Zinc transfer from transcription factor IIIA fingers to thionein clusters

Coordination of Zn2+ (and Cd2+) by prokaryotic metallothionein. Involvement of his-imidazole

In mammalian metallothionein Zn2+ is exclusively coordinated to Cys-thiolate to form clusters in which the metal is thermodynamically stable but also kinetically labile. By contrast, little is known about coordination to prokaryotic metallothionein, SmtA. 3 nmol of Zn2+ nmol-1 SmtA were displaced by 8 nmol of p-(hydroxymercuri)phenylsulfonate implicating eight of the nine Cys in the coordination of three metal ions. None of the Zn2+ associated with SmtA was accessible to 4-(2-pyridylazo)resorcinol prior to the addition of p-(hydroxymercuri)phenylsulfonate. An unusual feature of SmtA is the presence of three His residues, and we have investigated whether these contribute to metal coordination. Less Zn2+ was associated with purified SmtA(H40R/H49R/H55R), in which all three His residues were substituted with Arg, and approximately one equivalent of Zn2+ was immediately accessible to 4-(2-pyridylazo)resorcinol. Following incubation of SmtA with 111Cd, three 111Cd resonances were detected, two in a range expected for CdS4 and the third indicative of either CdNS3 or CdN2S2 coordination. Two-dimensional TOCSY 1H NMR and 111Cd-edited 1H NMR showed two His residues bound to 111Cd, confirming CdN2S2 coordination. The pH of half-dissociation of Zn2+ increased from 4.05 for SmtA to 5.37 for SmtA(H40R/H49R/H55R). Equivalent values for single His mutants SmtA(H40R), SmtA(H49R), and SmtA(H55R) were 4.62, 4.48, and 3.81, respectively, revealing that conversion of His40 or His49 to Arg impairs Zn2+ binding at the CdN2S2 and CdS4 sites. Only approximately two equivalents of Zn2+ were associated with purified SmtA(H49R). The appearance of a fourth 111Cd resonance at lower pH suggests that an alternative CdN2S2 site also exists.

Effect of redox conditions on the DNA-binding efficiency of the retinoic acid receptor zinc-finger

Retinoic acid and its derivatives are involved in many important biological processes. In the present study, we have shown that the DNA binding domain of the retinoic acid receptor, which contains two zinc fingers with the Zn(II) tetrahedrally coordinated by four Cys, is susceptible to intracellularly relevant oxidizing agents. In the presence of hydrogen peroxide or hypochlorite, the zinc-finger DNA binding activity was abolished in a concentration dependent manner. The loss of DNA binding activity was correlated with the release of Zn(II) from the zinc-finger motif as a consequence of Zn(II)-thiolate bond oxidation. A combination of glutathione and Zn(II) was able to restore the activity, suggesting that oxidation of the zinc-finger by hydrogen peroxide or hypochlorite resulted in the formation of disulfide bonds between the Cys present in the Zn(II)-binding motif. Our results indicate that in situations of oxidative-stress zinc-finger containing transcription factors may be particularly susceptible to oxidation, resulting in the disruption of control and regulation of gene expression.

A carboxyl-terminal Cys2/His2-type zinc-finger motif in DNA primase influences DNA content in Synechococcus PCC 7942

The DNA primase gene, dnaG, has been isolated from the cyanobacterium Synechococcus PCC 7942. It is not part of a macromolecular synthesis operon but is co-transcribed with pheT and located adjacent to the metallothionein divergon, smt. At the carboxyl terminus of this DnaG is a Cys2/His2 zinc-finger motif. The carboxyl-terminal 91 residues bound 65Zn and 0.95 g atom of Zn2+ mol-1 were detected with 4-(2-pyridylazo)resorcinol. Following exposure to Cd2+, 0.95 g atom of Cd2+ was displaced by 2 equivalents of p-(hydroxymercuri) phenylsulfonate mol-1, while only 0.03 g atom of Cd2+ was displaced mol-1 polypeptide missing the carboxyl-terminal (residue 592 onward) zinc-finger motif. Zn2+ caused an increase in intensity, and a reduction in wavelength, of Trp fluorescence at the tip of the predicted zinc-finger, while EDTA caused the converse. Cells containing a single chromosomal codon substitution (C597S), altering the zinc-finger, were generated by exploiting Zn2+-sensitive smt mutants and the proximity of dnaG to smt. Cells in which smt and dnaG(C597S) had integrated into the chromosome were selected via restored Zn2+ tolerance. Synechococcus PCC 7942 and its dnaG(C597S) mutant grew at equivalent rates, but the latter had a reduced number of chromosomes.

The elusive function of metallothioneins

Biochemistry and genetics are both required to elucidate the function of macromolecules. There is no question that metallothioneins (MTs) have unique biochemical properties, but genetic experiments have not substantiated the importance of MTs under physiological conditions. Even after thousands of studies describing the structure, biochemical characteristics, tissue distribution, induction, and consequences of genetic disruption and deliberate overexpression, the evolutionary forces that led to the initial appearance, gene duplications, and nearly ubiquitous expression of MTs remain enigmatic.

Modulation of DNA binding of a tramtrack zinc finger peptide by the metallothionein-thionein conjugate pair

The ability of metallothionein (MT) to modulate DNA binding by a two-finger peptide of Tramtrack (TTK), a CCHH zinc transcription factor, was investigated using metal-bound and metal-deficient forms of rabbit MT-2 and the TTK peptide. Thionein inhibited DNA binding by zinc-bound TTK, and Zn-MT restored DNA-binding by zinc-deficient apo-TTK. "Free" zinc at low concentrations was as effective as Zn-MT in restoring DNA binding by apopeptide but was inhibitory at concentrations equal to zinc bound to 2 mol eq and higher of Zn-MT. Substitution of cadmium for zinc reduced the affinity of the peptide for its DNA binding site. This effect was reversed by incubation with Zn-MT. The circular dichroic spectra of the TTK peptide indicated that zinc removal resulted in loss of alpha-helical structures, which are sites of DNA contact points. Reconstitution with cadmium resulted in stoichiometric substitution of 2 mol of Cd/mol of peptide but not recovery of alpha-helical structures. Incubation of Cd-TTK with Zn-MT restored the secondary structure expected for zinc-bound TTK. The ability of Zn-MT and thionein to restore or inhibit DNA-binding by TTK was associated with effects on the metallation status of the peptide and related alterations in its secondary structure.

Biphasic kinetics of Zn2+ removal from Zn metallothionein by nitrilotriacetate are associated with differential reactivity of the two metal clusters

We investigated the kinetics of nitrilotriacetate (NTA) extraction of Zn2+ from Zn7-metallothionein (MT) and a metal-hybrid derivative, Zn4Ag6MT, in which the Zn2+ and Ag+ ions occupy sites in the C-terminal alpha and N-terminal beta domains of the protein, respectively. Biphasic kinetics were observed for Zn7MT under pseudo-first-order conditions. Rate constants were (5.2 +/- 0.6) x 10(-3) and (1.0 +/- 0.3) x 10(-4)s-1 in 20 mM phosphate, 100 mM KF, pH 7.5 at 23 degrees C. In contrast, Zn4Ag6MT showed a single kinetic step with a rate constant of (2.9 +/- 0.4) x 10(-3)s-1. These results indicate that the biphasic reactivity of Zn7MT stems from differential susceptibility of the metal in the two metal-thiolate clusters to removal by competing ligands, with Zn2+ in the more stable alpha-domain cluster reacting faster than that in the less stable beta-domain cluster. Such behavior suggests that the structures of the two domains of mammalian MT may have evolved to assure that Cu binding does not compromise the structural characteristics that allow Zn to be rapidly transferred from MT to essential cellular ligands.

Dietary flavonoids interact with trace metals and affect metallothionein level in human intestinal cells

Flavonoids are natural compounds found in food items of plant origin. The study examined systematically the interaction of structurally diverse dietary flavonoids with trace metal ions and the potential impact of dietary flavonoids on the function of intestinal cells. Spectrum analysis was first performed to determine flavonoid-metal interaction in the buffer. Among the flavonoids tested, genistein, biochanin-A, naringin, and naringenin did not interact with any metal ions tested. Members of the flavonol family, quercetin, rutin, kaempferol, flavanol, and catechin, were found to interact with Cu(II) and Fe(III). On prolonged exposure, quercetin also interacted with Mn(II). Quercetin at 1:1 ratio to Cu(II) completely blocked the Cu-dependent color formation from hematoxylin. When quercetin was added to the growth medium of cultured human intestinal cells, Caco-2, the level of metal binding antioxidant protein, metallothionein, decreased. The effect of quercetin on metallothionein was dose- and time-dependent. Genistein and biochanin A, on the contrary, increased the level of metallothionein. The interaction between dietary flavonoids and trace minerals and the effect of flavonoids on metallothionein level imply that flavonoids may affect metal homeostasis and cellular oxidative status in a structure-specific fashion.

Effect of transmucosal fluid movement on zinc and copper absorption from the rat small intestine

1. The effects of transmucosal fluid movement on zinc and copper absorption from rat small intestine were investigated using an in situ recirculating perfusion method. 2. Transmucosal fluid movement was obtained by using hypo-, iso- or hyperosmotic perfusate containing various concentrations of sodium chloride. Linear correlations were obtained between net fluid movement and absorption rate constants, which were calculated from the perfusion decline of zinc (n = 15; r = 0.77) and copper (n = 15; r = 0.75). 3. The changes in serum levels of these metals did not correlate with the increase in the absorption rate. 4. The present study indicates that zinc and copper absorption is dependent on transmucosal fluid movement.

Dynamic regulation of copper uptake and detoxification genes in Saccharomyces cerevisiae

The essential yet toxic nature of copper demands tight regulation of the copper homeostatic machinery to ensure that sufficient copper is present in the cell to drive essential biochemical processes yet prevent the accumulation to toxic levels. In Saccharomyces cerevisiae, the nutritional copper sensor Mac1p regulates the copper-dependent expression of the high affinity Cu(I) uptake genes CTR1, CTR3, and FRE1, while the toxic copper sensor Ace1p regulates the transcriptional activation of the detoxification genes CUP1, CRS5, and SOD1 in response to copper. In this study, we characterized the tandem regulation of the copper uptake and detoxification pathways in response to the chronic presence of elevated concentrations of copper ions in the growth medium. Upon addition of CuSO4, mRNA levels of CTR3 were rapidly reduced to eightfold the original basal level whereas the Ace1p-mediated transcriptional activation of CUP1 was rapid and potent but transient. CUP1 expression driven by an Ace1p DNA binding domain-herpes simplex virus VP16 transactivation domain fusion was also transient, demonstrating that this mode of regulation occurs via modulation of the Ace1p copper-activated DNA binding domain. In vivo dimethyl sulfate footprinting analysis of the CUP1 promoter demonstrated transient occupation of the metal response elements by Ace1p which paralleled CUP1 mRNA expression. Analysis of a Mac1p mutant, refractile for copper-dependent repression of the Cu(I) transport genes, showed an aberrant pattern of CUP1 expression and copper sensitivity. These studies (i) demonstrate that the nutritional and toxic copper metalloregulatory transcription factors Mac1p and Ace1p must sense and respond to copper ions in a dynamic fashion to appropriately regulate copper ion homeostasis and (ii) establish the requirement for a wild-type Mac1p for survival in the presence of toxic copper levels.

Control of zinc transfer between thionein, metallothionein, and zinc proteins

Metallothionein (MT), despite its high metal binding constant (KZn = 3.2 x 10(13) M-1 at pH 7.4), can transfer zinc to the apoforms of zinc enzymes that have inherently lower stability constants. To gain insight into this paradox, we have studied zinc transfer between zinc enzymes and MT. Zinc can be transferred in both directions-i.e., from the enzymes to thionein (the apoform of MT) and from MT to the apoenzymes. Agents that mediate or enhance zinc transfer have been identified that provide kinetic pathways in either direction. MT does not transfer all of its seven zinc atoms to an apoenzyme, but apparently contains at least one that is more prone to transfer than the others. Modification of thiol ligands in MT zinc clusters increases the total number of zinc ions released and, hence, the extent of transfer. Aside from disulfide reagents, we show that selenium compounds are potential cellular enhancers of zinc transfer from MT to apoenzymes. Zinc transfer from zinc enzymes to thionein, on the other hand, is mediated by zinc-chelating agents such as Tris buffer, citrate, or glutathione. Redox agents are asymmetrically involved in both directions of zinc transfer. For example, reduced glutathione mediates zinc transfer from enzymes to thionein, whereas glutathione disulfide oxidizes MT with enhanced release of zinc and transfer of zinc to apoenzymes. Therefore, the cellular redox state as well as the concentration of other biological chelating agents might well determine the direction of zinc transfer and ultimately affect zinc distribution.

Zinc metabolism in the brain: relevance to human neurodegenerative disorders

Zinc is an important trace element in biology. An important pool of zinc in the brain is the one present in synaptic vesicles in a subgroup of glutamatergic neurons. In this form it can be released by electrical stimulation and may serve to modulate responses at receptors for a number of different neurotransmitters. These include both excitatory and inhibitory receptors, particularly the NMDA and GABA(A) receptors. This pool of zinc is the only form of zinc readily stained histochemically (the chelatable zinc pool), but constitutes only about 8% of the total zinc content in the brain. The remainder of the zinc is more or less tightly bound to proteins where it acts either as a component of the catalytic site of enzymes or in a structural capacity. The metabolism of zinc in the brain is regulated by a number of transport proteins, some of which have been recently characterized by gene cloning techniques. The intracellular concentration may be mediated both by efflux from the cell by the zinc transporter ZrT1 and by complexing with apothionein to form metallothlonein. Metallothionein may serve as the source of zinc for incorporation into proteins, including a number of DNA transcription factors. However, zinc is readily released from metallothionein by disulfides, increasing concentrations of which are formed under oxidative stress. Metallothionein is a very good scavenger of free radicals, and zinc itself can also reduce oxidative stress by binding to thiol groups, decreasing their oxidation. Zinc is also a very potent inhibitor of nitric oxide synthase. Increased levels of chelatable zinc have been shown to be present in cell cultures of immune cells undergoing apoptosis. This is very reminiscent of the zinc staining of neuronal perikarya dying after an episode of ischemia or seizure activity. Thus a possible role of zinc in causing neuronal death in the brain needs to be fully investigated. intraventricular injections of calcium EDTA have already been shown to reduce neuronal death after a period of ischemia. Pharmacological doses of zinc cause neuronal death, and some estimates indicate that extracellular concentrations of zinc could reach neurotoxic levels under pathological conditions. Zinc is released in high concentrations from the hippocampus during seizures. Unfortunately, there are contrasting observations as to whether this zinc serves to potentiate or decrease seizure activity. Zinc may have an additional role in causing death in at least some neurons damaged by seizure activity and be involved in the sprouting phenomenon which may give rise to recurrent seizure propagation in the hippocampus. In Alzheimer's disease, zinc has been shown to aggregate beta-amyloid, a form which is potentially neurotoxic. The zinc-dependent transcription factors NF-kappa B and Sp1 bind to the promoter region of the amyloid precursor protein (APP) gene. Zinc also inhibits enzymes which degrade APP to nonamyloidogenic peptides and which degrade the soluble form of beta-amyloid. The changes in zinc metabolism which occur during oxidative stress may be important in neurological diseases where oxidative stress is implicated, such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS). Zinc is a structural component of superoxide dismutase 1, mutations in which give rise to one form of familiar ALS. After HIV infection, zinc deficiency is found which may be secondary to immune-induced cytokine synthesis. Zinc is involved in the replication of the HIV virus at a number of sites. These observations should stimulate further research into the role of zinc in neuropathology.

Metallothionein: a potential marker for differentiating benign and neoplastic gastrointestinal lymphoid infiltrates

Metallothioneins (MT) are low molecular weight, metal-binding proteins. The induction of MT synthesis by cytokines, hormones and other cytotoxic agents indicates its role in cellular proliferation and differentiation as well as in cellular defense mechanisms. Previous studies have detected expression of MT in various human tumors by immunohistochemical staining. In certain cases the presence of MT in a tumor may be associated with its resistance against radiation and chemotherapeutic agents. Immunohistochemical staining of MT using a rabbit polyclonal anti-rat liver MT antibody was carried out in eight gastric, two small bowel and one large bowel lymphomas, and in ten control gastrointestinal (gastric, colonic and small bowel) biopsies or excised bowel segments with benign lymphoid infiltrates. The primary antibody against rat liver MT readily cross-reacts with human MT. The neoplastic cells in nine of 11 malignant lymphomas showed weak to intense staining for MT, mostly in cytoplasm. In these cells a few nuclei (less than 5% of all tumor cells) were stained positively for MT. The benign lymphocytes in the gastrointestinal excised specimens and biopsies were mostly negative; four cases showed few positive cells. The giant cells were also positive for MT in a Crohn's disease case. We conclude that the presence or absence of MT in lymphocytes, detected by immunohistochemistry, may indicate the growth patterns of these cells. The distinct pattern of MT staining in malignant lymphoma in our study is suggestive of a potential use of MT staining as a discriminator between benign and malignant lymphoid tissues.

Metal- and DNA-binding properties and mutational analysis of the transcription activating factor, B, of coliphage 186: a prokaryotic C4 zinc-finger protein

Coliphage 186 B is a 72-amino acid protein belonging to the Ogr family of analogous transcription factors present in P2-like phage, which contain a Cys-X2-Cys-X22-Cys-X4-Cys presumptive zinc-finger motif. The molecular characterization of these proteins has been hampered by their insolubility, a difficulty overcome in the present study by obtaining B as a soluble cadmium-containing derivative (CdB). Atomic absorption spectroscopy showed the presence of one atom of cadmium per molecule of purified CdB. The UV absorption spectrum revealed a shoulder at 250 nm, characteristic of CysS-Cd(II) ligand-to-metal charge-transfer transitions, and the difference absorption coefficient after acidification (delta epsilon 248, 24 mM-1 cm-1) indicated the presence of a Cd(Cys-S)4 center. Gel mobility shift analysis of CdB with a 186 late promoter demonstrated specific DNA-binding (KD, app 3-4 microM) and the protein was shown to activate transcription in vitro from a promoter-reporter plasmid construct. The B DNA-binding site was mapped by gel shift and DNAase I cleavage protection experiments to an area between-70 and -43 relative to the transcription start site, coincident with the consensus sequence, GTTGT-N8-TNANCCA, from -66 to -47 of the 186 and P2 late promoters. Inactive B point mutants were obtained in the putative DNA-binding loop of the N-terminal zinc-finger motif and in a central region thought to interact with the Escherichia coli RNA polymerase alpha-subunit. A truncated B mutant comprising the first 53 amino acids (B1-53) exhibited close to wild-type activity, showed a DNA-binding affinity similar to that of the full-length protein, and could be reconstituted with either Cd or Zn. Gel permeation analysis revealed that B1-53 was a majority dimeric species whereas wild-type B showed larger oligomers. 186 B therefore exhibits a potentially linear organization of functional regions comprising an N-terminal C4 zinc-finger DNA-binding region, a dispensable C-terminal region involved in protein self-association, and a central region that interacts with RNA polymerase.

Effect of non-toxic mercury, zinc or cadmium pretreatment on the capacity of human monocytes to undergo lipopolysaccharide-induced activation

1. Metal salts can inhibit cell activity through direct toxicity to critical cellular molecules and structures. On the other hand, they can also change cell behaviour by inducing specific genes (including genes encoding members of the metallothionein [MT] gene family). Therefore, transition metals may affect cell functions either by acting as a toxin, or by transmitting or influencing signals controlling gene expression. 2. To explore the latter possibility, we measured the ability of low, non-toxic metal pretreatment to alter immune cell behaviour. We previously found that pretreatment of human monocytes with zinc induces metallothionein gene expression and alters their capacity to undergo a bacterial lipopolysaccharide-induced respiratory burst. We showed here that cadmium and mercury salts, at concentrations that exert no discernible toxicity, inhibit activation of human monocytic leukemia (THP-1) cells. CdCl2 1 microM, ZnCl2 20-40 microM or HgCl2 2 microM pretreatment for 20 h induced MT-2 mRNA and total MT protein accumulation and had no effect on proliferation potential or metabolic activity, but significantly inhibited the ability of subsequent lipopolysaccharide treatment to induce the oxidative burst, increased adhesion to plastic, and MT-2 and interleukin-1 beta (IL-1 beta) mRNA accumulation. 3. The phenomenon of metal-induced suppression of monocyte activation, at metal concentrations that have no effect on cell viability, has important implications for assessment of acceptable levels of human exposure to cadmium, zinc and mercury.

Metallothionein in physiological and physiopathological processes

The multipurpose nature of MT that we have presented in this review has drawn attention from many different fields of research: biochemistry, molecular biology, toxicology, pharmacology, etc. In recent years, considerable advances have been made concerning the regulation of MT genes by metals. Little, however, is known at the molecular level about the mechanisms of MT induction by nonmetallic inducers such as growth factors. This is of particular interest since MT is highly expressed during liver regeneration, an event orchestrated by a series of growth stimulators and inhibitors. The significance of the nuclear distribution of MT in growing cells and what controls its translocation are questions that remain unanswered at the present time. The possibility that MT could participate in a DNA synthesis-related process through donation or abstraction of Zn to and from transcription factors has been inferred from in vitro studies. Such transfer mechanisms, however, have yet to be confirmed in vivo. Overexpression of MT is often accompanied by increased resistance towards a variety of alkylating agents and chemotherapeutic drugs. The mechanisms by which MT protects cells against these agents may depend on their distinct mode of toxic action. For some, MT cysteines can be the target of the direct attack from the parent compound. For others such as N-methyl-N-nitroso compounds, MT cysteines may serve as a sink for the reactive oxygen species now known to be derived from their metabolism. In either case, a primary consequence of such interactions is the release of the metals initially bound to MT. Therefore, the metal composition of MT appears to be an important factor to consider in determining the overall effect of MT in the resistance process.

Chinese hamster cells expressing antisense to metallothionein become spontaneous mutators

The functions of metallothioneins (MTs) have been debated for at least a decade. Because it seems unlikely that they evolved only to protect cells against exogenous heavy metals, it has been suggested that MTs have roles in scavenging reactive intermediates, controlling zinc and copper homeostasis, and controlling transfer of zinc to transcription factors and other proteins. Previously, we demonstrated that Chinese hamster G12 cells which overexpress MT have greatly reduced spontaneous mutation rates, suggesting that MT evolved to prevent spontaneous mutagenesis induced by free nuclear zinc ions. We have now isolated G12 transfectants which express antisense RNA to MT. Immunofluorescent staining reveals MT protein in both the nucleus and the cytoplasm in parental cells. A clone expressing high levels of antisense RNA (AMT30) shows reduced basal and induced levels of MT protein. AMT30 cells are hypersensitive to cadmium, zinc, copper and mercury chlorides as well as to menadione. Glutathione levels in AMT30 and G12 cells do not differ. AMT30 cells are spontaneous mutators, showing a spontaneous mutation rate 5-10 times that of G12 cells or G12 cells transfected with vector alone. Only transfectants which show a high level of MT antisense expression (i.e., AMT30) had greatly elevated spontaneous mutation rates. These results support our hypothesis that a major role of MT is to act as an endogenous antimutagen probably via scavenging of reactive intermediates in the nucleus. AMT30 cells should be useful in delineating the sources of spontaneous mutagenesis.

Inhibition of zinc-induced metallothionein mRNA accumulation by gonadotropin-releasing hormone in human hepatocarcinoma cell line HepG2

Recently we have demonstrated that the human hepatocellular carcinoma-derived cell lines, HepG2 and HuH7, contain gonadotropin-releasing hormone (gonadoliberin) receptors and respond to various molecular forms of gonadoliberin in terms of suppressed proliferation in vitro. This study provides the first demonstration that gonadoliberin inhibits the zinc-induced production of metallothionein mRNA in HepG2 and HuH7 cells. Administration of gonadoliberin agonist (gonadoliberin-A) inhibited the Zn-induced metallothionein mRNA level in a time-related and dose-related manner. The effect of gonadoliberin-A was found to be specific, because concomitant treatment with a gonadoliberin antagonist (gonadoliberin-ANT) blocked gonadoliberin-A inhibition of metallothionein mRNA accumulation. Furthermore, the gonadoliberin-A-induced inhibition of Zn-mediated metallothionein accumulation was found to correlate closely with suppresion of cell proliferation and [3H]thymidine uptake in these cells. It is known that the metal-binding protein metallothionein plays an important role in tumor cell pathobiology and resistance to chemotherapeutic drugs. The present findings may have important implications in the development of an effective chemotherapy for treatment of human liver cancer, in part, by improving the sensitivity of tumor cells through suppression of metallothionein production by gonadoliberin peptides.

RNA expression patterns of a type 2 metallothionein-like gene from rice

A type 2 metallothionein-like gene from rice, OsMT-2 (Oryza sativa metallothionein-like gene-2), was isolated in its cDNA form and sequenced. By northern analyses OsMT-2 expression was shown to be induced under stress by sucrose starvation, heat shock and, to a lesser extent, abscisic acid, but not excess metals, including copper. Its response to sucrose starvation was transient and different from OsMT-1, a type 1 metallothionein-like gene of rice inducible by copper. These results suggest that while OsMT-2 is also involved in cellular response to stress, its function may be complementary to that of OsMT-1.

Zinc- and iron-rubredoxins from Clostridium pasteurianum at atomic resolution: a high-precision model of a ZnS4 coordination unit in a protein

The Zn(Scys)4 unit is present in numerous proteins, where it assumes structural, regulatory, or catalytic roles. The same coordination is found naturally around iron in rubredoxins, several structures of which have been refined at resolutions of, or near to, 1 A. The fold of the small protein rubredoxin around its metal ion is an excellent model for many zinc finger proteins. Zn-substituted rubredoxin and its Fe-containing counterpart were both obtained as the products of the expression in Escherichia coli of the rubredoxin-encoding gene from Clostridium pasteurianum. The structures of both proteins have been refined with an anisotropic model at atomic resolution (1.1 A, R = 8.3% for Fe-rubredoxin, and 1.2 A, R = 9.6% for Zn-rubredoxin) and are very similar. The most significant differences are increased lengths of the M-S bonds in Zn-rubredoxin (average length, 2.345 A) as compared with Fe-rubredoxin (average length, 2.262 A). An increase of the CA-CB-SG-M dihedral angles involving Cys-6 and Cys-39, the first cysteines of each of the Cys-Xaa-Xaa-Cys metal binding motifs, has been observed. Another consequence of the replacement of iron by zinc is that the region around residues 36-46 undergoes larger displacements than the remainder of the polypeptide chain. Despite these changes, the main features of the FeS4 site, namely a local 2-fold symmetry and the characteristic network of N-H...S hydrogen bonds, are conserved in the ZnS4 site. The Zn-substituted rubredoxin provides the first precise structure of a Zn(Scys)4 unit in a protein. The nearly identical fold of rubredoxin around iron or zinc suggests that at least in some of the sites where the metal has mainly a structural role-e.g., zinc fingers-the choice of the relevant metal may be directed by its cellular availability and mobilization processes rather than by its chemical nature.

Dosage compensation in Drosophila and the "complex' world of transcriptional regulation

The purpose of this review is to draw attention to the mechanism of dosage compensation in Drosophila as a model for the study of the regulation of gene activity through the modulation of transcription. Dosage compensation resembles some mechanisms of transcriptional regulation, found in widely divergent organisms, that do not play a role in the activation of silent genes but determine the level of activity of genes that have been induced through the action of specific activators. It differs from other known regulatory mechanisms in that its effect is to achieve, on average, a twofold change in gene activity levels. This review introduces the notion that, in order to yield such a defined level of regulation, the mechanism of dosage compensation in Drosophila, and perhaps in Caenorhabditis as well, incorporates elements that govern both transcriptional enhancement and repression within the same multi-protein regulatory complex.

Modulation by nitric oxide of metalloprotein regulatory activities

In many cells, a nitric oxide (NO) synthase inducible by immunological stimuli produces a sustained flow of NO that lasts a long time. NO is a short-lived molecule but it is a diffusible ligand believed to be capable of reaching distal target sites. Further, several lines of evidence indicate that cysteine-rich motifs of metal-binding proteins, as well as redox-sensitive metal clusters of metalloproteins, are natural sensors of bioradicals like NO. In metalloregulatory proteins, metals are often conveniently located at binding sites and bound to cysteine residues. Accordingly, disruption of the metal-thiolate polymetallic clusters should trigger significant remodelling of the protein structure involved in regulation. We can therefore postulate that the nitrosation reaction occurring at metal centres or cysteine-rich motifs will preclude correct binding to regulatory sites. Several examples are given of metalloregulatory proteins whose metal is bound to thiols and may then become sensitive to NO. Recent observations indicate that in response to NO synthesis, iron regulatory protein, a eukaryotic bifunctional [Fe-S] protein, switches from acting as aconitase to being an RNA-binding regulator, and we suggest that the interplay between NO or a NO-derived molecule and metal clusters at critical allosteric sites may be a crucial component of the cellular response to environmental stress.

Interactions between metallothionein inducers in rat liver and primary cultures of rat hepatocytes

The interaction of Zn, stress and endotoxin on liver metallothionein (MT) regulation has been studied in the rat. Zn, stress and endotoxin increased liver MT levels significantly, by 12-, 5- and 8-fold, respectively. The previous administration of Zn to stress or endotoxin treatments increased MT levels by 35- and 42-fold, respectively, indicating a synergistic effect in both cases. In contrast, when liver MT was preinduced by stress, MT levels were further increased by endotoxin only in an additive manner. In another experiment where liver MT induction by stress was studied in control rats and in rats with preinduced MT by Zn, endotoxin or stress, it was found that Zn pretreated animals had higher MT-I mRNA levels than endotoxin- or stress-pretreated ones. No synergisms between dexamethasone, Zn, TNF and IFN were observed in primary culture of hepatocytes. These results suggest that the observed synergisms between Zn and other MT inducers in vivo in the liver is a consequence of increased Zn levels in the body and mobilization capacity, with concomitant MT synthesis.

In vivo and in vitro iron-replaced zinc finger generates free radicals and causes DNA damage

The estrogen receptor (ER) is a ligand-activated transcription factor whose DNA-binding domain (ERDBD) has eight cysteines, which coordinate two zinc atoms, forming two zinc finger-like structures. We demonstrate the capability of iron to replace zinc in zinc finger (hereby referred to as iron finger) both in vivo (using Escherichia coli BL21 (DE3)) and in vitro. Iron has the ability to substitute for zinc in the ERDBD as demonstrated by mobility shift and methylation interference assays of iron finger, which show specific recognition of the estrogen response element. The DNA binding constants for both in vivo and in vitro iron-replaced zinc fingers were similar to that of the native finger. Atomic absorption analysis revealed a ratio of 2:1 iron atoms/mol of ERDBD protein, as found for zinc in the crystal structure of native ERDBD. More importantly, we demonstrate that iron finger in the presence of H2O2 and ascorbate generates highly reactive free radicals, causing a reproducible cleavage pattern to the proximate DNA, the estrogen response element. The deoxyribose method, used to detect free radical species generated, and the resultant cleaved DNA ends, caused by iron finger, suggest that the free radicals generated are hydroxyl radicals. Due to the close proximity of the zinc finger to DNA, we postulate that iron-substituted zinc finger may generate free radicals while bound to genetic regulatory response elements, leading to adverse consequences such as iron-induced toxicity and/or carcinogenesis.

Expression of the type 2 metallothionein-like gene MT2 from Arabidopsis thaliana in Zn(2+)-metallothionein-deficient Synechococcus PCC 7942: putative role for MT2 in Zn2+ metabolism

Zn2+ proteins pervade metabolism and are essential for gene expression. However, no proteins have been ascribed the central roles of Zn2+ donation to, or removal from, metalloproteins, or Zn2+ storage in vegetative plant tissue. In animals, such functions have been proposed for metallothioneins. Plants contain multiple metallothionein-like genes but their predicted products, which differ significantly from animal metallothioneins, remain to be isolated from vegetative tissue and their roles are uncertain. The type 2 metallothionein-like gene from Arabidopsis, MT2, was expressed under the control of Zn2+-responsive elements derived from the cyanobacterial metallothionein divergon, smt. Zn2+-dependent expression of MT2 transcripts in Synechococcus PCC 7942 was confirmed by northern analysis. The Arabidopsis MT2 gene partly complemented Zn2+ hypersensitivity in mutants of Synechococcus PCC 7942 which are functionally deficient in an endogenous Zn2+-metallothionein gene, smtA. MT2 was also expressed as a recombinant fusion protein in Escherichia coli, purified and shown to bind Zn2+ in vitro. The mean pH of half displacement of Zn2+ from MT2 was estimated to be 5.05. This suggests that MT2 has a greater affinity for Zn2+ than phytochelatins. The results presented here reveal that MT2 is capable of binding Zn2+ in vitro, conferring tolerance to elevated [Zn2+] in vivo within cyanobacteria and is likely to compete with other polypeptides for cellular Zn2+ in planta.

Zinc supplementation is associated with improved neurologic recovery rate and visceral protein levels of patients with severe closed head injury

Sixty-eight patients were entered into a randomized, prospective, double-blinded controlled trial of supplemental zinc versus standard zinc therapy to study the effects of zinc supplementation on neurologic recovery and nutritional/metabolic status after severe closed head injury. One month after injury, the mortality rates in the standard zinc group and the zinc-supplemented group were 26 and 12%, respectively. Glasgow Coma Scale (GCS) scores of the zinc-supplemented group exceeded the adjusted mean GCS score of the standard group at day 28 (p = 0.03). Mean motor GCS score levels of the zinc-supplemented group were significantly higher on days 15 and 21 than those of the control group (p = 0.005, p = 0.02). This trend continued on day 28 of the study (p = 0.09). The groups did not differ in serum zinc concentration, weight, energy expenditure, or total urinary nitrogen excretion after hospital admission. Mean 24-h urine zinc levels were significantly higher in the zinc-supplemented group at days 2 (p = 0.0001) and 10 (p = 0.01) after injury. Mean serum prealbumin concentrations were significantly higher in the zinc-supplemented group (p = 0.003) at 3 weeks after injury. A similar pattern was found for mean serum retinol binding protein level (p = 0.01). A significantly larger number of patients in the standard zinc group had craniotomies for evacuation of hematoma; thus a bias may have been present. The results of this study indicate that zinc supplementation during the immediate postinjury period is associated with improved rate of neurologic recovery and visceral protein concentrations for patients with severe closed head injury.

Xenopus lipovitellin 1 is a Zn(2+)- and Cd(2+)-binding protein

This report discusses the identification of a Zn(2+)- and Cd(2+)-binding protein of Xenopus laevis that is abundant in vitellogenic oocytes and in embryos from fertilization to stage 46. Oocyte or embryo homogenates were fractionated by SDS-PAGE, blotted onto nitrocellulose, and probed with 65Zn2+ or 109Cd2+. The resulting autoradiograms showed binding of both radionuclides to a protein, designated pCdZn. Freon extraction of oocyte and embryo homogenates showed pCdZn to be a yolk protein. When pCdZn was isolated from oocyte homogenates by ammonium sulfate precipitation, delipidation, and chromatography, it co-purified with lipovitellin 1. The amino acid composition of pCdZn closely resembled the reported composition of lipovitellin 1 and the molecular weight of purified pCdZn (approximately 115 kD) corresponded to reported values for lipovitellin 1 (111-121 kD). Amino acid sequence analyses of five peptides derived from pCdZn yielded 94% identity to the reported sequence of lipovitellin 1, deduced from the DNA sequence of the Xenopus vitellogenin A2 precursor gene. Based on these findings, pCdZn was identified as lipovitellin 1. This study suggests that lipovitellin 1 is the major storage protein for zinc in mature oocytes and developing embryos of Xenopus laevis.

Purification and cation binding properties of the recombinant human S100 calcium-binding protein A3, an EF-hand motif protein with high affinity for zinc

The calcium-binding protein S100A3 is an unusual member of the S100 family, characterized by its very high content of Cys. In order to study the biochemical, cation-binding, and conformational properties, we produced and purified the recombinant human protein in Escherichia coli. The recombinant protein forms noncovalent homodimers, tetramers, and polymers in vitro with a subunit molecular weight of 11,712. The Zn(2+)-binding parameters of S100A3 were studied by equilibrium gel filtration and yielded a stoichiometry of four Zn2+ per monomer with a [Zn2+]0.5 of 11 microM and a Hill coefficient of 1.4 at physiological ionic strength. The affinity for Ca2+ is too low to be determined by direct methods (KCa > 10 mM). Ca(2+)- and Zn(2+)-binding can be followed by optical methods: the Trp-45 fluorescence is high in the metal-free form and addition of Zn2+ and Ca2+, but not of Mg2+, leads to a 4-fold quenching. Ca2+ and Zn2+ promote also quite similar conformational changes in the Tyr and Trp environment as monitored by difference spectrophotometry. Fluorescence titrations with Zn2+ confirmed that there is one set of high affinity binding sites with a [Zn2+]0.5 of 8 microM and a Hill coefficient of 1.3. Binding of Zn2+ to a second set of low affinity sites induces protein precipitation. Fluorescence titrations with Ca2+ confirmed the very low affinity of S100A3 for this ion with a [Ca2+]0.5 of 30 mM and slight negative cooperativity. Mg2+ has no effect on this binding curve. Of the 10 Cys residues in S100A3, 5 only are free thiols, and accessible to 5,5'-dithiobis(2-nitro-benzoic acid); they display a high reactivity in the metal-free and Ca2+ form, but a 20-fold lowered reactivity in the Zn2+ form of S100A3. Ca(2+)-binding promotes the formation of a solvent-accessible hydrophobic surface as monitored by the 60-fold fluorescence increase of 2-p-toluidinylnaphthalene-6-sulfonate, whereas Zn2+ has no noticeable influence. Our data indicate that Ca2+ and Zn2+ do not bind to the same sites and that under physiological conditions S100A3 is a Zn(2+)-binding rather than a Ca(2+)-binding protein; nevertheless, very specific conformational changes are introduced by either Ca2+ or Zn2+. Since no Zn(2+)-binding motif of known structure was identified in the primary sequence of S100A3, the results are suggestive for a novel Zn(2+)-binding motif.

Covalent sequestration of melphalan by metallothionein and selective alkylation of cysteines

Rabbit liver metallothionein-2 is shown to form covalent bonds with the anticancer agent melphalan, in support of the hypothesis that covalent sequestration by metallothionein constitutes one mechanism for the cross-resistance acquired by cancer patients to therapeutic alkylating agents. Among 20 cysteines in the 2-domain protein, 89% of the first alkylation reaction occurs with 2 that cochelate a zinc cation in the carboxy domain. Computer-supported docking studies indicate a favorable binding site for melphalan near these cysteine sulfhydryl groups. Although folded metallothionein-2 is resistant to trypsin cleavage, alkylation by 1 mol of melphalan allows cleavage by trypsin between the two globular domains.

Effects of teratogenic exposures to Zn2+, Cd2+, Ni2+, Co2+, and Cu2+ on metallothionein and metallothionein-mRNA contents of Xenopus embryos

Xenopus laevis embryos were analyzed for metallothionein by silver-saturation assay and metallothionein-mRNA by reverse transcriptase/polymerase chain reaction following exposures to the following metal chlorides at levels that caused > 95% malformations and < 7% mortality: Zn2+ (300 microM); Cd2+ (18 microM); Ni2+ (56 microM); Co2+ (1,800 microM); and Cu2+ (5.6 microM). At the beginning of the exposure (stages 8), metallothionein-mRNA and metallothionein levels averaged 2.0 x 10(6) copies/embryo and 19 pmol/embryo, respectively. In control embryos at stages 26, 36, 42, and 46, metallothionein-mRNA content averaged 9, 37, 104, and 97 copies x 10(6)/embryo, and metallothionein content averaged 6, 11, 15, and 18 pmol/embryo. In Zn(2+) -exposed embryos at the same stages, metallothionein-mRNA content averaged 116*, 11,400*, 3,210*, and 14 copies x 10(6)/embryo and metallothionein content averaged 10, 18*, 46*, and 90* pmol/embryo; in Cd(2+)-exposed embryos, metallothionein-mRNA content averaged 22, 7,170*, 1,783*, and 240 copies x 10(6)/embryo and metallothionein content averaged 8, 14, 33*, and 56* pmol/embryo, respectively (*P < 0.05 versus controls). Exposure-response curves (Cd2+, 1-18 microM; Zn2+, 3-300 microM) indicated that Cd2+ was 3- to 5-times more potent than Zn2+, based on metallothionein-mRNA response at stage 36 and metallothionein response at stage 46. In Ni(2+)-, Co(2+)-, or Cu(2+)-exposed embryos, metallothionein-mRNA and metallothionein contents did not differ significantly from controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyanobacterial metallothioneins: biochemistry and molecular genetics

Metallothioneins have been extensively studied in many different eukaryotes where they sequester, and hence detoxify, excess amounts of certain metal ions. However, the precise functions of many of these molecules are not fully understood. This article reviews literature concerning their namesakes in prokaryotes.

Early mouse preimplantation development is unaffected by microinjection of metallothionein antibodies

Polyclonal antibodies that cross-react with rodent metallothionein I (MT I) and metallothionein II (MT II) were microinjected in 1-cell and 2-cell mouse embryos, into either the cytoplasm or the nucleus. Regardless of the experimental treatment, mouse embryo development in vitro was not affected and most of the embryos cleaved normally until the morula stage. The results suggest that metallothionein is not essential for normal mouse early preimplantational development, in agreement with recent studies in mice with inactivated MT I and MT II genes.

Cell cycle regulation of metallothionein in human colonic cancer cells

Elevated levels of metallothionein (MT) found in rapidly growing tissues such as neonatal liver and various types of human tumors have suggested a role for MT in cell proliferation. To further explore this possibility we investigated the concentration of MT in human colonic cancer (HT-29) cells at different stages of proliferation by means of immunocytochemistry and competitive binding. MT is increased in subconfluent proliferating cells relative to growth-inhibited confluent cells, much as it is in growing tissues. Cycling cells synchronized with compactin, an inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, revealed an oscillation of cytoplasmic MT that reached a maximum in successive late G1 phases and at the G1/S transition. Individual phase of the cell cycle were assessed by [3H]thymidine incorporation and by immunofluorescence employing an antibody that detects a nuclear antigen associated with proliferation. An enzyme-linked immunosorbent assay was used to quantify the relative amounts of MT in homogenate supernatants of HT-29 cells. A 2- to 3-fold increase in MT in actively proliferating cells and the regulation of the protein during the mitotic cell cycle point to a physiological role for MT in cellular proliferation and suggest that it may also serve as a proliferation marker.

Developmental variability of metallothionein Mtn gene expression in the species of the Drosophila melanogaster subgroup

Developmental expression of the Drosophila melanogaster metallothionein Mtn gene has been analysed. Transcripts of this gene accumulate during the vitellogenic phase of oogenesis in a ring of follicular cells at the oocyte-nurse cell margin and in the follicular cells surrounding the oocyte. There is also strong expression of the Mtn gene during the second half of embryogenesis in hemocytes, the endoderm midgut, and Malpighian tubules. A banded expression pattern is observed transiently in the midgut at stage 13. The two Mtn alleles, Mtn and Mtn, show quantitative differences in their expression patterns. Copper intoxication of flies does not induce ectopic expression of the Mtn gene, but rather leads to over-expression of the gene in the structures where it is normally transcribed. Mtn transcription is not altered in homozygous mutants of four genes (lab, wg, dpp, bap) known to be involved in midgut morphogenesis. Expression of Mtn has been also studied in six other species of the melanogaster subgroup. This analysis demonstrates that regulation of Mtn gene transcription has changed during evolution of the Drosophila lineage. For example, Mtn is expressed specifically in the Malpighian tubules of D. melanogaster, while in D. mauritiana and D. sechellia the amnioserosa is a specific location of expression. Nonetheless, expression of Mtn in the midgut is common to the seven species, suggesting a basic role for the MTN protein during embryogenesis in this organ, possibly in the release of metallic ions from vitellogenins. In contrast, two genes also expressed in the embryonic midgut, lab and dFRA, display identical patterns in all species of the melanogaster subgroup. The diversity of Mtn patterns in closely related Drosophila species exemplifies the rapid evolution of a gene regulatory system.

Zinc metallochemistry in biochemistry

The chemically stable but stereochemically flexible, non-toxic nature of zinc combined with its amphoteric properties has permitted it to orchestrate a number of zinc-binding motifs critical to life processes. For zinc enzymes, catalytic, cocatalytic, and structural zinc sites exist. DNA-binding proteins have zinc fingers, twists, and clusters exist.

Metallothionein induction as a measure of response to metal exposure in aquatic animals

Metallothioneins (MTs) are considered central in the intracellular regulation of metals such as copper, zinc, and cadmium. Increased MT synthesis is associated with increased capacity for binding these metals and protection against metal toxicity. Recent advances in the biochemistry and molecular biology of MTs have facilitated research on MTs in aquatic species. For the bivalve mollusc Crassostrea virginica, a species frequently used in studies on the toxicology and environmental monitoring of metals, the primary structure for MT has been deduced from analysis of the proteins and cDNA. Procedures for analysis of MT synthesis and MT gene expression have been applied in studies of response to metal exposure. Induction of specific MT forms by Cd is concentration- and time-dependent. The levels of MT-bound metals exhibit a strong relationship with the cytosolic metal concentrations in a metal-exposed natural population of oysters. Ribonuclease protection assays using sequence-specific antisense RNA probes have shown that the MT mRNA structure in this natural population exhibits considerable individual variability in the 3'-untranslated region. Although yet to be substantiated, the possibility exists that the distribution of this variability may be related to the level of environmental metal contamination. One probe derived from the coding region is suitable for use in quantitative RPAs for oyster MT mRNAs.

Purification and characterisation of recombinant sea urchin metallothionein expressed in Escherichia coli

Metallothioneins (MT) are metalloproteins expressed tissue specifically during the development of the sea urchin, Strongylocentrotus pururatus. To explore their structural and functional features and to compare them with those of the evolutionary distant mammalian MTs, one isoform (MTA) was obtained as the cadmium-containing form, from synthetic cDNA heterologously expressed in Escherichia coli. The purified protein was identified as the desired product by a combination of peptide-map analysis, amino acid sequence analysis and ion-spray mass spectroscopy. The existence of seven 113Cd NMR resonances revealed that the recombinant protein binds seven Cd ions/molecule. The position of the NMR resonances (605-695 ppm) and the electronic absorption features suggest that the sea urchin MTA, like the mammalian MTs, possesses tetrahedrally coordinated cadmium-thiolate clusters. With its large Stokes' radius, sea urchin MTA resembles the mammalian forms, suggesting a comparable elongated molecular shape. Measurements by spectrophotometric pH titration of cadmium binding by the recombinant protein suggest that it possesses two metal-thiolate clusters of distinctly different stability. At pH 7 the average apparent association constant for Cd2+ in the clusters is about 20-times weaker in sea urchin MTA than in rabbit MT-2.

The significance of the nuclear and cytoplasmic localization of metallothionein in human liver and tumor cells

Metallothioneins are a group of low-molecular-weight intracellular proteins present in high levels in fetal mammalian livers, bound to zinc and copper. They are also present in two major isoforms in low basal levels in various organs of adults in several species. Although a number of functions have been proposed for metallothioneins, their major biological roles may be in the storage of zinc and copper during rapid growth and development, and also in the detoxification of certain toxic metals. In adult liver, metallothionein is mainly localized in the cytoplasm, it is localized also in the hepatocyte nuclei in human fetal liver and fetal and neonatal rat liver, as determined by immunohistochemical staining with a specific metallothionein antibody. Because of its high expression in fetal development, the potential role of metallothioneins in human tumors was investigated. The cellular localization of metallothionein was demonstrated in various human tumors such as thyroid tumors, testicular germ cell carcinoma, bladder transitional cell carcinomas, and salivary gland tumors. In most of these tumor tissues, metallothioneins were found in high levels in nucleus and cytoplasm in both benign and malignant tumors, although the proliferating edge of the malignant tumors showed most intense metallothionein staining. The expression of metallothionein is not universal to all tumor growth; its presence may depend on various factors, such as the type of tumor, cellular origin, morphological heterogeneity, or stage of growth. Human testicular seminomas, which are well differentiated, showed little expression of metallothionein irrespective of the staging, as compared to less well-differentiated embryonal carcinomas.(ABSTRACT TRUNCATED AT 250 WORDS)

Basal metallothionein in tumors: widespread presence of apoprotein

A survey has been conducted of solid and ascites tumors from mice and solid tumors in rats for the presence of metallothionein or metallothionein-like protein. In most tumors, a positive identification was made on the basis of Sephadex G-75 and HPLC-DEAE chromatography followed by competitive radioimmunoassay for metallothionein. Apometallothionein was revealed in a number of tumors for the first time by comparing the Sephadex G-75 chromatographic profiles of Zn in native cytosol and Cd in cytosol incubated briefly with CdCl2 to saturate free binding sites on the protein before Sephadex G-75 chromatography. In two cases unsaturation of metallothionein was correlated with a lack of zinc in the ascites fluid which supplies the tumor with zinc.

Transcription factor IIIA (TFIIIA): an update

Xenopus transcription factor, termed TFIIIA, is the first eukaryotic transcription factor purified to homogeneity and one of the most extensively characterized polymerase III gene factors at the levels both of the protein and its gene. It is an abundant protein in oocytes and is specifically required for the 5S RNA gene transcription. It promotes the formation of a stable transcription complex by first binding to the internal control region of the 5S RNA gene through its zinc finger motifs. It contains two structural domains and associates with 5S RNA to form 7S ribonucleoprotein particles in oocytes. Its expression is developmentally controlled at the level of transcription and translation. It participates in the assembly of active chromatin templates and, at least in part, is responsible for the differential expression of two kinds of 5S RNA genes in Xenopus.

Plant metallothioneins

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Transition metals in control of gene expression

Metalloproteins play structural and catalytic roles in gene expression. The metalloregulatory proteins are a subclass that exerts metal-responsive control of genes involved in respiration, metabolism, and metal-specific homeostasis or stress-response systems, such as iron uptake and storage, copper efflux, and mercury detoxification. Two allosteric mechanisms for control of gene expression were first discovered in metalloregulatory systems: an iron-responsive translational control mechanism for ferritin production and a mercury-responsive DNA-distortion mechanism for transcriptional control of detoxification genes. These otherwise unrelated mechanisms give rise to a rapid physiological response when metal ion concentrations exceed a dangerous threshold. Molecular recognition in these allosteric metal ion receptors is achieved through atypical coordination geometries, cluster formation, or complexes with prosthetic groups, such as sulfide and heme. Thus, many of the inorganic assemblies that otherwise buttress the structure of biopolymers or catalyze substrate transformation in active sites of enzymes have also been adapted to serve sensor functions in the metalloregulatory proteins. Mechanistic studies of these metal-sensor protein interactions are providing new insights into fundamental aspects of inorganic chemistry, molecular biology, and cellular physiology.

Molecular cloning and expression of a metallothionein mRNA in Xenopus laevis

Metallothioneins (MT) are expressed during early development in species of several groups. To understand MT function in developmental processes, we studied the MT system of Xenopus laevis, a model vertebrate species in experimental embryology. We first purified and sequenced the liver MT from copper-treated animals. This 62-amino-acid protein shares the main structural properties of known vertebrate MT, and is more closely related to avian than to fish or mammalian MT. Using this sequence, we designed oligonucleotide primers to amplify and isolate a MT clone from a XL2 cell line cDNA library. This 752-bp cDNA encodes a putative 62-amino-acid-long protein that is 100% identical with the sequenced MT. Zinc, cadmium, and copper ions are very efficient inducers of MT mRNA accumulation in Xenopus liver and cell lines.

Differential control of free calcium and free zinc levels in isolated bovine liver nuclei

Modulation of intracellular signal transduction by Ca2+ and possibly Zn2+ is based on an effective homoeostatic control of the corresponding free ion concentrations. We used the fluorescent indicator fura-2 to monitor concentrations of free Ca2+ and free Zn2+ in nuclei isolated from bovine liver. The nuclei showed an ATP-stimulated accumulation of intranuclear free Ca2+, which was inhibited in the presence of the Ca(2+)-pump inhibitor thapsigargin. Furthermore, uptake and intranuclear levels of free Zn2+ were measured after incubation with different extranuclear Zn2+ concentrations. There was no stimulating effect of ATP on Zn2+ uptake. Our data suggest that the levels of intranuclear free Ca2+ and free Zn2+ are controlled differentially. A distinct regulation of free ion levels in the nucleus may contribute to the specific control of nuclear events associated with gene transcription and cell differentiation.

Zinc rapidly induces a metal response element-binding factor

Metal activation of metallothionein gene transcription is mediated by specific promoter sequences, termed metal regulatory elements (MREs). Nuclear extracts prepared from various human cell lines were assayed for their capacity to bind to a synthetic human MREa (hMREa) oligomer. Electrophoretic mobility-shift assays with extracts from control cells detected a single hMREa-containing complex. Addition to the growth medium of zinc, cadmium, or copper--metals known to induce MT biosynthesis in vivo--resulted in the rapid but reversible appearance of a second distinct hMREa-protein complex in all cell lines studied. This result was not seen when the metals were added directly to the extracts from control cells. DNA-binding protein blotting, UV crosslinking, and electroelution experiments were used to characterize the two hMREa-binding factors, termed BF1 and BF2. MRE-BF1 has an apparent molecular mass of approximately 86 kDa and binds to the hMREa in control cells, whereas MRE-BF2 consists of two molecules of approximately 28 kDa and binds to the hMREa in metal-treated cells. EDTA and o-phenanthroline inhibited binding of both factors to hMREa in a dose-dependent manner, indicating that a metal atom or atoms are essential for interaction of the factors with DNA.

The interactions of zinc, nickel, and cadmium with Xenopus transcription factor IIIA, assessed by equilibrium dialysis

Transcription factor IIIA (TFIIIA) was isolated from Xenopus ovary and treated with 1,10-phenanthroline to remove zinc. The interactions of apoTFIIIA with Zn2+, Ni2+, and Cd2+ were studied by equilibrium dialysis under anaerobic conditions (pH 7.0, 25 degrees C), using 65ZnCl2, 63NiCl2, and 109CdCl2 as the radioligands. The data for binding of Zn2+, Ni2+, and Cd2+ to apoTFIIIA were best-fitted by a model with two classes of binding sites. For Zn2+, the apparent dissociation constants (KdlZn and Kd2Zn) for the high- and low-affinity sites were 1.0 x 10(-8) and 2.6 x 10(-5) M; the apparent binding capacities of the two classes were 0.8 +/- 0.5 and 9.6 +/- 0.3 g-atoms of Zn/mol; the Hill coefficient was 1.18, consistent with positive cooperativity of Zn-binding sites. For Ni2+, the apparent KdlNi and Kd2Ni values were 2.3 x 10(-5) and 5.2 x 10(-4) M; the apparent binding capacities were 2.3 +/- 0.6 and 8.6 +/- 0.6 g-atoms of Ni/mol; the Hill coefficient was 1.20, consistent with positive cooperativity of Ni-binding sites. For Cd2+, the apparent KdlCd and Kd2Cd values were 2.8 x 10(-6) and 1.6 x 10(-4) M; the apparent binding capacities were 0.9 +/- 0.3 and 2.4 +/- 0.5 g-atoms of Cd/mol; the Hill coefficient was 0.53, consistent with negative cooperativity or heterogeneity of Cd-binding sites. This study has the following significance: First, it helps to resolve a controversy about the zinc content of purified TFIIIA. Second, it shows that the KdlZn of apoTFIIIA is less than the reported KdZn of thionein, consistent with the hypothesis that thionein modulates gene expression by competing with TFIIIA and other Zn-finger proteins for intracellular Zn2+ stores. Third, it confirms previous indirect evidence that the affinity of apoTFIIIA for Zn2+ is much greater than for Cd2+, and that the affinity for Cd2+ is greater than for Ni2+.