were removed, and possibly participate in the light-induced electron transfer reaction in PS I.

Chemical rescue of site-modified amino acids using externally supplied organic molecules represents a powerful method to investigate structure-function relationships in proteins. Here we provide definitive evidence that aryl and alkyl thiolates, reagents typically used for in vitro iron-sulfur cluster reconstitutions, serve as rescue ligands to a site-specifically modified [4Fe-4S](1+,2+) cluster in PsaC, a bacterial dicluster ferredoxin-like subunit of Photosystem I. PsaC binds two low-potential [4Fe-4S](1+,2+) clusters termed F(A) and F(B). In the C13G/C33S variant of PsaC, glycine has replaced cysteine at position 13 creating a protein that is missing one of the ligating amino acids to iron-sulfur cluster F(B). Using a variety of analytical techniques, including non-heme iron and acid-labile sulfur assays, and EPR, resonance Raman, and Mössbauer spectroscopies, we showed that the C13G/C33S variant of PsaC binds two [4Fe-4S](1+,2+) clusters, despite the absence of one of the biological ligands. (19)F NMR spectroscopy indicated that the external thiolate replaces cysteine 13 as a substitute ligand to the F(B) cluster. The finding that site-modified [4Fe-4S](1+,2+) clusters can be chemically rescued with external thiolates opens new opportunities for modulating their properties in proteins. In particular, it provides a mechanism to attach an additional electron transfer cofactor to the protein via a bound, external ligand.

A 4.4-kb HindIII fragment, encoding an unusual rubredoxin (denoted RubA), a homolog of the Synechocystis sp. PCC 6803 gene slr2034 and Arabidopsis thaliana HCF136, and the psbEFLJ operon, was cloned from the cyanobacterium Synechococcus sp. PCC 7002. Inactivation of the slr2034 homolog produced a mutant with no detectable phenotype and wild-type photosystem (PS) II levels. Inactivation of the rubA gene of Synechococcus sp. PCC 7002 produced a mutant unable to grow photoautotrophically. RubA and PS I electron transport activity were completely absent in the mutant, although PS II activity was ϳ80% of the wild-type level. RubA contains a domain of ϳ50 amino acids with very high similarity to the rubredoxins of anaerobic bacteria and archaea, but it also contains a region of about 50 amino acids that is predicted to form a flexible hinge and a transmembrane ␣-helix at its C terminus. Overproduction of the water-soluble rubredoxin domain in Escherichia coli led to a product with the absorption and EPR spectra of typical rubredoxins. RubA was present in thylakoid but not plasma membranes of cyanobacteria and in chloroplast thylakoids isolated from spinach and Chlamydomonas reinhardtii. Fractionation studies suggest that RubA might transiently associate with PS I monomers, but no evidence for an association with PS I trimers or PS II was observed. PS I levels were significantly lower than in the wild type (ϳ40%), but trimeric PS I complexes could be isolated from the rubA mutant. These PS I complexes completely lacked the stromal subunits PsaC, PsaD, and PsaE but contained all membrane-intrinsic subunits. The three missing proteins could be detected immunologically in whole cells, but their levels were greatly reduced, and degradation products were also detected. Our results indicate that RubA plays a specific role in the biogenesis of PS I.

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 A 1 ؊ 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 A 1 . 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 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 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 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-A 1 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 .