Copper and the ACE1 regulatory protein reversibly induce yeast metallothionein gene transcription in a mouse extract

Copper Utilization, Regulation, and Acquisition by Aspergillus fumigatus

Copper is an essential micronutrient for the opportunistic human pathogen, Aspergillus fumigatus. Maintaining copper homeostasis is critical for survival and pathogenesis. Copper-responsive transcription factors, AceA and MacA, coordinate a complex network responsible for responding to copper in the environment and determining which response is necessary to maintain homeostasis. For example, A. fumigatus uses copper exporters to mitigate the toxic effects of copper while simultaneously encoding copper importers and small molecules to ensure proper supply of the metal for copper-dependent processes such a nitrogen acquisition and respiration. Small molecules called isocyanides recently found to be produced by A. fumigatus may bind copper and partake in copper homeostasis similarly to isocyanide copper chelators in bacteria. Considering that the host uses copper as a microbial toxin and copper availability fluctuates in various environmental niches, understanding how A. fumigatus maintains copper homeostasis will give insights into mechanisms that facilitate the development of invasive aspergillosis and its survival in nature.

Transcriptional Regulation of Gene Expression by Metalloproteins

FNR and SoxR are transcriptional regulators containing an iron–sulfur cluster. The iron–sulfur cluster in FNR acts as an oxygen sensor by reacting with oxygen. The structural change of the iron–sulfur cluster takes place when FNR senses oxygen, which regulates the transcriptional regulator activity of FNR through the change of the quaternary structure. SoxR contains the [2Fe–2S] cluster that regulates the transcriptional activator activity of SoxR. Only the oxidized SoxR containing the [2Fe–2S]2+ cluster is active as the transcriptional activator. CooA is a transcriptional activator containing a protoheme that acts as a CO sensor. CO is a physiological effector of CooA and regulates the transcriptional activator activity of CooA. In this review, the biochemical and biophysical properties of FNR, SoxR, and CooA are described.

Copper Acquisition and Utilization in Fungi

Fungal cells colonize and proliferate in distinct niches, from soil and plants to diverse tissues in human hosts. Consequently, fungi are challenged with the goal of obtaining nutrients while simultaneously elaborating robust regulatory mechanisms to cope with a range of availability of nutrients, from scarcity to excess. Copper is essential for life but also potentially toxic. In this review we describe the sophisticated homeostatic mechanisms by which fungi acquire, utilize, and control this biochemically versatile trace element. Fungal pathogens, which can occupy distinct host tissues that have their own intrinsic requirements for copper homeostasis, have evolved mechanisms to acquire copper to successfully colonize the host, disseminate to other tissues, and combat host copper bombardment mechanisms that would otherwise mitigate virulence.

Development of a tightly regulated and highly responsive copper-inducible gene expression system and its application to control of flowering time

KEY MESSAGE

A newly developed copper-inducible gene expression system overcame the mixed results reported earlier, worked well both in cultured cells and a whole plant, and enabled to control flowering timing. Copper is one of the essential microelements and is readily taken up by plants. However, to date, it has rarely been used to control the expression of genes of interest, probably due to the inefficiency of the gene expression systems. In this study, we successfully developed a copper-inducible gene expression system that is based on the regulation of the yeast metallothionein gene. This system can be applied in the field and regulated at approximately one-hundredth of the rate used for registered copper-based fungicides. In the presence of copper, a translational fusion of the ACE1 transcription factor with the VP16 activation domain (VP16AD) of herpes simplex virus strongly activated transcription of the GFP gene in transgenic Arabidopsis. Interestingly, insertion of the To71 sequence, a 5'-untranslated region of the 130k/180k gene of tomato mosaic virus, upstream of the GFP gene reduced the basal expression of GFP in the absence of copper to almost negligible levels, even in soil-grown plants that were supplemented with ordinary liquid nutrients. Exposure of plants to 100 μM copper resulted in an over 1,000-fold induction ratio at the transcriptional level of GFP. This induction was copper-specific and dose-dependent with rapid and reversible responses. Using this expression system, we also succeeded in regulating floral transition by copper treatment. These results indicate that our newly developed copper-inducible system can accelerate gene functional analysis in model plants and can be used to generate novel agronomic traits in crop species.

Effective induction of pblac1 laccase by copper ion in Polyporus brumalis ibrc05015

Polyporus brumalis ibrc05015 is a strain capable of high laccase (Lac) production. Among several inducers, 0.25 mM copper was most effective for Lac production. One of the Lacs induced by copper was PbLac1, and its transcription was induced within 60 min after copper addition. The promoter region of pblac1 contained six putative metal response elements and one Ace1 consensus cis-element. We cloned the P. brumalis PbAce1 transcription factor, a homologue of Saccharomyces cerevisiae transcription factor Ace1, which regulates metallothionein genes in response to excess copper. PbAce1 complemented the function of Ace1 in an S. cerevisiae Δace strain. The conserved N-terminal copper-fist DNA binding domain of PbAce1 was required for complementation. In the PbAce1 complemented Δace1 strain, the pblac1 promoter was constitutively expressed at a high level, independent of copper concentration. PbAce1 has two Cys-rich repeat motifs (PbC1 and PbC2), which are similar to the Cys-rich repeat domain in metallothionein proteins, and are uniquely conserved in the C-terminal domain of basidiomycetous Ace1 sequences. These C-terminal domains could be involved in copper sensing and concentration-dependent Lac production in basidiomycetous fungi.

Yeast copper-dependent transcription factor ACE1 enhanced copper stress tolerance in Arabidopsis

Copper is essential but toxic in excess for aerobic organisms. Yeast transcription factor ACE1 functions as a sensor for copper and an inducer for the transcription of CUP1. In addition, ACE1 can activate the transcription of superoxide dismutase gene (sod1) in response to copper. In this study, we introduced the yeast ACE1 into Arabidopsis and analyzed its function in plant. Under high copper stress, the transgenic plants over-expressing ACE1 showed higher survival rate than the wild-type. We also found that over-expression of ACE1 in Arabidopsis increased the activities of SOD and POD, which were beneficial to the cell in copper buffering. Excess copper would suppress the expression of chlorophyll biosynthetic genes in Arabidopsis, RT-PCR analysis revealed that over-expression of ACE1 decrease the suppression. Together, our results indicate that ACE1 may play an important role in response to copper stress in Arabidopsis.

Gene expression profiling and phenotype analyses ofS. cerevisiaein response to changing copper reveals six genes with new roles in copper and iron metabolism

Exhaustive microarray time course analyses of Saccharomyces cerevisiae during copper starvation and copper excess reveal new aspects of metal-induced gene regulation. Aside from identifying targets of established copper- and iron-responsive transcription factors, we find that genes encoding mitochondrial proteins are downregulated and that copper-independent iron transport genes are preferentially upregulated, both during prolonged copper deprivation. The experiments also suggest the presence of a small regulatory iron pool that links copper and iron responses. One hundred twenty-eight genes with putative roles in metal metabolism were further investigated by several systematic phenotype screens. Of the novel phenotypes uncovered, hsp12-Δ and arn1-Δ display increased sensitivity to copper, cyc1-Δ and crr1-Δ show resistance to high copper, vma13-Δ exhibits increased sensitivity to iron deprivation, and pep12-Δ results in reduced growth in high copper and low iron. Besides revealing new components of eukaryotic metal trafficking pathways, the results underscore the previously determined intimate links between iron and copper metabolism and mitochondrial and vacuolar function in metal trafficking. The analyses further suggest that copper starvation can specifically lead to downregulation of respiratory function to preserve iron and copper for other cellular processes.

Activity of metal-responsive transcription factor 1 by toxic heavy metals and H2O2 in vitro is modulated by metallothionein

Metallothioneins are small, cysteine-rich proteins that avidly bind heavy metals such as zinc, copper, and cadmium to reduce their concentration to a physiological or nontoxic level. Metallothionein gene transcription is induced by several stimuli, notably heavy metal load and oxidative stress. Transcriptional induction of metallothionein genes is mediated by the metal-responsive transcription factor 1 (MTF-1), an essential zinc finger protein that binds to specific DNA motifs termed metal-response elements. In cell-free DNA binding reactions with nuclear extracts, MTF-1 requires elevated zinc concentrations for efficient DNA binding but paradoxically is inactivated by other in vivo inducers such as cadmium, copper, and hydrogen peroxide. Here we have developed a cell-free, MTF-1-dependent transcription system which accurately reproduces the activation of metallothionein gene promoters not only by zinc but also by these other inducers. We found that while transcriptional induction by zinc can be achieved by elevated zinc concentration alone, induction by cadmium, copper, or H2O2 additionally requires the presence of zinc-saturated metallothionein. This is explained by the preferential binding of cadmium or copper to metallothionein or its oxidation by H2O2; the concomitant release of zinc in turn leads to the activation of transcription factor MTF-1. Conversely, thionein, the metal-free form of metallothionein, inhibits activation of MTF-1. The release of zinc from cellular components, including metallothioneins, and the sequestration of zinc by newly produced apometallothionein might be a basic mechanism to regulate MTF-1 activity upon cellular stress.

Targeted histone acetylation at the yeast CUP1 promoter requires the transcriptional activator, the TATA boxes, and the putative histone acetylase encoded by SPT10

The relationship between chromatin remodeling and histone acetylation at the yeast CUP1 gene was addressed. CUP1 encodes a metallothionein required for cell growth at high copper concentrations. Induction of CUP1 with copper resulted in targeted acetylation of both H3 and H4 at the CUP1 promoter. Nucleosomes containing upstream activating sequences and sequences farther upstream were the targets for H3 acetylation. Targeted acetylation of H3 and H4 required the transcriptional activator (Ace1p) and the TATA boxes, suggesting that targeted acetylation occurs when TATA-binding protein binds to the TATA box or at a later stage in initiation. We have shown previously that induction results in nucleosome repositioning over the entire CUP1 gene, which requires Ace1p but not the TATA boxes. Therefore, the movement of nucleosomes occurring on CUP1 induction is independent of targeted acetylation. Targeted acetylation of both H3 and H4 also required the product of the SPT10 gene, which encodes a putative histone acetylase implicated in regulation at core promoters. Disruption of SPT10 was lethal at high copper concentrations and correlated with slower induction and reduced maximum levels of CUP1 mRNA. These observations constitute evidence for a novel mechanism of chromatin activation at CUP1, with a major role for the TATA box.

Mitochondrial functions and aging

In the filamentous ascomycete Podospora anserina mitochondria play a major role in lifespan control. Since the function of these organelles depends on a large number of individual components it is no surprise that a complex network of interacting branches of individual molecular pathways is involved in this process. Recently, the nuclear encoded transcription factor GRISEA was found to significantly affect mitochondrial functions. GRISEA is involved in the control of cellular copper homeostasis. Most importantly, the high affinity uptake of copper from the environment is controlled by this transcription factor. Once copper has entered the cell, it becomes distributed to different compartments and different target molecules. This process depends on a group of proteins, termed copper chaperones. PaCOX17, a homologue of the yeast copper chaperone yCOX17, appears to be involved in copper delivery to mitochondria. Most importantly, the metal is crucial for the assembly and the function of complex IV of the respiratory chain. However, although P. anserina is an obligate aerobe and therefore depends on mitochondrial energy transduction, impairments in the copper delivery pathway are not lethal. This is due to the induction of a molecular back-up system able to compensate for deficiencies in complex IV. The system utilizes an alternative oxidase (PaAOX) which uses iron instead of copper as a cofactor. The alternative respiratory pathway is characterized by a decreased ATP generation but, most significantly, also a decrease in the production of reactive oxygen species. Consequently, molecular damage is reduced which contributes to an increased lifespan of this type of mutant. In addition, modifications in the availability of cellular copper have other relevant consequences. Most significantly, the characteristic age-related rearrangements occurring in the mitochondrial DNA of wild-type strains of P. anserina were found to be dependent on the availability of copper.

DNA sequence plays a major role in determining nucleosome positions in yeast CUP1 chromatin

The role of DNA sequence in determining nucleosome positions in vivo was investigated by comparing the positions adopted by nucleosomes reconstituted on a yeast plasmid in vitro using purified core histones with those in native chromatin containing the same DNA, described previously. Nucleosomes were reconstituted on a 2.5 kilobase pair DNA sequence containing the yeast TRP1ARS1 plasmid with CUP1 as an insert (TAC-DNA). Multiple, alternative, overlapping nucleosome positions were mapped on TAC-DNA. For the 58 positioned nucleosomes identified, the relative positioning strengths and the stabilities to salt and temperature were determined. These positions were, with a few exceptions, identical to those observed in native, remodeled TAC chromatin containing an activated CUP1 gene. Only some of these positions are utilized in native, unremodeled chromatin. These observations suggest that DNA sequence is likely to play a very important role in positioning nucleosomes in vivo. We suggest that events occurring in yeast CUP1 chromatin determine which positions are occupied in vivo and when they are occupied.

Copper-regulatory domain involved in gene expression

Copper ion homeostasis in yeast is maintained through regulated expression of genes involved in copper ion uptake, Cu(I) sequestration, and defense against reactive oxygen intermediates. Positive and negative copper ion regulation is observed, and both effects are mediated by Cu(I)-sensing transcription factors. The mechanism of Cu(I) regulation is distinct for transcriptional activation versus transcriptional repression. Cu(I) activation of gene expression in S. cerevisiae and C. glabrata occurs through Cu-regulated DNA binding. The activation process involves Cu(I) cluster formation within the regulatory domain in Ace1 and Amt1. Cu(I) binding stabilizes a specific conformation capable of high-affinity interaction with specific DNA promoter sequences. Cu(I)-activated transcription factors are modular proteins in which the DNA-binding domain is distinct from the domain that mediates transcriptional activation. The all-or-nothing formation of the polycopper cluster permits a graded response of the cell to environmental copper. Cu(I) triggering may involve a metal exchange reaction converting Ace1 from a Zn(II)-specific conformer to a clustered Cu(I) conformer. The Cu(I) regulatory domain occurs in transcription factors from S. cerevisiae and C. glabrata. Sequence homologs are also known in Y. lipolytica and S. pombe, although no functional information is available for these candidate regulatory molecules. The presence of the Cu(I) regulatory domain in four distinct yeast strains suggests that this Cu-responsive domain may occur in other eukaryotes. Cu-mediated repression of gene expression in S. cerevisiae occurs through Cu(I) regulation of Mac1. Cu(I) binding to Mac1 appears to inhibit the transactivation domain. The Cu(I) specificity of this repression is likely to arise from formation of a polycopper thiolate cluster.

Analysis of copper-induced metallothionein expression using autonomously replicating plasmids in Candida glabrata

Candida glabrata strains and a stable plasmid were developed that were suitable for analysis of copper-inducible expression from promoters of the three metallothionein (MT) genes. The two homologous MTII genes, MTIIa and MTIIb, encode the same polypeptide but are differentially induced by copper salts. MTIIb is more highly inducible than MTIIa and cells harboring a single MTIIb exhibit a greater resistance to copper salts compared to cells harboring a single MTIIa. The differential copper inducibility was mapped to sequences between -03 and -292 upstream of the MT coding sequences. Expression of MTI is highly Cu-regulated, but this MT gene confers much less resistance than MTII genes.

Regulation of Saccharomyces cerevisiae catalase gene expression by copper

Treatment of Saccharomyces cerevisiae cells with copper induces the activity of Cu/Zn superoxide dismutase (SOD) and catalase. To understand the level at which Cu regulates catalase, the expression of the S. cerevisiae CTA1 (encoding the peroxisomal catalase A) and CTT1 (encoding the cytosolic catalase T) genes was monitored as a function of Cu treatment. Copper was found to specifically induce transcription of CTT1, but not CTA1, mRNA. Moreover, genetic and biochemical studies demonstrate that this induction is independent of the ACE1 Cu trans-activator controlling the expression of yeast Cu/Zn SOD and metallothionein genes. Copper regulation of CTT1 thus appears to represent a novel metal regulatory pathway in S. cerevisiae cells.

The Saccharomyces cerevisiae CDC25 gene product binds specifically to catalytically inactive ras proteins in vivo

Genetic data suggest that the yeast cell cycle control gene CDC25 is an upstream regulator of RAS2. We have been able to show for the first time that the guanine nucleotide exchange proteins Cdc25 and Sdc25 from Saccharomyces cerevisiae bind directly to their targets Ras1 and Ras2 in vivo. Using the characteristics of the yeast Ace1 transcriptional activator to probe for protein-protein interaction, we found that the CDC25 gene product binds specifically to wild-type Ras2 but not to the mutated Ras2Val-19 and Ras2 delta Val-19 proteins. The binding properties of Cdc25 to Ras2 were strongly diminished in yeast cells expressing an inactive Ira1 protein, which normally acts as a negative regulator of Ras activity. On the basis of these data, we propose that the ability of Cdc25 to interact with Ras2 proteins is strongly dependent on the activation state of Ras2. Cdc25 binds predominantly to the catalytically inactive GDP-bound form of Ras2, whereas a conformational change of Ras2 to its activated GTP-bound state results in its loss of binding affinity to Cdc25.

The Saccharomyces cerevisiae CDC25 gene product binds specifically to catalytically inactive ras proteins in vivo

Genetic data suggest that the yeast cell cycle control gene CDC25 is an upstream regulator of RAS2. We have been able to show for the first time that the guanine nucleotide exchange proteins Cdc25 and Sdc25 from Saccharomyces cerevisiae bind directly to their targets Ras1 and Ras2 in vivo. Using the characteristics of the yeast Ace1 transcriptional activator to probe for protein-protein interaction, we found that the CDC25 gene product binds specifically to wild-type Ras2 but not to the mutated Ras2Val-19 and Ras2 delta Val-19 proteins. The binding properties of Cdc25 to Ras2 were strongly diminished in yeast cells expressing an inactive Ira1 protein, which normally acts as a negative regulator of Ras activity. On the basis of these data, we propose that the ability of Cdc25 to interact with Ras2 proteins is strongly dependent on the activation state of Ras2. Cdc25 binds predominantly to the catalytically inactive GDP-bound form of Ras2, whereas a conformational change of Ras2 to its activated GTP-bound state results in its loss of binding affinity to Cdc25.

The gene for cadmium metallothionein from a cadmium-resistant yeast appears to be identical to CUP1 in a copper-resistant strain

A cadmium-resistant strain of Saccharomyces cerevisiae produces a cadmium metallothionein with the same characteristics as the copper metallothionein that is encoded by CUP1 in a copper-resistant strain. The structural gene for metallothionein from the cadmium-resistant strain resembles CUP1 in terms of the fragmentation patterns generated by restriction enzymes. Furthermore, the gene may be amplified as 2.0 kb repeating units in both the cadmium-resistant and the copper-resistant strains. However, transformants with a plasmid that carried the metallothionein gene from the cadmium-resistant strain were resistant to copper but not to cadmium. It appears that the same metallothionein gene, CUP1, is amplified in both cadmium- and copper-resistant yeasts. However, the mechanism for the cadmium-specific inducibility of the gene may be restricted to the cadmium-resistant strain.

Metal-regulated transcription in eukaryotes

This review has summarized many of the major aspects of metal-regulated gene transcription in eukaryotic organisms as they are currently understood at the mechanistic level. Clearly, metals represent a class of important transcriptional effector molecules which regulate gene expression in different ways and both by activation or repression of gene transcription. To date, studies of metal-regulated transcription in fungi have resulted in the most detailed description of the structure, function and mechanisms of action of eukaryotic metal-responsive transcription factors. Recently, significant progress has been made in higher eukaryotic systems through the biochemical detection and purification of MRE binding proteins which may represent MRTFs. Additionally, perhaps fungi will be exploited for their genetics and ease of manipulation to clone and functionally analyze cDNAs for MRTFs from higher eukaryotes. The isolation of cDNAs for higher eukaryotic MRTFs will provide important tools for answering a number of interesting questions in metal-regulated gene transcription. How do higher eukaryotes activate MT gene transcription in response to a broad range of environmental metals? What are the tissue distributions of MRTFs and how does their activity correlate with the exposure of different tissues to varying concentrations of metals? What are the identities of other genes regulated by MRTFs and why are such genes metal-responsive? A comprehensive understanding of the detailed mechanisms for metal-regulated transcription will ultimately require an understanding of how eukaryotic cells sense, transport, distribute and remove metals from their environment. These questions provide an interesting and exciting area of investigation for geneticists, physiologists, molecular biologists, biophysicists and biochemists now and in the future.

The cloned promoter of the human DNA beta-polymerase gene contains a cAMP response element functional in HeLa cells

The mammalian DNA beta-polymerase (beta-pol) gene is constitutively expressed in cultured cells as a function of growth stage and DNA replication, but is expressed in rodents in a tissue-specific fashion. As revealed by transient expression experiments with wild-type and mutated beta-pol promoter fusion genes, the cloned human beta-pol promoter is transcriptionally regulated by signals acting through the single palindromic sequence (GT-GACGTCAC) known as an ATF/CRE-binding site centered at position -45 in the core promoter. Although the mere presence of the ATF/CRE palindromic sequence in a promoter does not always confer cAMP responsiveness or protein binding over and around the ATF/CRE sequence, we find that agents that increase cAMP levels (forskolin and IBMX) in HeLa cells activate the beta-pol promoter; activation also can be observed by coexpression of the protein kinase A catalytic subunit. Experiments with mutagenized beta-pol promoters indicate that the ATF/CRE-binding site mediates these effects. Thus, the ATF/CRE-binding site in the context of this TATA-less constitutive promoter is able to respond to the kinase A signal transduction pathway.

The transactivating domain of the c-Jun proto-oncoprotein is required for cotransformation of rat embryo cells

The nuclear phosphoprotein c-Jun, encoded by the proto-oncogene c-jun, is a major component of the AP-1 complex. A potent transcriptional regulator, c-jun is also able to transform normal rat embryo cells in cooperation with an activated c-Ha-ras gene. By deletion analysis, we identified the regions of c-Jun encoding transformation and transactivation functions. Our studies indicate that there is a direct correlation between the ability of the c-Jun protein to activate transcription and cotransform rat embryo cells. The regions involved in these functions include the conserved leucine zipper/DNA binding domain and an effector domain near its N terminus. This N-terminal region spans amino acids 61 to 146 of the c-Jun protein and is highly conserved among all Jun family members. These results support the hypothesis that c-Jun transforms cells by stimulating the expression of transformation-mediating genes.

The transactivating domain of the c-Jun proto-oncoprotein is required for cotransformation of rat embryo cells

The nuclear phosphoprotein c-Jun, encoded by the proto-oncogene c-jun, is a major component of the AP-1 complex. A potent transcriptional regulator, c-jun is also able to transform normal rat embryo cells in cooperation with an activated c-Ha-ras gene. By deletion analysis, we identified the regions of c-Jun encoding transformation and transactivation functions. Our studies indicate that there is a direct correlation between the ability of the c-Jun protein to activate transcription and cotransform rat embryo cells. The regions involved in these functions include the conserved leucine zipper/DNA binding domain and an effector domain near its N terminus. This N-terminal region spans amino acids 61 to 146 of the c-Jun protein and is highly conserved among all Jun family members. These results support the hypothesis that c-Jun transforms cells by stimulating the expression of transformation-mediating genes.

Copper in biological systems. A report from the 6th Manziana Conference, September 23-27, 1990

Enzymes and proteins: AO, amine oxidase; and as proposed in reference 3, BSAO, bovine serum AO; SSAO, swine serum AO; SKDAO, swine kidney AO; PSAO, pea seedling AO; APAO, arthrobacter P1AO; MADH, methylamine dehydrogenase; AAO, ascorbic acid oxidase; alpha-AE, alpha-amidating enzyme; Az, azurin; COX, cytochrome c oxidase; CP, ceruloplasmin; DBH, dopamine beta-hydroxylase; GO, galactose oxidase; Hc, hemocyanin; MT, metallotheonein; NIR, nitrite reductase; SOD, superoxide dismutase. Cofactors: Dopa, 3,4 dihydroxyphenylalanine; Topa, 3,4,6 trihydroxyphenyl-alanine; PLP, pyridoxal-phosphate; PQQ, pyrroloquinolinequinone. Reagents: DDC, diethyldithiocarbamate; DMG, diaminoguanidine; DMSA, dimercaptosuccinic acid; NTA, nitrilotriacetic acid. Technique-related: XANES, x-ray absorption near edge spectroscopy; EXAFS, extended x-ray absorption fine structure; ENDOR, electron-nuclear double resonance; ESEEM, electron spin echo envelope modulation; CD, circular dichroism; MCD, magnetic circular dichroism; NMRD, nuclear magnetic resonance dispersion; nqi, nuclear quadrupole interaction; DSC, differential scanning calorimetry.

A nuclear factor binds to the metal regulatory elements of the mouse gene encoding metallothionein-I

The ability of vertebrate metallothionein (MT) genes to be induced by heavy metals is controlled by metal regulatory elements (MREs) present in the promoter in multiple, non-identical copies. The binding specificity of the mouse L-cell nuclear factor(s) that interact with the element MREd of the mouse MT-I gene was analyzed by in vitro footprinting, protein blotting, and UV cross-linking assays. In vitro footprinting analyses revealed that synthetic oligodeoxynucleotides (oligomers) corresponding to the metal regulatory elements MREa, MREb, MREc, MREd and MREe of the mouse MT-I gene, as well as the MRE4 of the human MT-IIA gene and the MREa of the trout MT-B gene, all competed for the nuclear protein species binding to the MREd region of the mouse MT-I gene, the MREe oligomer being the weakest competitor. In addition, protein blotting experiments revealed that a nuclear protein of 108 kDa, termed metal element protein-1 (MEP-1), which specifically binds with high affinity to mouse MREd, binds with different affinities to the other mouse MRE elements, mimicking their relative transcriptional strength in vivo: MREd greater than or equal to MREa = MREc greater than MREb greater than MREe greater than MREf. Similarly, human MRE4 and trout MREa bind to MEP-1. A protein similar in size to MEP-1 was also detected in HeLa-cell nuclear extracts. In UV cross-linking experiments the major protein species, complexed with mouse MREd oligomers, migrated on a denaturating gel with an apparent Mr of 115,000 and was detected using each of the mouse MRE oligomers tested. These results show that a mouse nuclear factor can bind to multiple MREs in mouse, trout, and human MT genes.

Role of heat shock transcription factor in yeast metallothionein gene expression

The induction of Saccharomyces cerevisiae metallothionein gene transcription by Cu and Ag is mediated by the ACE1 transcription factor. In an effort to detect additional stimuli and factors that regulate metallothionein gene transcription, we isolated a Cu-resistant suppressor mutant of an ACE1 deletion strain. Even in the absence of metals, the suppressor mutant exhibited high basal levels of metallothionein gene transcription that required upstream promoter sequences. The suppressor gene was cloned, and its predicted product was shown to correspond to yeast heat shock transcription factor with a single-amino-acid substitution in the DNA-binding domain. The mutant heat shock factor bound strongly to metallothionein gene upstream promoter sequences, whereas wild-type heat shock factor interacted weakly with the same region. Heat treatment led to a slight but reproducible induction of metallothionein gene expression in both wild-type and suppressor strains, and Cd induced transcription in the mutant strain. These studies provide evidence for multiple pathways of metallothionein gene transcriptional regulation in S. cerevisiae.

Role of heat shock transcription factor in yeast metallothionein gene expression

The induction of Saccharomyces cerevisiae metallothionein gene transcription by Cu and Ag is mediated by the ACE1 transcription factor. In an effort to detect additional stimuli and factors that regulate metallothionein gene transcription, we isolated a Cu-resistant suppressor mutant of an ACE1 deletion strain. Even in the absence of metals, the suppressor mutant exhibited high basal levels of metallothionein gene transcription that required upstream promoter sequences. The suppressor gene was cloned, and its predicted product was shown to correspond to yeast heat shock transcription factor with a single-amino-acid substitution in the DNA-binding domain. The mutant heat shock factor bound strongly to metallothionein gene upstream promoter sequences, whereas wild-type heat shock factor interacted weakly with the same region. Heat treatment led to a slight but reproducible induction of metallothionein gene expression in both wild-type and suppressor strains, and Cd induced transcription in the mutant strain. These studies provide evidence for multiple pathways of metallothionein gene transcriptional regulation in S. cerevisiae.

Spectroscopic characterization of the copper(I)-thiolate cluster in the DNA-binding domain of yeast ACE1 transcription factor

A polypeptide containing the amino-terminal region of ACE1 (residues 1-122; 122*), the activator of yeast Cu-metallothionein gene transcription, shows charge-transfer and metal-centered UV absorption bands, and orange luminescence which are characteristic of Cu-cysteinyl thiolate cluster structures. These spectral features are abolished by the Cu(I) complexing agents CN- and diethyldithiocarbamate or exposure to acid, but not by the Cu(II) chelator, EDTA. Binding of the polypeptide to its specific DNA recognition site, but not to calf-thymus double-stranded DNA, induces quenching of its Tyr and Cu-S cluster luminescence emission. The CD spectrum is characteristic of a tightly folded structure that may be organized around the Cu cluster.

Metal ion resistance in fungi: molecular mechanisms and their regulated expression

One stress response in cells is the ability to survive in an environment containing excessive concentrations of metal ions. This paper reviews current knowledge about cellular and molecular mechanisms involved in the response and adaptation of various fungal species to metal stress. Most cells contain a repertoire of mechanisms to maintain metal homeostasis and prevent metal toxicity. Roles played by glutathione, related (gamma-EC)nG peptides, metallothionein-like polypeptides, and sulfide ions are discussed. In response to cellular metal stress, the biosynthesis of some of these molecules are metalloregulated via intracellular metal sensors. The identify of the metal sensors and the role of metal ions in the regulation of biosynthesis of metallothionein and (gamma-EC)nG peptides are subjects of much current attention and are discussed herein.

Cloned yeast and mammalian transcription factor TFIID gene products support basal but not activated metallothionein gene transcription

Transcription factor IID (TFIID), the "TATA binding factor," is thought to play a key role in the regulation of eukaryotic transcriptional initiation. We have studied the role of TFIID in the transcription of the yeast metallothionein gene, which is regulated by the copper-dependent activator protein ACE1. Both basal and induced transcription of the metallothionein gene require TFIID and a functional TATA binding site. Crude human and mouse TFIID fractions, prepared from mammalian cells, respond to stimulation by ACE1. In contrast, human and yeast TFIID proteins expressed from the cloned genes do not respond to ACE1, except in the presence of wheat germ or yeast total cell extracts. These results indicate that the cloned TFIID gene products lack a component(s) or modification(s) that is required for regulated as compared to basal transcription.

A single amino acid change in CUP2 alters its mode of DNA binding

CUP2 is a copper-dependent transcriptional activator of the yeast CUP1 metallothionein gene. In the presence of Cu+ and Ag+) ions its DNA-binding domain is thought to fold as a cysteine-coordinated Cu cluster which recognizes the palindromic CUP1 upstream activation sequence (UASc). Using mobility shift, methylation interference, and DNase I and hydroxyl radical footprinting assays, we examined the interaction of wild-type and variant CUP2 proteins produced in Escherichia coli with the UASc. Our results suggest that CUP2 has a complex Cu-coordinated DNA-binding domain containing different parts that function as DNA-binding elements recognizing distinct sequence motifs embedded within the UASc. A single-amino-acid substitution of cysteine 11 with a tyrosine results in decreased Cu binding, apparent inactivation of one of the DNA-binding elements and a dramatic change in the recognition properties of CUP2. This variant protein interacts with only one part of the wild-type site and prefers to bind to a different half-site from the wild-type protein. Although the variant has about 10% of wild-type DNA-binding activity, it appears to be completely incapable of activating transcription.

A single amino acid change in CUP2 alters its mode of DNA binding

CUP2 is a copper-dependent transcriptional activator of the yeast CUP1 metallothionein gene. In the presence of Cu+ and Ag+) ions its DNA-binding domain is thought to fold as a cysteine-coordinated Cu cluster which recognizes the palindromic CUP1 upstream activation sequence (UASc). Using mobility shift, methylation interference, and DNase I and hydroxyl radical footprinting assays, we examined the interaction of wild-type and variant CUP2 proteins produced in Escherichia coli with the UASc. Our results suggest that CUP2 has a complex Cu-coordinated DNA-binding domain containing different parts that function as DNA-binding elements recognizing distinct sequence motifs embedded within the UASc. A single-amino-acid substitution of cysteine 11 with a tyrosine results in decreased Cu binding, apparent inactivation of one of the DNA-binding elements and a dramatic change in the recognition properties of CUP2. This variant protein interacts with only one part of the wild-type site and prefers to bind to a different half-site from the wild-type protein. Although the variant has about 10% of wild-type DNA-binding activity, it appears to be completely incapable of activating transcription.