Purification, some properties and amino acid sequence of Thermus thermophilus HB8 ferredoxin

Rapid and Sensitive Amino-Acid Sequencing of Cloning Thermus thermophilus HB8 Ferredoxin by Proteomics

Recombinant holo Thermus thermophilus [7Fe-8S] ferredoxin was synthesized by cloning from Thermus thermophilus HB8 gene. A specific sequence (Pro-His-Val-Ile) at the N-terminus of the recombinant ferredoxin was determined by a rapid and highly sensitive mass spectral method using a novel Ru(II) Edman reagent, [(tpy)Ru(tpy-C6H4-NCS)](PF6)2 (tpy=terpyridine). The formation of the recombinant holoTtFd was established by the characteristic absorptions and CD extrema as [7Fe-8S] ferredoxin. The catalytic electron-transfer reactivity of the [7Fe-8S] ferredoxin between ferredoxin-NADP+ reductase and cytochrome c was recognized.

Structural conservation of the isolated zinc site in archaeal zinc-containing ferredoxins as revealed by x-ray absorption spectroscopic analysis and its evolutionary implications

The zfx gene encoding a zinc-containing ferredoxin from Thermoplasma acidophilum strain HO-62 was cloned and sequenced. It is located upstream of two genes encoding an archaeal homolog of nascent polypeptide-associated complex alpha subunit and a tRNA nucleotidyltransferase. This gene organization is not conserved in several euryarchaeoteal genomes. The multiple sequence alignments of the zfx gene product suggest significant sequence similarity of the ferredoxin core fold to that of a low potential 8Fe-containing dicluster ferredoxin without a zinc center. The tightly bound zinc site of zinc-containing ferredoxins from two phylogenetically distantly related Archaea, T. acidophilum HO-62 and Sulfolobus sp. strain 7, was further investigated by x-ray absorption spectroscopy. The zinc K-edge x-ray absorption spectra of both archaeal ferredoxins are strikingly similar, demonstrating that the same zinc site is found in T. acidophilum ferredoxin as in Sulfolobus sp. ferredoxin, which suggests the structural conservation of isolated zinc binding sites among archaeal zinc-containing ferredoxins. The sequence and spectroscopic data provide the common structural features of the archaeal zinc-containing ferredoxin family.

Delta T 14/Delta D 15 Azotobacter vinelandii ferredoxin I: creation of a new CysXXCysXXCys motif that ligates a [4Fe-4S] cluster

In clostridial-type ferredoxins, each of the two [4Fe-4S]2+/+ clusters receives three of its four ligands from a CysXXCysXXCys motif. Azotobacter vinelandii ferredoxin I (AvFdI) is a seven-iron ferredoxin that contains one [4Fe-4S]2+/+ cluster and one [3Fe-4S]+/0 cluster. During the evolution of the 7Fe azotobacter-type ferredoxins from the 8Fe clostridial-type ferredoxins, one of the two motifs present changed to a CysXXCysXXXXCys motif, resulting in the inability to form a 4Fe cluster and the appearance of a 3Fe cluster in that position. In a previous study, we were unsuccessful in using structure as a guide in designing a 4Fe cluster in the 3Fe cluster position of AvFdI. In this study, we have reversed part of the evolutionary process by deleting two residues between the second and third cysteines. UV/Vis, CD, and EPR spectroscopies and direct electrochemical studies of the purified protein reveal that this DeltaT14/DeltaD15 FdI variant is an 8Fe protein containing two [4Fe-4S]2+/+ clusters with reduction potentials of -466 and -612 mV versus SHE. Whole-cell EPR shows that the protein is present as an 8Fe protein in vivo. These data strongly suggest that it is the sequence motif rather than the exact sequence or the structure that is critical for the assembly of a 4Fe cluster in that region of the protein. The new oxygen-sensitive 4Fe cluster was converted in partial yield to a 3Fe cluster. In known ferredoxins and enzymes that contain reversibly interconvertible [4Fe-4S]2+/+ and [3Fe-4S]+/0 clusters, the 3Fe form always has a reduction potential ca. 200 mV more positive than the 4Fe cluster in the same position. In contrast, for DeltaT14/DeltaD15 FdI, the 3Fe and 4Fe clusters in the same location have extremely similar reduction potentials.

Solution structure of the oxidized Fe7S8 ferredoxin from the thermophilic bacterium Bacillus schlegelii by 1H NMR spectroscopy

The solution structure of the paramagnetic seven-iron ferredoxin from Bacillus schlegelii in its oxidized form has been determined by 1H NMR. The protein, which contains 77 amino acids, is thermostable. Seventy-two residues and 79% of all theoretically expected proton resonances have been assigned. The structure has been determined through torsion angle dynamics calculations with the program DYANA, using 966 meaningful NOEs (from a total of 1305), hydrogen bond constraints, and NMR derived dihedral angle constraints for the cluster ligating cysteines, and by using crystallographic information to build up the two clusters. Afterwards, restrained energy minimization and restrained molecular dynamics were applied to each conformer of the family. The final family of 20 structures has RMSD values from the mean structure of 0.68 A for the backbone atoms and of 1.16 A for all heavy atoms. The contributions to the thermal stability of the B. schlegelii ferredoxin are discussed by comparing the present structure to that of the less stable Azotobacter vinelandii ferredoxin I which is the only other available structure of a bacterial seven-iron ferredoxin. It is proposed that the hydrophobic interactions and the hydrogen bond network linking the N-terminus and the C-terminus together and a high number of salt bridges contribute to the stability.

A stable intermediate product of the archaeal zinc-containing 7Fe ferredoxin from Sulfolobus sp. strain 7 by artificial oxidative conversion

Irreversible conversion of the purified zinc-containing 7Fe ferredoxin from the thermoacidophilic archaeon Sulfolobus sp. strain 7 was found to occur under aerobic conditions at pH 5.0 and at 4 degrees C. This process accompanied a substantial increase of the electron paramagnetic resonance signal attributed to a [3Fe-4S]1+ cluster, and the converted form containing approximately 6 Fe/Zn (mol/mol) had a net charge different from that of the native 7Fe ferredoxin. These data provide evidence for the formation of a stable intermediate product of the archaeal ferredoxin having two [3Fe-4S] clusters and a zinc center by artificial oxidative degradation. This further explains the discrepancy that a zinc center and two [3Fe-4S] clusters (but not a zinc center and one [3Fe-4S] cluster plus one [4Fe-4S] cluster) are observed in the crystal structure at pH 5.0.

Novel zinc-containing ferredoxin family in thermoacidophilic archaea

The dicluster-type ferredoxins from the thermoacidophilic archaea such as Thermoplasma acidophilum and Sulfolobus sp. are known to contain an unusually long extension of unknown function in the N-terminal region. Recent x-ray structural analysis of the Sulfolobus ferredoxin has revealed the presence of a novel zinc center, which is coordinated by three histidine ligand residues in the N-terminal region and one aspartate in the ferredoxin core domain. We report here the quantitative metal analyses together with electron paramagnetic resonance and resonance Raman spectra of T. acidophilum ferredoxin, demonstrating the presence of a novel zinc center in addition to one [3Fe-4S] and one [4Fe-4S] cluster (Fe/Zn = 6.8 mol/mol). A phylogenetic tree constructed for several archaeal monocluster and dicluster type ferredoxins suggests that the zinc-containing ferredoxins of T. acidophilum and Sulfolobus sp. form an independent subgroup, which is more distantly related to the ferredoxins from the hyperthermophiles than those from the methanogenic archaea, indicating the existence of a novel group of ferredoxins, namely, a "zinc-containing ferredoxin family" in the thermoacidophilic archaea. Inspection of the N-terminal extension regions of the archaeal zinc-containing ferredoxins suggested strict conservation of three histidine and one aspartate residues as possible ligands to the novel zinc center.

Aerobic inactivation of fumarate reductase from Escherichia coli by mutation of the [3Fe-4S]-quinone binding domain

Fumarate reductase from Escherichia coli functions both as an anaerobic fumarate reductase and as an aerobic succinate dehydrogenase. A site-directed mutation of E. coli fumarate reductase in which FrdB Pro-159 was replaced with a glutamine or histidine residue was constructed and overexpressed in a strain of E. coli lacking a functional copy of the fumarate reductase or succinate dehydrogenase complex. The consequences of these mutations on bacterial growth, assembly of the enzyme complex, and enzymatic activity were investigated. Both mutations were found to have no effect on anaerobic bacterial growth or on the ability of the enzyme to reduce fumarate compared with the wild-type enzyme. The FrdB Pro-159-to-histidine substitution was normal in its ability to oxidize succinate. In contrast, however, the FrdB Pro-159-to-Gln substitution was found to inhibit aerobic growth of E. coli under conditions requiring a functional succinate dehydrogenase, and furthermore, the aerobic activity of the enzyme was severely inhibited upon incubation in the presence of its substrate, succinate. This inactivation could be prevented by incubating the mutant enzyme complex in an anaerobic environment, separating the catalytic subunits of the fumarate reductase complex from their membrane anchors, or blocking the transfer of electrons from the enzyme to quinones. The results of these studies suggest that the succinate-induced inactivation occurs by the production of hydroxyl radicals generated by a Fenton-type reaction following introduction of this mutation into the [3Fe-4S] binding domain. Additional evidence shows that the substrate-induced inactivation requires quinones, which are the membrane-bound electron acceptors and donors for the succinate dehydrogenase and fumarate reductase activities. These data suggest that the [3Fe-4S] cluster is intimately associated with one of the quinone binding sites found n fumarate reductase and succinate dehydrogenase.

Characterization of a 2[4Fe-4S] ferredoxin obtained by chemical insertion of the Fe-S clusters into the apoferredoxin II from Rhodobacter capsulatus

The Rhodobacter capsulatus ferredoxin II (FdII) belongs to a family of 7Fe ferredoxins containing one [3Fe-4S] cluster and one [4Fe-4S] cluster. This protein, encoded by the fdxA gene, has been overproduced in Escherichia coli as a soluble apoferredoxin. The purified recombinant protein was subjected to reconstitution experiments by chemical incorporation of the Fe-S clusters under anaerobic conditions. A brown protein was obtained, the formation of which was dependent upon the complete unfolding of the polypeptide prior to incorporation of iron and sulfur atoms. The yield of the reconstituted product was higher when the reaction was carried out at slightly basic pH. The reconstituted ferredoxin was purified and shown to be distinct from the native [7Fe-8S] ferredoxin, based on several biochemical and spectroscopic criteria. In the oxidized state, EPR revealed the quasi-absence of [3Fe-4S] cluster. 1H-NMR spectroscopic analyses provided evidence that the protein was reconstituted as a 2[4Fe-4S] ferredoxin. This conclusion was further supported by the determination by electrospray mass spectrometry of the molecular mass of the reconstituted protein, which matched within 2 Da to the mass of the FdII polypeptide incremented of eight atoms each of iron and sulfur. Exposure of the reconstituted protein to air resulted in a fast and irreversible oxidative denaturation of the Fe-S clusters, without formation of [7Fe-8S] form. Unlike the natural 7Fe ferredoxin, the reconstituted ferredoxin appeared incompetent in an electron-transfer assay coupled to nitrogenase activity. The fact that the apoFdII was reconstituted as a highly unstable 8Fe ferredoxin instead of the 7Fe naturally occurring FdII is discussed in relation to the results obtained with other types of ferredoxins.

Rhodobacter sphaeroides rdxA, a homolog of Rhizobium meliloti fixG, encodes a membrane protein which may bind cytoplasmic [4Fe-4S] clusters

In the photosynthetic bacterium Rhodobacter sphaeroides, a chromosomal gene, rdxA, which encodes a 52-kDa protein, was found to be homologous to fixG, the first gene of a Rhizobium meliloti nitrogen fixation operon on the pSym plasmid (D. Kahn, M. David, O. Domergue, M.-L. Daveran, J. Ghai, P. R. Hirsch, and J. Batut, J. Bacteriol. 171:929-939, 1989). The deduced amino acid sequences of RdxA and FixG are 53% identical and 73% similar; sequence analyses suggested that each has five transmembrane helices and a central region resembling bacterial-type ferredoxins. Translational fusion proteins with an alkaline phosphatase reporter group were expressed in both R. sphaeroides and Escherichia coli and were used to assess the membrane topology of RdxA. Its ferredoxinlike sequence, which may bind two [4Fe-4S] centers, was found to be cytoplasmically located. Genetic disruptions showed that rdxA is not essential for nitrogen fixation in R. sphaeroides. Immediately downstream of rdxA, an open reading frame (ORFT2) that encoded a 48-kDa protein was found. This DNA sequence was not homologous to any region of the R. meliloti fixG operon. The N-terminal sequence of the ORFT2 gene product resembled amino acid sequences found in members of the GntR family of regulatory proteins (D. J. Haydon and J. R. Guest, FEMS Microbiol. Lett. 79:291-296, 1991). The rdxA gene was localized to the smaller of two R. sphaeroides chromosomes, upstream of and divergently transcribed from hemT, which encodes one of two 5-aminolevulinate synthase isozymes. The rdxA and hemT genes may share a transcriptional regulatory region. Southern hybridization analysis demonstrated the presence of an rdxA homolog on the R. sphaeroides large chromosome. The functions of this homolog, like those of rdxA, remain to be determined, but roles in oxidation-reduction processes are likely.

[3Fe-4S] to [4Fe-4S] cluster conversion in Escherichia coli fumarate reductase by site-directed mutagenesis

Site-directed mutants of Escherichia coli fumarate reductase in which FrdB Cys204, Cys210, and Cys214 were individually replaced by Ser and in which Val207 was replaced by Cys were constructed and overexpressed in a strain of E. coli lacking a wild-type copy of fumarate reductase and succinate dehydrogenase. The consequences of these mutations on bacterial growth, enzymatic activity, and the EPR properties of the constituent iron-sulfur clusters were investigated. The FrdB Cys204Ser, Cys210Ser, and Cys214Ser mutations result in enzymes with negligible activity that have dissociated from the membrane and consequently are incapable of supporting cell growth under conditions requiring a functional fumarate reductase. EPR studies indicate that these effects are associated with loss of both the [3Fe-4S] and [4Fe-4S] clusters, centers 3 and 2, respectively. In contrast, the FrdB Val207Cys mutation results in a functional membrane-bound enzyme that is able to support growth under anaerobic and aerobic conditions. However, EPR studies indicate that the indigenous [3Fe-4S]+,0 cluster (Em = -70 mV), center 3, has been replaced by a much lower potential [4Fe-4S]2+,+ cluster (Em = -350 mV), indicating that the primary sequence of the polypeptide determines the type of clusters assembled. The results of these studies afford new insights into the role of centers 2 and 3 in mediating electron transfer from menaquinol, the residues that ligate these clusters, and the intercluster magnetic interactions in the wild-type enzyme.

Does ferredoxin I (Azotobacter) represent a novel class of DNA-binding proteins that regulate gene expression in response to cellular iron(II)?

Azotobacter vinelandii (Av) and chroococcum (Ac) ferredoxin I contain [3Fe-4S]1 + 0 and [4Fe-4S]2+1+ clusters, when isolated aerobically, which undergo one-electron redox cycles at potentials of -460 +/- 10 mV (vs SHE) at pH 8.3 and -645 +/- 10 mV, respectively. The X-ray structure of Fd I (Av) reveals that the N-terminal half of the polypeptide folds as a sandwich of beta-strands which enclose the iron-sulphur clusters. The C-terminal sequence contains an amphiphilic alpha-helix of four turns which lies on the surface of the beta-barrel. Fd I (Av) controls expression of an unknown protein of Mr approximately 18,000. Fd I (Ac) will complex iron(II) avidly above pH approximately 8.0 only when the [3Fe-4S] cluster is reduced and provided that cellular nucleic acid is bound. Fd I (Ac) rigorously purified from nucleic acid does not undergo iron(II) uptake. These facts, together with recent evidence that the interconversion process [3Fe-4S]0 + Fe2+----[4Fe-4S]2+ in the iron-responsive element binding protein (IRE-BP) of eukaryotic cells is controlling protein expression at the level of mRNA [1991, Cell 64, 4771; 1991, Nucleic Acid Res. 19, 1739] leads to the following hypothesis. Fd I is a DNA-binding protein which interacts by single alpha-helix binding in the wide groove of DNA. The binding is regulated by iron(II) levels in the cell. The 7Fe form binds to DNA and represses gene expression. Only the DNA-bound form of the 7Fe Fd I will take up iron(II), not the form free in solution. Iron(II) becomes bound when the [3Fe-4S] cluster is reduced. The 8Fe Fd I thus generated no longer binds DNA and the gene is de-repressed. Sequence comparisons and the crystal structure suggests that the two central turns of the alpha-helix are important elements of the DNA-recognition process and that residues Gln69 and Glu73, which lie on the outer surface of the helix, hydrogen-bond with specific base pairs.

Amino acid sequence and molecular modelling of a thermostable two (4Fe-4S) ferredoxin from the archaebacterium Methanococcus thermolithotrophicus

The amino acid sequence of a two (4Fe-4S) ferredoxin from the methanogenic bacterium Methanococcus thermolithotrophicus (FdMt) has been determined. This thermostable protein comprises 60 amino acid residues (Mr 6541) and two (4Fe-4S) clusters chelated to the protein through the eight cysteines. FdMt contains a relatively high number of lysines [5], threonines [4] and valines [10]. The three-dimensional molecular model generated from the Peptococcus aerogenes X-ray structure keeps the characteristic overall ferredoxin folding thanks to complementary substitutions of residues of the hydrophobic core. The major structural features of the model are the different environments of both clusters, and the patch of three lysines at one end of the molecule. The possible role of several structural factors in the thermostability of the protein is discussed.

Purification and characterization of a 7Fe ferredoxin from Streptomyces griseus

A ferredoxin has been purified from Streptomyces griseus grown in soybean flour-containing medium. The homogeneous protein has a molecular weight near 14,000 as determined by both PAGE and size exclusion chromatography. The iron and labile sulfide content is 6-7 atoms/mole protein. EPR spectroscopy of native S. griseus ferredoxin shows an isotropic signal at g = 2.01 which is typical of [3Fe-4S]1+ clusters and which quantitates to 0.9 spin/mole. Reduction of the ferredoxin by excess dithionite at pH 8.0 produces an EPR silent state with a small amount of a g = 1.95 type signal. Photoreduction in the presence of deazaflavin generates a signal typical of [4Fe-4S]1+ clusters at much higher yields (0.4-0.5 spin/mole) with major features at g-values of 2.06, 1.94, 1.90 and 1.88. This latter EPR signal is most similar to that seen for reduced 7Fe ferredoxins, which contain both a [3Fe-4S] and [4Fe-4S] cluster. In vitro reconstitution experiments demonstrate the ability of the S. griseus ferredoxin to couple electron transfer between spinach ferredoxin reductase and S. griseus cytochrome P-450soy for NADPH-dependent substrate oxidation. This represents a possible physiological function for the S. griseus ferredoxin, which if true, would be the first functional role demonstrated for a 7Fe ferredoxin.

Primary structure of a 7Fe ferredoxin from Streptomyces griseus

The complete primary structure of a Streptomyces griseus (ATCC 13273) 7Fe ferredoxin, which can couple electron transfer between spinach ferredoxin reductase and S. griseus cytochrome P-450soy for NADPH-dependent substrate oxidation, has been determined by Edman degradation of the whole protein and peptides derived by Staphylococcus aureus V8 proteinase and trypsin digestion. The protein consists of 105 amino acids and has a calculated molecular weight, including seven irons and eight sulfurs, of 12,291. The ferredoxin sequence is highly homologous (73%) to that of the 7Fe ferredoxin from Mycobacterium smegmatis. The N-terminal half of the sequence, which is the Fe-S clusters binding domain, has more than 50% homology with other 7Fe ferredoxins. In particular, the seven cysteines known from the crystal structure of Azotobacter vinelandii ferredoxin I to be involved in binding the two Fe-S clusters are conserved.

A novel and remarkably thermostable ferredoxin from the hyperthermophilic archaebacterium Pyrococcus furiosus

The archaebacterium Pyrococcus furiosus is a strict anaerobe that grows optimally at 100 degrees C by a fermentative-type metabolism in which H2 and CO2 are the only detectable products. A ferredoxin, which functions as the electron donor to the hydrogenase of this organism was purified under anaerobic reducing conditions. It had a molecular weight of approximately 12,000 and contained 8 iron atoms and 8 cysteine residues/mol but lacked histidine or arginine residues. Reduction and oxidation of the ferredoxin each required 2 electrons/mol, which is consistent with the presence of two [4Fe-4S] clusters. The reduced protein gave rise to a broad rhombic electronic paramagnetic resonance spectrum, with gz = 2.10, gy = 1.86, gx = 1.80, and a midpoint potential of -345 mV (at pH 8). However, this spectrum represented a minor species, since it quantitated to only approximately 0.3 spins/mol. P. furiosus ferredoxin is therefore distinct from other ferredoxins in that the bulk of its iron is not present as iron-sulfur clusters with an S = 1/2 ground state. The apoferredoxin was reconstituted with iron and sulfide to give a protein that was indistinguishable from the native ferredoxin by its iron content and electron paramagnetic resonance properties, which showed that the novel iron-sulfur clusters were not artifacts of purification. The reduced ferredoxin also functioned as an electron donor for H2 evolution catalyzed by the hydrogenase of the mesophilic eubacterium Clostridium pasteurianum. P. furiosus ferredoxin was resistant to denaturation by sodium dodecyl sulfate (20%, wt/vol) and was remarkably thermostable. Its UV-visible absorption spectrum and electron carrier activity to P. furiosus hydrogenase were unaffected by a 12-h incubation of 95 degrees C.

Spectroscopic studies of the seven-iron-containing ferredoxins from Azotobacter vinelandii and Thermus thermophilus

The seven-iron-containing ferredoxins from Azotobacter vinelandii and Thermus thermophilus have been investigated by low-temperature magnetic circular dichroism (MCD) and electron paramagnetic resonance (EPR) spectroscopies and room temperature ultraviolet-visible absorption spectroscopy. The results confirm the presence of one trinuclear and one tetranuclear iron-sulfur cluster in both ferredoxins and facilitate comparison of the electronic and magnetic properties of the oxidized and reduced [3Fe-xS] clusters. MCD magnetization data are consistent with an S = 2 ground state for both reduced [3Fe-xS] clusters, but indicate differences in the rhombicity of the zero-field splittings. The data permit rationalization of the absence of a delta M = 4 EPR transition for the reduced [3Fe-xS] cluster in A. vinelandii ferredoxin I. Spectroscopic studies of anaerobically isolated A. vinelandii ferredoxin I do not support the hypothesis that the [3Fe-xS] cluster arises as a result of aerial oxidative damage to a [4Fe-4S] cluster during isolation. The possibility that two distinct forms of [3Fe-xS] clusters can exist in A. vinelandii ferredoxin I was investigated by spectroscopic studies as a function of pH. The results reveal two distinct and interconvertible forms of the reduced [3Fe-xS] cluster, but do not permit rationalization of the inconsistencies in the structural data that have been reported for the oxidized clusters.

New perspectives on bacterial ferredoxin evolution

Recent evidence indicates that a gene transposition event occurred during the evolution of the bacterial ferredoxins subsequent to the ancestral intrasequence gene duplication. In light of this new information, the relationships among the bacterial ferredoxins were reexamined and an evolutionary tree consistent with this new understanding was derived. The bacterial ferredoxins can be divided into several groups based on their sequence properties; these include the clostridial-type ferredoxins, the Azotobacter-type ferredoxins, and a group containing the ferredoxins from the anaerobic, green, and purple sulfur bacteria. Based on sequence comparison, it was concluded that the amino-terminal domain of the Azotobacter-type ferredoxins, which contains the novel 3Fe:3S cluster binding site, is homologous with the carboxyl-terminal domain of the ferredoxins from the anaerobic photosynthetic bacteria. A number of ferredoxin sequences do not fit into any of the groups described above. Based on sequence properties, these sequences can be separated into three groups: a group containing Methanosarcina barkeri ferredoxin and Desulfovibrio desulfuricans ferredoxin II, a group containing Desulfovibrio gigas ferredoxin and Clostridium thermoaceticum ferredoxin, and a group containing Desulfovibrio africanus ferredoxin I and Bacillus stearothermophilus ferredoxin. The last two groups differ from all of the other bacterial ferredoxins in that they bind only one Fe:S cluster per polypeptide, whereas the others bind two. Sequence examination indicates that the second binding site has been either partially or completely lost from these ferredoxins. Methanosarcina barkeri ferredoxin and Desulfovibrio desulfuricans ferredoxin II are of interest because, of all the ferredoxins whose sequences are presently known, they show the strongest evidence of internal gene duplication.(ABSTRACT TRUNCATED AT 400 WORDS)

Purification, amino-acid sequence and some properties of the ferredoxin isolated from Bacillus acidocaldarius

Ferredoxin was isolated from the aerobic, thermophilic and acidophilic bacterium Bacillus acidocaldarius and its sequence of 78 amino acids completely determined by automated Edman degradation of the protein and of peptides derived from chemical cleavage between aspartic acid and proline and from enzymatic digestions. The optical spectrum of the oxidized protein has a broad maximum around 400 nm. The ferredoxin is thermostable: its absorbance begins to decrease only at incubation over 71 degrees C. The number of iron and inorganic sulphur atoms per molecule was determined to be 5.3 and 5.0, respectively. The calculated molar extinction coefficient was 23 000 M-1 X cm-1, the molecular mass of the apoferredoxin 8 872 Da. Contrary to all expectations, the sequence of B. acidocaldarius ferredoxin shows very little homology to that of B. stearothermophilus but closely resembles that of Thermus thermophilus.

Complete amino acid sequence of the 4Fe-4S, thermostable ferredoxin from Clostridium thermoaceticum

The complete amino acid sequence of the 4Fe-4S ferredoxin from the thermophilic bacterium Clostridium thermoaceticum has been determined. The protein is extremely thermostable and is the only known clostridial ferredoxin to contain a single [4Fe-4S] cluster. The sequence totals 63 residues and includes the first tryptophan (Trp-26) reported for a clostridial ferredoxin, and other amino acids not commonly found in clostridial or clostridial-like ferredoxins: methionine (Met-1), histidine (His-33), arginine (Arg-49), and leucine (Leu-9, -19, and -31). Sequence homology to clostridial and other 8Fe-8S ferredoxins is limited to eight to nine residues at the amino-terminal sulfhydryl grouping (Cys-10, -13, -16, and -20) and two to five residues in the carboxyterminal region. This ferredoxin is, thus, sequentially distinct from all known clostridial ferredoxins and from other bacterial ferredoxins in both the 8Fe-8S and 4Fe-4S classes.