Amino acid sequence of the bacterioferritin (cytochrome b1) of Escherichia coli-K12

Molecular Engineering of E. coli Bacterioferritin: A Versatile Nanodimensional Protein Cage

Currently, intense interest is focused on the discovery and application of new multisubunit cage proteins and spherical virus capsids to the fields of bionanotechnology, drug delivery, and diagnostic imaging as their internal cavities can serve as hosts for fluorophores or bioactive molecular cargo. Bacterioferritin is unusual in the ferritin protein superfamily of iron-storage cage proteins in that it contains twelve heme cofactors and is homomeric. The goal of the present study is to expand the capabilities of ferritins by developing new approaches to molecular cargo encapsulation employing bacterioferritin. Two strategies were explored to control the encapsulation of a diverse range of molecular guests compared to random entrapment, a predominant strategy employed in this area. The first was the inclusion of histidine-tag peptide fusion sequences within the internal cavity of bacterioferritin. This approach allowed for the successful and controlled encapsulation of a fluorescent dye, a protein (fluorescently labeled streptavidin), or a 5 nm gold nanoparticle. The second strategy, termed the heme-dependent cassette strategy, involved the substitution of the native heme with heme analogs attached to (i) fluorescent dyes or (ii) nickel-nitrilotriacetate (NTA) groups (which allowed for controllable encapsulation of a histidine-tagged green fluorescent protein). An in silico docking approach identified several small molecules able to replace the heme and capable of controlling the quaternary structure of the protein. A transglutaminase-based chemoenzymatic approach to surface modification of this cage protein was also accomplished, allowing for future nanoparticle targeting. This research presents novel strategies to control a diverse set of molecular encapsulations and adds a further level of sophistication to internal protein cavity engineering.

Perspectives for Photobiology in Molecular Solar Fuels

This paper presents an overview of the prospects for bio-solar energy conversion. The Global Artificial Photosynthesis meeting at Lord Howe Island (14–18 August 2011) underscored the dependence that the world has placed on non-renewable energy supplies, particularly for transport fuels, and highlighted the potential of solar energy. Biology has used solar energy for free energy gain to drive chemical reactions for billions of years. The principal conduits for energy conversion on earth are photosynthetic reaction centres – but can they be harnessed, copied and emulated? In this communication, we initially discuss algal-based biofuels before investigating bio-inspired solar energy conversion in artificial and engineered systems. We show that the basic design and engineering principles for assembling photocatalytic proteins can be used to assemble nanocatalysts for solar fuel production.

The physiological role of ferritin-like compounds in bacteria

Iron, as the ferrous or ferric ion, is essential for the life processes of all eukaryotes and most prokaryotes; however, the element is toxic when in excess of that needed for cellular homeostasis. Ferrous ions can react with metabolically generated hydrogen peroxide to yield toxic hydroxyl radicals that in turn degrade lipids, DNA, and other cellular biomolecules. Mechanisms have evolved in living systems for iron detoxification and for the removal of excess ferrous ions from the cytosol. These detoxification mechanisms involve the oxidation of excess ferrous ions to the ferric state and storage of the ferric ions in ferritin-like proteins. There are at least three types of ferritin-like proteins in bacteria: bacterial ferritin, bacterioferritin, and dodecameric ferritin. These bacterial proteins are related to the ferritins found in eukaryotes. The structure and physical characteristics of the ferritin-like compounds have been elucidated in several bacteria. Unfortunately, the physiological roles of the bacterial ferritin-like compounds have been less thoroughly studied. A few studies conducted with mutants indicated that ferritin-like compounds can protect bacterial cells from iron overload, serve as an iron source when iron is limited, protect the bacterial cells against oxidative stress and/or protect DNA against enzymatic or oxidative attack. There is very little information available concerning the roles that ferritin-like compounds might play in the survival of bacteria in food, water, soil, or eukaryotic host environments.

An antifungal protein from the marine bacterium streptomyces sp. Strain AP77 is specific for Pythium porphyrae, a causative agent of red rot disease in Porphyra spp

A novel antifungal protein (SAP) was found in the culture supernatant of a marine bacterium, Streptomyces sp. strain AP77, and was purified. This protein was characterized by chemical, biochemical, and biological analyses. By using gel filtration, the molecular mass of SAP was estimated to be 160 kDa. Structural analysis of SAP by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and matrix-assisted laser desorption ionization-time of flight mass spectrometry suggested that SAP is composed of three heterologous protein subunits of 41.7 kDa (SAP1), 21.7 kDa (SAP2), and 18.7 kDa (SAP3) at a molar ratio of 1:1:5 (or 1:1:6). N-terminal amino acid sequence analysis and a homology search revealed that SAP1, SAP2, and SAP3 exhibit 64.3, 68.4, and 86.7% similarity to three Streptomyces coelicolor polypeptides, puromycin resistance protein (Pur8), a conserved hypothetical protein, and bacterioferritin, respectively. The MIC of purified SAP against Pythium porphyrae was determined to be 1.6 microg/disk, whereas no inhibitory effect was observed at concentrations up to 100 microg/disk against most of the fungal and bacterial strains tested; the only exception was relatively strong antifungal activity against Pythium ultimum (MIC, 6.3 microg/disk). In vitro and in vivo toxicity tests demonstrated that SAP showed no toxicity against Porphyra yezoensis cells, human normal dermal fibroblasts, and mice at doses up to 700 microg/ml (for 24 h), 250 microg/ml (for 12 h), and 75 mg/kg (for 35 days), respectively. SAP was labile when it was subjected to a heated-air drying treatment, which is a great advantage in food production procedures. These results indicated that Streptomyces sp. strain AP77 might be useful as a gene source for safe transgenic Porphyra breeding for tolerance to Pythium infection.

Iron storage in bacteria

Iron is an essential nutrient for nearly all organisms but presents problems of toxicity, poor solubility and low availability. These problems are alleviated through the use of iron-storage proteins. Bacteria possess two types of iron-storage protein, the haem-containing bacterioferritins and the haem-free ferritins. These proteins are widespread in bacteria, with at least 39 examples known so far in eubacteria and archaebacteria. The bacterioferritins and ferritins are distantly related but retain similar structural and functional properties. Both are composed of 24 identical or similar subunits (approximately 19 kDa) that form a roughly spherical protein (approximately 450 kDa, approximately 120 A diameter) containing a large hollow centre (approximately 80 A diameter). The hollow centre acts as an iron-storage cavity with the capacity to accommodate at least 2000 iron atoms in the form of a ferric-hydroxyphosphate core. Each subunit contains a four-helix bundle which carries the active site or ferroxidase centre of the protein. The ferroxidase centres endow ferrous-iron-oxidizing activity and are able to form a di-iron species that is an intermediate in the iron uptake, oxidation and core formation process. Bacterioferritins contain up to 12 protoporphyrin IX haem groups located at the two-fold interfaces between pairs of two-fold related subunits. The role of the haem is unknown, although it may be involved in mediating iron-core reduction and iron release. Some bacterioferritins are composed of two subunit types, one conferring haem-binding ability (alpha) and the other (beta) bestowing ferroxidase activity. Bacterioferritin genes are often adjacent to genes encoding a small [2Fe-2S]-ferredoxin (bacterioferritin-associated ferredoxin or Bfd). Bfd may directly interact with bacterioferritin and could be involved in releasing iron from (or delivering iron to) bacterioferritin or other iron complexes. Some bacteria contain two bacterioferritin subunits, or two ferritin subunits, that in most cases co-assemble. Others possess both a bacterioferritin and a ferritin, while some appear to lack any type of iron-storage protein. The reason for these differences is not understood. Studies on ferritin mutants have shown that ferritin enhances growth during iron starvation and is also involved in iron accumulation in the stationary phase of growth. The ferritin of Campylobacter jejuni is involved in redox stress resistance, although this does not appear to be the case for Escherichia coli ferritin (FtnA). No phenotype has been determined for E. coli bacterioferritin mutants and the precise role of bacterioferritin in E. coli remains uncertain.

Spectroscopic studies of cobalt(II) binding to Escherichia coli bacterioferritin

The iron storage protein bacterioferritin (BFR) consists of 24 identical subunits, each containing a dinuclear metal binding site called the ferroxidase center, which is essential for fast iron core formation. Cobalt(II) binding to wild-type and site-directed variants of Escherichia coli BFR was studied by optical and magnetic techniques. Data from absorption spectroscopy demonstrate the binding of two cobalt(II) ions per subunit of wild-type and heme-free BFR, each with a pseudotetrahedral or pentacoordinate geometry, and EPR studies show that the two cobalt(II) ions are weakly magnetically coupled. Studies of variants of BFR in which a single glutamic acid residue at the ferroxidase center is replaced by alanine confirm that this is the site of cobalt(II) binding, since the altered centers bind only one cobalt(II) ion. This work shows that the electroneutrality of the ferroxidase center is preserved on binding a pair of divalent metal ions. Optical and EPR data show that cobalt(II) binding to BFR exhibits positive cooperativity, with an average Kd of approximately 1 x 10(-5) M. The favored filling of the ferroxidase center with pairs of metal ions may have mechanistic implications for the iron(II) binding process. Discrimination against oxidation of single iron(II) ions avoids odd electron reduction products of oxygen.

Charge compensated binding of divalent metals to bacterioferritin: H+ release associated with cobalt(II) and zinc(II) binding at dinuclear metal sites

Divalent metal ion binding to the bacterial iron-storage protein, bacterioferritin (BFR), which contains a dinuclear metal binding site within each of its 24 subunits, was investigated by potentiometric and spectrophotometric methods. Cobalt(II) and zinc(II) were found to bind at both high- and low-affinity sites. Cobalt(II) binding at the high-affinity site was observed at a level of two per subunit with the release of approximately 1.6 protons per metal ion, thus confirming the dinuclear metal centre as the high-affinity site. Zinc(II) binding at the dinuclear centre (high-affinity site) resulted in the release of approximately 2 protons per metal ion, but exhibited a binding stoichiometry which indicated that not all dinuclear centres were capable of binding two zinc(II) ions. Competition data showed that binding affinities for the dinuclear centre were in the order zinc(II) > cobalt(II), and also confirmed the unexpected stoichiometry of zinc(II) binding. This work emphasises the importance of charge neutrality at the dinuclear centre.

Construction of a ferritin-deficient mutant of Campylobacter jejuni: contribution of ferritin to iron storage and protection against oxidative stress

The ferritin-encoding gene (cft) of Campylobacter jejuni was cloned and sequenced. The nucleotide sequence of cft had a 501 bp open reading frame for a protein with 167 amino acids and a predicted molecular mass of 19 180 Da, and showed a high similarity to that of Helicobacter pylori and Escherichia coli ferritin genes. To determine the biological function of ferritin in C. jejuni, a ferritin-deficient mutant was constructed. The growth of ferritin-deficient strain SNA 1 was clearly inhibited under iron deprivation. The ferritin-deficient mutant was more sensitive to killing by H2O2 and paraquat than the isogenic parent strain. These findings demonstrate that ferritin in C. jejuni makes a significant contribution to both iron storage and protection from intracellular iron overload, and resulting iron-mediated oxidative stress.

Cloning and sequencing of the bacterioferritin gene of Brucella melitensis 16M strain

The 40 N-terminal amino acids of the 20 kDa antigen A2 from Brucella melitensis were sequenced and showed important similarities with 4 bacterioferrins. A monoclonal antibody raised against this antigen cross-reacted with Escherichia coli bacterioferritin. Hybridization of two sets of degenerate primers with B. melitensis HindIII-digested genomic DNA identified a 3.8 kb fragment. This fragment was shown to contain a bacterioferritin gene (bfr) encoding a 161-amino acid protein. The sequence of the Brucella bacterioferritin is 69% similar to that of E. coli, and many of the ferroxidase centre and haem-ligation residues are conserved.

Iron metabolism in Rhodobacter capsulatus. Characterisation of bacterioferritin and formation of non-haem iron particles in intact cells

The water-soluble cytochrome b557 from the photosynthetic bacterium Rhodobacter capsulatus was purified and shown to have the properties of the iron-storage protein bacterioferritin. The molecular mass of R. capsulatus bacterioferritin is 428 kDa and it is composed of a single type of 18-kDa subunit. The N-terminal amino acid sequence of the bacterioferritin subunit shows 70% identity to the sequence of bacterioferritin subunits from Escherichia coli, Nitrobacter winogradskyi, Azotobacter vinelandii and Synechocystis PCC 6803. The absorbance spectrum of reduced bacterioferritin shows absorbance maxima at 557 nm (alpha band), 526 nm (beta band) and 417 nm (Soret band) from the six haem groups/molecule. Antibody assays reveal that bacterioferritin is located in the cytoplasm of R. capsulatus, and its levels stay relatively constant during batch growth under aerobic conditions when the iron concentration in the medium is kept constant. Iron deficiency leads to a decrease in bacterioferritin and iron overload leads to an increase. Bacterioferritin from R. capsulatus has an amorphous iron-oxide core with a high phosphate content (900-1000 Fe atoms and approximately 600 phosphates/bacterioferritin molecule). Mössbauer spectroscopy indicates that in both aerobically and anaerobically (phototrophically) grown cells bacterioferritin with an Fe3+ core is formed, suggesting that iron-core formation in vivo may not always require molecular oxygen.

Structure of a unique twofold symmetric haem-binding site

Bacterioferritin of Escherichia coli, also known as cytochrome b1, is a hollow, nearly spherical shell made up of 24 identical protein subunits and 12 haems. We have solved this structure in a tetragonal crystal form at 2.9 A resolution. We find that each haem is bound in a pocket formed by the interface between a pair of symmetry-related subunits. The quasi-twofold axis of the haem is closely aligned with the local twofold axis relating these subunits. The axial ligands of the haem are sulphurs of two equivalent methionyl residues (Met 52) from the symmetry-related subunits. A cluster of four water molecules is trapped in the gap between the upper edge of the haem and two extended protein loops which close off the haem from the outer aqueous environment. This is the first structure of a bis-methionine ligated haem-binding site and the first case of a twofold symmetric haem-binding site.

Overproduction, purification and characterization of the Escherichia coli ferritin

Recent studies have indicated that Escherichia coli possesses at least two iron-storage proteins, the haem-containing bacterioferritin and ferritin. The ferritin protein has been amplified 600-fold to 11-14% of total cell protein in a bfr mutant and purified to homogeneity with an overall yield of 13%. The cellular ferritin content remained relatively constant throughout the growth cycle and amplification was accompanied by a 2.5-fold increase in cellular iron content. The isolated ferritin contained 5-20 non-haem iron atoms/holomer and resembled the eukaryotic ferritins rather than the prokaryotic bacterioferritins in containing no haem. The 24 subunits of this ferritin (M(r) 19,400) assemble into a spherical protein shell (12 +/- 1 nm diameter, M(r) 465,000) which sequesters at least 2000 iron atoms in vitro to form an electron-dense iron core of 7.9 +/- 1 nm diameter. Electron-microscopic and Mössbauer spectroscopic studies with iron-loaded ferritin showed that the core can be either crystalline (ferrihydrite) or amorphous, depending on the absence or presence of phosphate, respectively. Mössbauer spectroscopy with intact E. coli revealed a novel-high spin Fe(II) component which is enhanced in bacteria amplified for ferritin but not in the parental strain. Western blotting showed that ferritin and bacterioferritin are immunologically distinct proteins. E. coli is thus an organism containing both a ferritin and a bacterioferritin and the relative roles of the two iron-storage proteins are discussed in this study.

Kinetic and structural characterization of an intermediate in the biomineralization of bacterioferritin

The mechanism by which iron-storage proteins take up and oxidise iron(II) is not understood. We show by rapid-kinetic and EPR measurements that iron uptake, in vitro, by a bacterial iron-storage protein, bacterioferritin, involves at least three kinetically distinguishable phases: phase 1, the binding of Fe(II) ions, probably at a dimeric iron ferroxidase centre; phase 2, oxidation of the Fe(II) dimer and production of mononuclear Fe(III); and phase 3, iron core formation.

An EPR investigation of non-haem iron sites in Escherichia coli bacterioferritin and their interaction with phosphate. A study using nitric oxide as a spin probe

EPR studies of bacterioferritin (BFR), an iron-storage protein of Escherichia coli [1993, Biochem. J. 292, 47-56], have revealed the presence of non-haem iron (III) (NHI) sites within the protein coat which may be involved in iron uptake and release. When nitric oxide was used as an EPR spin probe of the Fe(II) state of the NHI sites, two distinct mononuclear NHI species were found. Under certain conditions, an iron dimer was also observed. The reaction of phosphate with NHI species has been investigated. Results point to a function for this anion in core nucleation.

Haem and non-haem iron sites in Escherichia coli bacterioferritin: spectroscopic and model building studies

The bacterioferritin (BFR) of Escherichia coli is an iron-storage protein containing 24 identical subunits and between three and 11 protohaem IX groups per molecule. Titration with additional haem gave a maximum loading of 12-14 haems per molecule. The e.p.r. spectra and magnetic c.d. spectra of the protein-bound haem show it to be low-spin Fe(III), and coordinated by two methionine residues as previously reported for BFRs isolated from Pseudomonas aeruginosa and Azotobacter vinelandii [Cheesman, Thomson, Greenwood, Moore and Kadir, Nature (London) (1990) 346, 771-773]. A recent sequence alignment indicated that BFR may be structurally related to ferritin. The molecular model proposed for E. coli BFR has a four-alpha-helix-bundle subunit conformation and a quaternary structure similar to those of mammalian ferritins. In this model there are two types of hydrophobic pocket within which two methionine residues are correctly disposed to bind haem. The e.p.r. spectra also reveal a monomeric non-haem Fe(III) species with spin, S = 5/2. On the basis of sequence comparisons, a ferroxidase centre has recently been proposed to be present in BFR [Andrews, Smith, Yewdall, Guest and Harrison (1991) FEBS Lett. 293, 164-168] and the possibility that this Fe(III) ion may reside at or near the ferroxidase centre is discussed.

Overproduction, purification and characterization of the bacterioferritin of Escherichia coli and a C-terminally extended variant

The bacterioferritin (BFR) of Escherichia coli is an iron-sequestering haemoprotein composed of 24 identical polypeptide chains forming an approximately spherical protein shell with a central iron-storage cavity. BFR and BFR-lambda, a variant with a 14-residue C-terminal extension, have been amplified (120-fold and 50-fold, respectively), purified by a new procedure and characterized. The overproduced BFR exhibited properties similar to those of natural BFR, but the iron content (25-75 non-haem Fe atoms/molecule) was 13-39-fold lower. Two major assembly states of BFR were detected, a 24-subunit protein (tetracosamer) and a novel haem-containing subunit dimer. BFR-lambda subunits assembled into tetracosamers having the same external-surface properties as BFR, presumably because their C-terminal extensions project into and occupy about 60% of the central cavity. As a result, BFR-lambda failed totake up iron under conditions that allowed incorporation into BFR in vitro. The haem content of BFR-lambda (1-2 haems/tetracosamer) was lower than that of BFR (3.5-10.5 haems/tetracosamer) and this, together with a difference in the visible spectra of the two haemoproteins, suggested that the C-terminal extensions in BFR-lambda perturb the haem-binding pockets. A subunit dimer form of BFR-lambda was not detected. A combination of Mössbauer spectroscopy and electron diffraction showed that the BFR loaded with iron in vitro has a ferrihydrite-like iron core, whereas the in-vivo loaded protein has an amorphous core.

E.p.r. and magnetic circular dichroism spectroscopic characterization of bacterioferritin from Pseudomonas aeruginosa and Azotobacter vinelandii

The e.p.r. and magnetic circular dichroism (m.c.d.) spectra of bacterioferritin (BFR) extracted from Pseudomonas aeruginosa and Azotobacter vinelandii have been studied over a wide temperature range down to liquid-helium temperature. The e.p.r. spectra show the presence of low-spin Fe3+ haem with g values of 2.86, 2.32, 1.48 (P. aeruginosa) and 2.88, 2.31, 1.46 (A. vinelandii), in both the presence and absence of the BFR core. Together with evidence from the porphyrin-to-Fe3+ charge-transfer band at 2240 and 2270 nm the axial haem ligands are identified as two methionines. The low-temperature m.c.d. spectra in the region 300-1000 nm of P. aeruginosa and A. vinelandii BFR are identical with one another and unaffected by removal of the iron core. Hence it can be concluded that the presence of the iron core has no detectable effect on the electronic states and on the stereochemistry of the haem group. This was unexpected, in view of the observations by Watt, Frankel, Papaefthymiou, Spartalian & Stiefel [(1986) Biochemistry 25, 4330-4336] that the redox potential of the haem group in A. vinelandii BFR shifts from -475 mV to -225 mV on removal of the core. The e.p.r. spectra of holoBFR show a broad symmetrical derivative-shaped band centred at g = 2.0 which decreases in bandwidth as the temperature is raised. This signal is assigned to the uncompensated electron spins of the iron core.

Structure, function, and evolution of ferritins

The ferritins of animals and plants and the bacterioferritins (BFRs) have a common iron-storage function in spite of differences in cytological location and biosynthetic regulation. The plant ferritins and BFRs are more similar to the H chains of mammals than to mammalian L chains, with respect to primary structure and conservation of ferroxidase center residues. Hence they probably arose from a common H-type ancestor. The recent discovery in E. coli of a second type of iron-storage protein (FTN) resembling ferritin H chains raises the question of what the relative roles of these two proteins are in this organism. Mammalian L ferritins lack ferroxidase centers and form a distinct group. Comparison of the three-dimensional structures of mammalian and invertebrate ferritins, as well as computer modeling of plant ferritins and of BFR, indicate a well conserved molecular framework. The characterisation of numerous ferritin homopolymer variants has allowed the identification of some of the residues involved in iron uptake and an investigation of some of the functional differences between mammalian H and L chains.

Unification of the ferritin family of proteins

Ferritin is the iron-storage protein of eukaryotic organisms. The nucleotide sequence encoding Azotobacter vinelandii bacterioferritin, a hemoprotein, was determined. The deduced amino acid sequence reveals a high degree of identity with Escherichia coli bacterioferritin and a striking similarity to eukaryotic ferritins. Moreover, derivation of a global alignment shows that virtually all key residues specifying the unique structural motifs of eukaryotic ferritin are conserved or conservatively substituted in the A. vinelandii sequence. The alignment suggests specific methionine residues as heme-binding ligands in bacterioferritins. The overall sequence similarity with conservation of key structural residues implies that all ferritins form a unified family of proteins. The results implicate ferritins as proteins potentially common to all aerobic organisms and as such useful in taxonomic classification, evolutionary analysis, and environmental monitoring.

Purification, characterization and function of bacterioferritin from the cyanobacterium Synechocystis P.C.C. 6803

Storage and buffering of iron is achieved by a class of proteins, the ferritins, widely distributed throughout the living kingdoms. All ferritins have in common their three-dimensional structure and their ability to store large amounts of iron in their central cavity. However, eukaryotic ferritins from plants and animals and bacterioferritins have no sequence similarity, and besides non-haem iron bacterioferritins contain haem residues whereas eukaryotic ferritins do not. In this paper we report the first purification and characterization of a bacterioferritin from a cyanobacterium. It has a molecular mass of 400 kDa and is built up from 19 kDa subunits. Its N-terminal sequence shows 73% identity with that of the Escherichia coli bacterioferritin subunit. It contains 2300 atoms of iron and 1500 molecules of phosphate per ferritin molecule and 0.25 haem residue per subunit; the alpha-peak of the cytochrome has its maximum at 559 nm. In contrast with what is known for eukaryotic ferritins, we found that bacterioferritin from Synechocystis is not inducible by iron under the conditions that we have tested and that it has a constant concentration whatever the iron status of the cells, even at very low iron concentration. Bacterioferritin from Synechocystis P.C.C. 6803 is fully assembled in vivo and it is shown by labelling with 59Fe that it is able to load iron in vitro as well as in vivo. Bacterioferritin from Synechocystis is shown to have an iron-buffering function while the bulk of cellular iron is found associated with a pool of low-molecular-mass electronegative molecules. The role of Synechocystis bacterioferritin in iron metabolism is discussed.

Bacterioferritins and ferritins are distantly related in evolution. Conservation of ferroxidase-centre residues

Iron-storage proteins can be divided into two classes; the bacterioferritins and ferritins. In spite of many apparent structural and functional analogies, no significant amino acid sequence similarity has been detected previously. This report now reveals a distant evolutionary relationship between bacterioferritins and ferritins derived by 'Profile Analysis'. Optimum alignment of bacterioferritin and ferritin sequences suggests that key residues of the ferroxidase centres of ferritins are conserved in bacterioferritins.

Influence of site-directed modifications on the formation of iron cores in ferritin

The structure and crystal chemical properties of iron cores of reconstituted recombinant human ferritins and their site-directed variants have been studied by transmission electron microscopy and electron diffraction. The kinetics of Fe uptake have been compared spectrophotometrically. Recombinant L and H-chain ferritins, and recombinant H-chain variants incorporating modifications in the threefold (Asp131----His or Glu134----Ala) and fourfold (Leu169----Arg) channels, at the partially buried ferroxidase sites (Glu62,His65----Lys,Gly), a putative nucleation site on the inner surface (Glu61,Glu64,Glu67----Ala), and both the ferroxidase and nucleation sites (Glu62,His65----Lys,Gly and Glu61,Glu64,Glu67----Ala), were investigated. An additional H-chain variant, incorporating substitution of the last ten C-terminal residues for those of the L-chain protein, was also studied. Most of the proteins assimilated iron to give discrete electron-dense cores of the Fe(III) hydrated oxide, ferrihydrite (Fe2O3.nH2O). No differences were observed for variants modified in the three- or fourfold channels compared with the unmodified H-chain ferritin. The recombinant L-chain ferritin and H-chain variant depleted of the ferroxidase site, however, showed markedly reduced uptake kinetics and comprised cores of increased diameter and regularity. Depletion of the inner surface Glu residues, whilst maintaining the ferroxidase site, resulted in a partially reduced rate of Fe uptake and iron cores of wider particle size distribution. Modification of both ferroxidase and inner surface Glu residues resulted in complete inhibition of iron uptake and deposition. No cores were observed by electron microscopy although negative staining showed that the protein shell was intact. The general requirement of an appropriate spatial charge density across the cavity surface rather than specific amino acid residues could explain how, in spite of an almost complete lack of identity between the amino acid sequences of bacterioferritin and mammalian ferritins, ferrihydrite is deposited within the cavity of both proteins under similar reconstitution conditions.

Mycobacterium paratuberculosis antigen D: characterization and evidence that it is a bacterioferritin

By using a combination of agarose and polyacrylamide gel electrophoresis, Mycobacterium paratuberculosis antigen D was resolved from a crude sonicated preparation of the organism and characterized as a component with a molecular mass of approximately 400,000 Da. While this component was composed mainly of protein, with unusually high proportions of glutamic acid and leucine, it was resistant to digestion with a number of proteolytic enzymes. Structural detail revealed by electron microscopy, amino acid sequence data, and the demonstration of a Soret band in its absorption spectrum indicated that antigen D was similar to an Escherichia coli bacterioferritin.

Physical, chemical and immunological properties of the bacterioferritins of Escherichia coli, Pseudomonas aeruginosa and Azotobacter vinelandii

The 70-amino-acid-residue N-terminal sequence of the bacterioferritin (BFR) of Azotobacter vinelandii was determined and shown to be highly similar to the N-terminal sequences of the Escherichia coli and Nitrobacter winogradskyi bacterioferritins. Electrophoretic and immunological analyses further indicate that the bacterioferritins of E. coli, A. vinelandii and Pseudomonas aeruginosa are closely related. A novel, two-subunit assembly state that predominates over the 24-subunit form of BFR at low pH was demonstrated. The results indicate that the bacterioferritins form a family of proteins that are distinct from the ferritins of plants and animals.

Bacterial 4-alpha-helical bundle cytochromes

The biological functions of cytochrome c' and bacterioferritin, both haemoproteins with a common 4-alpha-helical bundle structure, are discussed and an example given of one of Kamen's laws, namely: comparative studies of prokaryotic cytochromes and their eukaryotic counterparts are useful. In the present case, the comparison is between bacterioferritin and its animal counterpart, haemoferritin.

Cloning and sequencing of an Escherichia coli K12 gene which encodes a polypeptide having similarity to the human ferritin H subunit

Using lambda phage clones containing segments of the Escherichia coli K12 chromosome as hybridization probes, we found one gene at 42 min on the E. coli chromosome map, the expression of which was affected by RNase III. The sequence of the DNA fragment containing this gene (gen-165) revealed the presence of an open reading frame encoding a polypeptide of 165 amino acid residues. The amino acid sequence deduced from the nucleotide sequence exhibited a remarkable similarity to that of the human ferritin H chain.

Bis-methionine axial ligation of haem in bacterioferritin from Pseudomonas aeruginosa

The iron-containing bacterioferritins contain the protoporphyrin IX haem group. It has been established that Escherichia coli cytochrome b1, cytochrome b557 and bacterioferritin are identical. The optical spectra at room temperature of the haem group show it to be predominantly low-spin in both the ferrous and ferric states. The nature of the axial ligands binding the haem group to the polypeptide has, however, remained unknown. Low-spin, bis-coordinate haem centres in proteins typically have a role in rapid electron transfer as redox changes at the metal ion lead to little structural rearrangement. There are only four amino acids with side-chains that have ligand field strengths sufficient to generate the low-spin state of haem, namely, histidine, lysine, methionine and cysteine. Hence there are, potentially, ten different pairs of these four ligands which could be discovered in electron transfer haemoproteins. To date only three have been established with certainty. They are bis-histidine, as in mammalian cytochrome b5, methionine-histidine, typified by cytochrome c and lysine-histidine, recently recognized by spectroscopic methods in cytochrome f. Here we report the electron paramagnetic resonance and near infrared magnetic circular dichroism spectra of the oxidized state of Ps. aeruginosa bacterioferritin which enable the axial ligands to be identified as the thioether side chains of two methionine residues, a ligation scheme not previously reported for haem in any protein.

Cloning, sequencing, and mapping of the bacterioferritin gene (bfr) of Escherichia coli K-12

The bacterioferritin (BFR) of Escherichia coli K-12 is an iron-storage hemoprotein, previously identified as cytochrome b1. The bacterioferritin gene (bfr) has been cloned, sequenced, and located in the E. coli linkage map. Initially a gene fusion encoding a BFR-lambda hybrid protein (Mr 21,000) was detected by immunoscreening a lambda gene bank containing Sau3A restriction fragments of E. coli DNA. The bfr gene was mapped to 73 min (the str-spc region) in the physical map of the E. coli chromosome by probing Southern blots of restriction digests of E. coli DNA with a fragment of the bfr gene. The intact bfr gene was then subcloned from the corresponding lambda phage from the gene library of Kohara et al. (Y. Kohara, K. Akiyama, and K. Isono, Cell 50:495-508, 1987). The bfr gene comprises 474 base pairs and 158 amino acid codons (including the start codon), and it encodes a polypeptide having essentially the same size (Mr 18,495) and N-terminal sequence as the purified protein. A potential promoter sequence was detected in the 5' noncoding region, but it was not associated with an "iron box" sequence (i.e., a binding site for the iron-dependent Fur repressor protein). BFR was amplified to 14% of the total protein in a bfr plasmid-containing strain. An additional unidentified gene (gen-64), encoding a relatively basic 64-residue polypeptide and having the same polarity as bfr, was detected upstream of the bfr gene.