Evidence for the presence of a [2Fe-2S] ferredoxin in bean sprouts

Ferredoxin-dependent CO2 fixation in bean sprouts

Extracts of bean sprouts are capable of reducing ferredoxin and of catalyzing the incorporation of bicarbonate and acetyl coenzyme A into an organic compound that is likely to be pyruvate, in a reaction that requires reduced ferredoxin. The rate of the reaction, the first known for which ferredoxin appears to serve as the direct reductant for CO2 fixation in a higher plant, depends on the concentrations of both ferredoxin and bicarbonate, with half-maximal rates being observed at ferredoxin and bicarbonate concentrations of 0.8 microM and 200 microM, respectively.

Characterization of two low-potential cytochromes from bean sprouts

Two cytochromes have been isolated from chlorophyll-free bean sprouts, purified and characterized. The more abundant cytochrome was purified to apparent homogeneity and exhibits visible region absorbance maxima at 416, 520 and 550 nm in the reduced form and at 410 and 530 nm in the oxidized form. Although Resonance Raman spectra of this cytochrome closely resemble those of c-type cytochromes, pyridine hemochromogen analysis suggests that this cytochrome may contain a variant of heme c as its prosthetic group. The cytochrome has an apparent molecular mass of 12.5 kDa, an isoelectric point > 9.0 and a midpoint oxidation-reduction potential (Em) of -130 mV at pH 8.0. The less abundant of the two cytochromes, which was not completely purified, exhibits absorbance maxima at 438 and 560 nm in the reduced form and at 411 nm in the oxidized form and was shown to contain heme c as a prosthetic group. This cytochrome, which may also contain FAD, has an apparent molecular mass of approx. 38 kDa, an isoelectric point > 9.0 and Em = -300 mV. Preliminary results indicate that both cytochromes can form electrostatically-stabilized complexes with ferredoxin, suggesting the possibility that one or both of the cytochromes may participate in low-potential, non-photosynthetic electron transfer pathways involving ferredoxin.

Promoter and leader regions involved in the expression of the Arabidopsis ferredoxin A gene

Ferredoxin is a nuclear-encoded protein that is involved in a variety of electron transfer reactions in both photosynthetic and non-photosynthetic plastids. We show here that the expression of the ferredoxin A gene (FedA) in Arabidopsis thaliana is light-regulated, with its mRNA level increased 4.5-fold by transfer of dark-grown seedlings to white light for 3 h. A portion of this light regulation is mediated by phytochrome through a very low fluence type of response. In addition, it is likely that another photoreceptor(s) is also involved. The FedA promoter confers a light- and tissue-regulated expression pattern when fused to the beta-glucuronidase (GUS) and luciferase reporter genes, indicating that the gene is transcriptionally regulated. No evidence of cis-acting light-regulatory elements within the 5' untranslated leader region of the gene was detected. Nevertheless, elements within this leader are required for full activity since its deletion reduces expression both in the light and dark by 25-fold. This region includes a sequence, ACAAAA, which is also present in the 5' untranslated leader of the other three ferredoxin leaders that have been sequenced and in the leaders of 31 other plant genes or about 8% of all plant genes in the GenBank database. In addition to mediating light regulation, the FedA promoter also directs GUS expression in aerial tissues at 70-fold higher levels than in roots. GUS activity staining in aerial tissues is observed in both photosynthetic and non-photosynthetic cells. These data indicate that the FedA promoter also carries the information for expression of this gene in a highly tissue- and cell-specific manner.

The interaction of ferredoxin and glutamate synthase: cross-linking and immunological studies

The water-soluble carbodiimide, N-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) serves as an effective reagent for cross-linking spinach leaf ferredoxin and the ferredoxin-dependent spinach leaf enzyme, glutamate synthase. The cross-linked complex was functional in the absence of added ferredoxin, suggesting that ferredoxin is cross-linked to glutamate synthase at the physiological binding site on the enzyme for this iron-sulfur protein electron donor. The ferredoxin:glutamate synthase stoichiometry of the cross-linked complex was estimated to be 2:1. The absorbance spectrum of the oxidized, cross-linked complex was very similar to that of an electrostatically stabilized, noncovalent, 2:1 complex of the two proteins. An antibody raised against spinach NADP+ reductase, which recognizes a ferredoxin-binding site on glutamate synthase, does not recognize the cross-linked ferredoxin-glutamate synthase complex. This implies that the ferredoxin-binding sites on the two enzymes are structurally similar enough so that an antibody raised against one of these ferredoxin-dependent enzymes recognizes an epitope at the ferredoxin-binding site of the second enzyme. Cross-linking of ferredoxin to its binding site on glutamate synthase renders this epitope inaccessible to the antibody.

Novel forms of ferredoxin and ferredoxin-NADP reductase from spinach roots

Ferredoxin and the enzyme catalyzing its reduction by NADPH, ferredoxin-NADP reductase (ferredoxin-NADP+ oxidoreductase or FNR), were found to be present in roots of spinach (Spinacia oleracea). Localization experiments with endosperm of germinating castor beans (Ricinus communis), a classical nonphotosynthetic tissue for cell fractionation studies, confirmed that ferredoxin and FNR are localized in the plastid fraction. Both proteins were purified from spinach roots and found to resemble their leaf counterparts in activity, spectral properties, and complex formation, but to differ in amino acid composition and amino terminal sequence. The results indicate that the primary structures of the FNR and ferredoxin of spinach roots differ from that of the corresponding leaf proteins. Together with earlier findings, the present results provide evidence that nonphotosynthetic plastids, including those of roots, are capable of reducing ferredoxin with heterotrophically generated NADPH.

Characterization of a ferredoxin:NADP+ oxidoreductase from a nonphotosynthetic plant tissue

A flavoprotein with properties similar to those of ferredoxin:NADP+ oxidoreductases found in the leaves of higher plants has been purified to apparent homogeneity from bean sprouts, a nonphotosynthetic plant tissue. The absorbance and circular dichroism spectra of the bean sprout protein are similar to those of spinach leaf ferredoxin:NADP+ oxidoreductase and an antibody raised against the spinach enzyme recognized the bean sprout enzyme. The bean sprout enzyme catalyzed ferredoxin-dependent electron transfer from NADPH to equine cytochrome c at a high rate but, unlike the spinach enzyme, exhibited little NADPH to 2,6-dichlorophenol indophenol diaphorase activity. The bean sprout enzyme forms a 1:1 electrostatically stabilized complex with ferredoxins isolated from either bean sprouts or spinach leaves.