Expression of a yeast metallothionein gene family is activated by a single metalloregulatory transcription factor

Metallothioneins of the urochordate Oikopleura dioica have Cys-rich tandem repeats, large size and cadmium-binding preference

Oikopleura dioica has the longest metallothionein described so far, made of repeats generated by a modular and step-wise evolution.

Metals in fungal virulence

Metals are essential for life, and they play a central role in the struggle between infecting microbes and their hosts. In fact, an important aspect of microbial pathogenesis is the 'nutritional immunity', in which metals are actively restricted (or, in an extended definition of the term, locally enriched) by the host to hinder microbial growth and virulence. Consequently, fungi have evolved often complex regulatory networks, uptake and detoxification systems for essential metals such as iron, zinc, copper, nickel and manganese. These systems often differ fundamentally from their bacterial counterparts, but even within the fungal pathogens we can find common and unique solutions to maintain metal homeostasis. Thus, we here compare the common and species-specific mechanisms used for different metals among different fungal species-focusing on important human pathogens such as Candida albicans, Aspergillus fumigatus or Cryptococcus neoformans, but also looking at model fungi such as Saccharomyces cerevisiae or A. nidulans as well-studied examples for the underlying principles. These direct comparisons of our current knowledge reveal that we have a good understanding how model fungal pathogens take up iron or zinc, but that much is still to learn about other metals and specific adaptations of individual species-not the least to exploit this knowledge for new antifungal strategies.

Copper Resistance in Aspergillus nidulans Relies on the PI-Type ATPase CrpA, Regulated by the Transcription Factor AceA

Copper homeostasis has been extensively studied in mammals, bacteria, and yeast, but it has not been well-documented in filamentous fungi. In this report, we investigated the basis of copper tolerance in the model fungus Aspergillus nidulans. Three genes involved in copper homeostasis have been characterized. First, crpA the A. nidulans ortholog of Candida albicans CaCRP1 gene encoding a P_I-type ATPase was identified. The phenotype of crpA deletion led to a severe sensitivity to Cu⁺² toxicity and a characteristic morphological growth defect in the presence of high copper concentration. CrpA displayed some promiscuity regarding metal species response. The expression pattern of crpA showed an initial strong elevation of mRNA and a low continuous gene expression in response to long term toxic copper levels. Coinciding with maximum protein expression level, CrpA was localized close to the cellular surface, however protein distribution across diverse organelles suggests a complex regulated trafficking process. Secondly, aceA gene, encoding a transcription factor was identified and deleted, resulting in an even more extreme copper sensitivity than the ΔcrpA mutant. Protein expression assays corroborated that AceA was necessary for metal inducible expression of CrpA, but not CrdA, a putative metallothionein the function of which has yet to be elucidated.

The PathoYeastract database: an information system for the analysis of gene and genomic transcription regulation in pathogenic yeasts

We present the PATHOgenic YEAst Search for Transcriptional Regulators And Consensus Tracking (PathoYeastract - http://pathoyeastract.org) database, a tool for the analysis and prediction of transcription regulatory associations at the gene and genomic levels in the pathogenic yeasts Candida albicans and C. glabrata. Upon data retrieval from hundreds of publications, followed by curation, the database currently includes 28 000 unique documented regulatory associations between transcription factors (TF) and target genes and 107 DNA binding sites, considering 134 TFs in both species. Following the structure used for the YEASTRACT database, PathoYeastract makes available bioinformatics tools that enable the user to exploit the existing information to predict the TFs involved in the regulation of a gene or genome-wide transcriptional response, while ranking those TFs in order of their relative importance. Each search can be filtered based on the selection of specific environmental conditions, experimental evidence or positive/negative regulatory effect. Promoter analysis tools and interactive visualization tools for the representation of TF regulatory networks are also provided. The PathoYeastract database further provides simple tools for the prediction of gene and genomic regulation based on orthologous regulatory associations described for other yeast species, a comparative genomics setup for the study of cross-species evolution of regulatory networks.

Co-dependent recruitment of Ino80p and Snf2p is required for yeast CUP1 activation

In yeast, Ace1p-dependent induction of CUP1 is responsible for protecting cells from copper toxicity. Although the mechanism of yeast CUP1 induction has been studied intensively, it is still uncertain which chromatin remodelers are involved in CUP1 transcriptional activation. Here, we show that yeast cells are inviable in the presence of copper when either chromatin remodeler, Ino80p or Snf2p, is not present. This inviability is due to the lack of CUP1 expression in ino80Δ and snf2Δ cells. Subsequently, we observe that both Ino80p and Snf2p are present at the promoter and they are responsible for recruiting chromatin remodeling activity to the CUP1 promoter under induced conditions. These results suggest that they directly participate in CUP1 transcriptional activation. Furthermore, the codependent recruitment of both INO80 and SWI/SNF depends on the presence of the transcriptional activator, Ace1p. We also demonstrate that both remodelers are required to recruit RNA polymerase II and targeted histone acetylation, indicating that remodelers are recruited to the CUP1 promoter before RNA polymerase II and histone acetylases. These observations provide evidence for the mechanism of CUP1 induction. As such, we propose a model that describes novel insight into the order of events in CUP1 activation.

Chromatin repositioning activity and transcription machinery are both recruited by Ace1p in yeast CUP1 activation

The relationship among transcriptional activators, nucleosome repositioning activity and transcription machinery at the yeast CUP1 gene was addressed. CUP1 encodes a cysteine-rich, copper-binding metallothionein that protects cells against copper toxicity through its ability to sequester copper. The induction of CUP1 requires the presence of Ace1p and the binding of Ace1p at the CUP1 promoter during activation provides evidence that Ace1p is directly involved in CUP1 induction. Furthermore, transcriptional activation of CUP1 resulted in nucleosome repositioning at the CUP1 promoter and sequences further downstream in the coding region, suggesting a gene-wide chromatin remodeling activity. Such remodeling activity depends on the presence of transcription activator Ace1p. The recruitment of RNA polymerase II also requires the presence of Ace1p. Therefore, these observations provide insight into the molecular mechanism of CUP1 activation.

Charting the travels of copper in eukaryotes from yeast to mammals

Throughout evolution, all organisms have harnessed the redox properties of copper (Cu) and iron (Fe) as a cofactor or structural determinant of proteins that perform critical functions in biology. At its most sobering stance to Earth's biome, Cu biochemistry allows photosynthetic organisms to harness solar energy and convert it into the organic energy that sustains the existence of all nonphotosynthetic life forms. The conversion of organic energy, in the form of nutrients that include carbohydrates, amino acids and fatty acids, is subsequently released during cellular respiration, itself a Cu-dependent process, and stored as ATP that is used to drive a myriad of critical biological processes such as enzyme-catalyzed biosynthetic processes, transport of cargo around cells and across membranes, and protein degradation. The life-supporting properties of Cu incur a significant challenge to cells that must not only exquisitely balance intracellular Cu concentrations, but also chaperone this redox-active metal from its point of cellular entry to its ultimate destination so as to avert the potential for inappropriate biochemical interactions or generation of damaging reactive oxidative species (ROS). In this review we chart the travels of Cu from the extracellular milieu of fungal and mammalian cells, its path within the cytosol as inferred by the proteins and ligands that escort and deliver Cu to intracellular organelles and protein targets, and its journey throughout the body of mammals. This article is part of a Special Issue entitled: Cell Biology of Metals.

Selective recognition of metal ions by metalloregulatory proteins

Significant advances have been made over the past decade on illustrating structural and functional features underneath the selective metal ion recognition by various proteins in nature. These efforts led to fruitful information regarding mechanisms that bacteria employed on the regulation, transportation, and utilization of main group and essential transition metal ions, and the detoxification of hazardous heavy metal ions. Here we summarize the recent advancement on the understanding of selective recognition of transition and heavy metal ions by metalloregulatory proteins. An emphasis is placed on demonstrating the molecular level mechanism of selective metal ion recognition in bacteria. Other types of metal sensory proteins will also be briefly reviewed.

A surprisingly large RNase P RNA in Candida glabrata

We have found an extremely large ribonuclease P (RNase P) RNA (RPR1) in the human pathogen Candida glabrata and verified that this molecule is expressed and present in the active enzyme complex of this hemiascomycete yeast. A structural alignment of the C. glabrata sequence with 36 other hemiascomycete RNase P RNAs (abbreviated as P RNAs) allows us to characterize the types of insertions. In addition, 15 P RNA sequences were newly characterized by searching in the recently sequenced genomes Candida albicans, C. glabrata, Debaryomyces hansenii, Eremothecium gossypii, Kluyveromyces lactis, Kluyveromyces waltii, Naumovia castellii, Saccharomyces kudriavzevii, Saccharomyces mikatae, and Yarrowia lipolytica; and by PCR amplification for other Candida species (Candida guilliermondii, Candida krusei, Candida parapsilosis, Candida stellatoidea, and Candida tropicalis). The phylogenetic comparative analysis identifies a hemiascomycete secondary structure consensus that presents a conserved core in all species with variable insertions or deletions. The most significant variability is found in C. glabrata P RNA in which three insertions exceeding in total 700 nt are present in the Specificity domain. This P RNA is more than twice the length of any other homologous P RNAs known in the three domains of life and is eight times the size of the smallest. RNase P RNA, therefore, represents one of the most diversified noncoding RNAs in terms of size variation and structural diversity.

Defined structural changes occur in a nucleosome upon Amt1 transcription factor binding

Here, we study the binding of the transcription factor Amt1 to its recognition site near the dyad of a highly positioned nucleosome. We find that the DNA binding domain of Amt1 binds to nucleosomes with only threefold reduced affinity compared to free DNA. We show by fluorescence resonance energy transfer that factor binding at the nucleosomal dyad is accompanied by the partial dissociation of the DNA ends from the histone octamer surface; however, no dissociation or subtle rearrangements of histone subunits is observed. A poly(dA.dT) DNA sequence element adjacent to the transcription factor binding site appears to facilitate factor binding, but is not essential. The methods that we describe here characterize for the first time the subtle structural changes that occur upon transcription factor binding to nucleosomes, and demonstrate the ability of the nucleosome to structurally adapt in response to outside influences.

Identification and functional expression of tahA, a filamentous fungal gene involved in copper trafficking to the secretory pathway in Trametes versicolor

In this study, cDNA and genomic clones encoding a homologue of the yeast gene anti-oxidant 1 (ATX1) from the white-rot fungus Trametes versicolor, a basidiomycete known to produce several laccase isoenzymes involved in lignin degradation, were identified. This gene, named Trametes ATX homologue (tahA), encodes a protein of 7.9 kDa with 56% identity to the yeast Atx1p sequence. Two different alleles of tahA were obtained that differed mainly in their intervening sequences and in a 425 nt insertion located 183 nt upstream of the transcription start site. tahA is present as one copy per haploid nucleus in T. versicolor, as shown by Southern analysis. Expression of tahA cDNA restored high-affinity iron uptake in a deltaatx1 yeast strain and oxygen sensitivity in a deltasod1 deltasod2 yeast strain, showing that tahA is also a functional homologue of ATX1. The inability of tahA to rescue the deltasod1 phenotype on copper-deficient medium indicated that tahA function is copper-dependent. Sequence analysis of the tahA promoter revealed several motifs that were similar to the conserved motifs found in the copper-regulated metallothionein and Cu, Zn superoxide dismutase genes, CUP1 and SOD1, of Saccharomyces cerevisiae, Neurospora crassa and Candida glabrata. In contrast to its yeast homologue ATX1, tahA is induced under elevated copper concentrations in the medium (>0.25 micro M CuSO(4)) and repressed under copper starvation. The transcription of tahA was analysed in response to copper and iron, and after adding xenobiotica. The results are discussed in relevance to laccase expression.

Microbial metallothioneins

Bacterial metallothioneins bind, sequester and buffer excess intracellular zinc. At present, the vast majority of the available experimental data relate to cyanobacterial metallothionein, SmtA, from Synechococcus PCC 7942. SmtA is required for normal resistance to zinc and smtA-mediated zinc resistance has been used as a selectable marker. The imidazole groups of histidine residues, in addition to the thiol groups of cysteine residues, co-ordinate zinc in bacterial metallothioneins. The structure of bacterial metallothionein must facilitate some discrimination between 'adventitious' and 'adventageous' zinc-binding sites such that under excess zinc conditions metal is predominantly scavenged from the former. It remains unclear whether or not bacterial metallothionein also acts as a zinc store that supplies zinc-requiring proteins or if under some conditions it deactivates a subset of proteins via zinc removal. Expression of smtA is induced in response to elevated concentrations of zinc via the action of SmtB. SmtB has some sequence similarity to the arsenic responsive repressor ArsR and genes encoding related proteins are present in many bacterial genomes. Metal perception by SmtB differs from ArsR. The latter contains a characteristic Cys-Val-Cys motif associated with a DNA-binding helix-turn-helix (the ArsR motif), while the former contains metal-binding motifs associated with a carboxyl-terminal alpha-helix that forms the interface between SmtB dimers (the SmtB motif). Some SmtB-ArsR family proteins, including the zinc sensor ZiaR from the cyanobacterium Synechocystis PCC 6803, have the metal-sensory motifs of both SmtB and ArsR. The mechanisms of action, and the features that allow discrimination between different metal ions by these sensors, are discussed.

The Candida glabrata Amt1 copper-sensing transcription factor requires Swi/Snf and Gcn5 at a critical step in copper detoxification

The yeast Candida glabrata rapidly autoactivates transcription of the AMT1 gene in response to potentially toxic copper levels through the copper-inducible binding of the Amt1 transcription factor to a metal response element (MRE) within a positioned nucleosome. Our previous studies have characterized the role of a 16 bp homopolymeric dA:dT DNA structural element in facilitating rapid Amt1 access to the AMT1 promoter nucleosomal MRE. In this study, we have used the genetically more facile yeast Saccharomyces cerevisiae to identify additional cellular factors that are important for promoting rapid autoactivation of the AMT1 gene in response to toxic copper levels. We demonstrate that the Swi/Snf nucleosome remodelling complex and the histone acetyltransferase Gcn5 are both essential for AMT1 gene autoregulation, and that the requirement for these chromatin remodelling factors is target gene specific. Chromatin accessibility measurements performed in vitro and in vivo indicate that part of the absolute requirement for these factors is derived from their involvement in facilitating nucleosomal access to the AMT1 promoter MRE. Additionally, these data implicate the involvement of Swi/Snf and Gcn5 at multiple levels of AMT1 gene autoregulation.

Regulation of metallothionein gene expression by oxidative stress and metal ions

The metallothioneins (MT) are small, cysteine-rich heavy metal-binding proteins which participate in an array of protective stress responses. Although a single essential function of MT has not been demonstrated, MT of higher eukaryotes evolved as a mechanism to regulate zinc levels and distribution within cells and organisms. These proteins can also protect against some toxic metals and oxidative stress-inducing agents. In mice, among the four known MT genes, the MT-I and -II genes are most widely expressed. Transcription of these genes is rapidly and dramatically up-regulated in response to zinc and cadmium, as well as in response to agents which cause oxidative stress and/or inflammation. The six zinc-finger metal-responsive transcription factor MTF-1 plays a central role in transcriptional activation of the MT-I gene in response to metals and oxidative stress. Mutation of the MTF-1 gene abolishes these responses, and MTF-1 is induced to bind to the metal response elements in proximal MT promoter in cells treated with zinc or during oxidative stress. The exact molecular mechanisms of action of MTF-1 are not fully understood. Our studies suggest that the DNA-binding activity of MTF-1 in vivo and in vitro is reversibly activated by zinc interactions with the zinc-finger domain. This reflects heterogeneity in the structure and function of the six zinc fingers. We hypothesize that MTF-1 functions as a sensor of free zinc pools in the cell. Changes in free zinc may occur in response to chemically diverse inducers. MTF-1 also exerts effects on MT-I gene transcription which are independent of a large increase in MTF-1 DNA-binding activity. For example, cadmium, which has little effect on the DNA-binding activity of MTF-1 in vivo or in vitro, is a more potent inducer of MT gene expression than is zinc. The basic helix-loop-helix-leucine zipper protein, USF (upstream stimulatory factor family), also plays a role in regulating transcription of the mouse MT-I gene in response to cadmium or H2O2. Expression of dominant negative USF-1 or deletion of its binding site from the proximal promoter attenuates induction of the mouse MT-I gene. USF apparently functions in this context by interacting with as yet unidentified proteins which bind to an antioxidant response element which overlaps the USF-binding site (USF/ARE). Interestingly, this composite element does not participate in the induction of MT-I gene transcription by zinc or redox-cycling quinones. Thus, regulation of the mouse MT-I gene by metals and oxidative stress involves multiple signaling pathways which depend on the species of metal ion and the nature of the oxidative stress.

Functional analysis of a homopolymeric (dA-dT) element that provides nucleosomal access to yeast and mammalian transcription factors

Eukaryotic organisms ranging from yeast to humans maintain a large amount of genetic information in the highly compact folds of chromatin, which poses a large DNA accessibility barrier to rapid changes in gene expression. The ability of the yeast Candida glabrata to survive copper insult requires rapid transcriptional autoactivation of the AMT1 copper-metalloregulatory transcription factor gene. The kinetics of AMT1 autoactivation is greatly enhanced by homopolymeric (dA-dT) element (A16)-mediated nucleosomal accessibility for Amt1p to a metal response element in this promoter. Analysis of the nucleosomal positional requirements for the A16 element reveal an impaired ability of the A16 element to stimulate AMT1 autoregulation when positioned downstream of the metal response element within the nucleosome, implicating an inherent asymmetry to the nucleosome positioned within the AMT1 promoter. Importantly, we demonstrate that the A16 element functions to enhance nucleosomal access and hormone-stimulated transcriptional activation for the mammalian glucocorticoid receptor, in a rotational phase-dependent manner. These data provide compelling evidence that nucleosomal homopolymeric (dA-dT) elements provide enhanced DNA access to diverse classes of transcription factors and suggest that these elements may function in this manner to elicit rapid transcriptional responses in higher eukaryotic organisms.

Metal-ion regulation of gene expression in yeast

Metal-responsive transcription factors exist in yeast to modulate expression of genes that encode proteins involved in cellular uptake of copper, iron and zinc ions. These signal transduction pathways function in the cellular regulation of the intracellular concentration of free metal ions. A second component of metal homeostasis is the regulation of metal-ion binding through protein-mediated metallation. Copper-specific chaperones exist in yeast that route copper ions to the site of biosynthesis of copper-metalloenzymes.

Copper-binding motifs in catalysis, transport, detoxification and signaling

Copper is required for many biological processes but is toxic at high cellular concentrations, so levels in the cell must be strictly controlled. Copper-binding motifs have been identified and characterized in many proteins. The way in which copper is coordinated by these motifs is important for the transport and distribution of intracellular copper and for the effective functioning of copper-dependent enzymes.

Yeast metallothionein gene expression in response to metals and oxidative stress

Metals and oxygen are chemically linked in biological systems. Metals and oxygen play important roles in enzymatic reactions, metabolism, and signal transduction; however, metals and oxygen react to form highly toxic oxygen-derived free radical species. In this review we focus on the use of yeast cells, as unicellular eukaryotic model systems, to conduct studies aimed at understanding fundamental mechanisms for the sensation and protective responses to toxic metals and oxygen-derived radicals via the activation of yeast metallothionein gene expression.

Copper-binding proteins in ectomycorrhizal fungi

The presence of copper-binding proteins produced in response to added copper was examined in isolates of Laccaria laccata (Scop, ex Fr.) Cooke and Paxillus involutus (Batsch ex Fr.) Fr. taken from copper-contaminated and uncontaminated sites, and in a single isolate of Scleroderma citrinum Pers. from a contaminated site. Two isolates of Laccaria (GLac4 and ELacl) grew better in 1-5 miu and 2-5 niM copper than a third (Lac3G) and were considered to be more tolerant. Amongst five isolates of P. involutus, three (WJPax2R, GPaxRSp2 and Pax4) were capable of growth in media containing 4-0 mM copper and were regarded as tolerant. All isolates of both Laccaria and Paxillus were capable of some growth in 2-5 mM copper, but S. citrinum was much more copper-sensitive and the concentration had to be reduced at least 10-fold before any growth occurred. Tolerance of isolates was not related to whether they were taken from copper-contaminated or uncontaminated sites. Copper-binding proteins were detected in response to copper in the culture media in the two tolerant isolates of Laccaria (GLac4 and ELacl) but not in the least tolerant isolate. In Paxillus, similar proteins were found in two tolerant isolates (GPaxRSp2 and Pax4) but not in WJPax2R, which was also regarded as tolerant, nor in any of the less tolerant isolates. Copper-binding proteins were not detected in S. citrinum. The copper-binding protein purified from the Laccaria isolate ELacl appeared as a single band in modified SDS-PAGE electrophoresis. Its molecular mass and spectral characteristics were consistent with it being a metallothionein.

A specialized nucleosome modulates transcription factor access to a C. glabrata metal responsive promoter

The ability of DNA binding transcription factors to access cis-acting promoter elements is critical for transcriptional responses. We demonstrate that rapid transcriptional autoactivation by the Amt1 Cu metalloregulatory transcription factor from the opportunistic pathogenic yeast Candida glabrata is dependent on rapid metal-induced DNA binding to a single metal response element (MRE). In vivo footprinting and chromatin-mapping experiments demonstrate that the MRE and a homopolymeric (dA x dT) element adjacent to the MRE are packaged into a positioned nucleosome that exhibits homopolymeric (dA x dT)-dependent localized distortion. This distortion is critical for rapid Amt1 binding to the MRE, for Cu-dependent AMT1 gene transcription, and for C. glabrata cells to mount a rapid transcriptional response to Cu for normal metal detoxification. The AMT1 promoter represents a novel class of specialized nucleosomal structures that links rapid transcriptional responses to the biology of metal homeostasis.

Molecular biology of iron acquisition in Saccharomyces cerevisiae

In recent years, significant advances have been made in our understanding of the mechanism and regulation of elemental iron transport in the eukaryote Saccharomyces cerevisiae. This organism employs two distinct iron-transport systems, depending on the bioavailability of the metal. In iron-replete environments, a low-affinity transport system (K(m) = 30 microM) is used to acquire iron. This system may also be used to acquire other metals including cobalt and cadmium. When environmental iron is limiting, a high-affinity (K(m) = 0.15 microM) iron-transport system is induced. Genetic studies in S. cerevisiae have identified multiple genes involved in both iron-transport systems. Cell-surface reductases, FRE1 and FRE2, provide ferrous iron for both systems. A non-ATP-dependent transmembrane transporter (FET4) has been identified as the main component of low-affinity transport. One gene identified to date as part of the high-affinity transport system is FET3, which shows high sequence and functional homology to multicopper oxidases. Accessory genes required for the functioning of this transport system include a plasma-membrane copper transporter (CTR1), an intracellular copper transporter (CCC2), and a putative transcription factor (AFT1). The mechanism by which these genes act in concert to ensure iron accumulation in S. cerevisiae presents an intriguing picture, drawing parallels with observations made in the human system almost 40 years ago.

Toxic metal-responsive gene transcription

Metals play a dual role in biological systems, serving as essential co-factors for a wide range of biochemical reactions yet these same metals may be extremely toxic to cells. To cope with the stress of increases in environmental metal concentrations, eukaryotic cells have developed sophisticated toxic metal sensing proteins which respond to elevations in metal concentrations. This signal is transmitted to stimulate the cellular transcriptional machinery to activate expression of metal detoxification and homeostasis genes. This review summarizes our current understanding of the biochemical and genetic mechanisms which underlie cellular responses to toxic metals via metalloregulatory transcription factors.

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.

Cloning of the Candida glabrata TRP1 and HIS3 genes, and construction of their disruptant strains by sequential integrative transformation

The Candida glabrata (Cg) TRP1 and HIS3 genes have been isolated by complementation of the Saccharomyces cerevisiae (Sc) trp1 and his3 mutants, respectively. Cg TRP1 encodes a polypeptide of 217 amino acids (aa), whose aa sequence is 58% identical to that of Sc TRP1. Cg HIS3 encodes a polypeptide of 210 aa, whose aa sequence is 73% identical to that of the Sc HIS3. Both Cg TRP1 and HIS3 were disrupted by sequential integrative transformation where the Sc URA3 was used as a selection marker for transformation. The resulting auxotrophic strain of his3- and trp1- was used to examine the ability of the Sc genes to complement the Cg mutations; Sc HIS3 and TRP1 complemented the Cg his3- and trp1- mutations, respectively.

Structure, organization and expression of the metallothionein gene family in Arabidopsis

Metallothioneins (MTs) are cysteine-rich proteins required for heavy metal tolerance in animals and fungi. Recent results indicate that plants also possess functional metallothionein genes. Here we report the cloning and characterization of five metallothionein genes from Arabidopsis thaliana. The position of the single intron in each gene is conserved. The proteins encoded by these genes can be divided into two groups (MT1 and MT2) based on the presence or absence of a central domain separating two cysteine-rich domains. Four of the MT genes (MT1a, MT1c, MT2a and MT2b) are transcribed in Arabidopsis. Several lines of evidence suggest that the fifth gene, MT1b, is inactive. There is differential regulation of the MT gene family. MT1 mRNA is expressed highly in roots, moderately in leaves and is barely detected in inflorescences and siliques. MT2a and MT2b mRNAs are more abundant in leaves, inflorescences and in roots from mature plants, but are also detected in roots of young plants, and in siliques. MT2a mRNA is strongly induced in seedlings by CuSO4, whereas MT2b mRNA is relatively abundant in this tissue and levels increase only slightly upon exposure to copper. MT1a and MT1c are located within 2 kb of each other and have been mapped to chromosome I. MT1b and MT2b map to separate loci on chromosome V, and MT2a is located on chromosome III. The locations of these MT genes are different from that of CAD1, a gene involved in cadmium tolerance in Arabidopsis.

Distinct regions of Cu(I).ACE1 contact two spatially resolved DNA major groove sites

The interaction between the Cu(I).ACE1 (CuACE1) transcription factor and its DNA binding site in the yeast metallothionein gene was studied by systematically altering the DNA sequence through base substitution, modification, and deletions as well as by altering the protein structure through chemical modification. We show here that CuACE1 is comprised of two distinct domains that contact DNA through minor groove interactions located between two major groove interaction sites. The minor groove interactions are shown to be critical for formation of a stable CuACE1.DNA complex. The NH2-terminal segment of ACE1 is shown to contact the 5'-most distal major groove site.

Mutants of Saccharomyces cerevisiae sensitive to oxidative and osmotic stress

Although oxidative stress is involved in many human diseases, little is known of its molecular basis in eukaryotes. In a genetic approach, S. cerevisiae was used to identify elements involved in oxidative stress. By using hydrogen peroxide as an agent for oxidative stress, 34 mutants were identified. All mutants were recessive and fell into 16 complementation groups (pos1 to pos16 for peroxide sensitivity). They corresponded to single mutations as shown by a 2:2 segregation pattern. Enzymes reportedly involved in oxidative stress, such as glucose-6-phosphate dehydrogenase, glutathione reductase, superoxide dismutase, as well as glutathione concentrations, were investigated in wild-type and mutant-cells. One complementation group lacked glucose-6-phosphate dehydrogenase and was shown to be allelic to the glucose-6-phosphate dehydrogenase structural gene ZWF1/MET19. In other mutants all enzymes supposedly involved in oxidative-stress resistance were still present. However, several mutants showed strongly elevated levels of glutathione reductase, gluconate-6-phosphate dehydrogenase and glucose-6-phosphate dehydrogenase. One complementation group, pos9, was highly sensitive to oxidative stress and revealed the same growth phenotype as the previously described yap1/par1 mutant coding for the yeast homologue of mammalian transcriptional activator protein, c-Jun, of the proto-oncogenic AP-1 complex. However, unlike par1 mutants, which showed diminished activities of oxidative-stress enzymes and glutathion level, the pos9 mutants did not reveal any such changes. In contrast to other recombinants between pos mutations and par1, the sensitivity did not further increase in par1 pos9 recombinants, which may indicate that both mutations belong to the same regulating circuit.(ABSTRACT TRUNCATED AT 250 WORDS)

Production of metallothionein in copper- and cadmium-resistant strains of Saccharomyces cerevisiae

Certain mutants of the yeast Saccharomyces cerevisiae show copper or cadmium resistance. Both copper- and cadmium-resistant strains produce the same metallothionein with 53 amino acid residues which causes metal detoxification by chelating copper or cadmium. The metal detoxification role is the only known function of the metallothionein in yeast. The MT is encoded by the CUP1 gene on chromosome VIII which is expressed by induction with metals. The CUP1 is amplified to 3-14 copies with 2 kb-tandem-repeat units in the metal-resistant strains, whereas the wild-type strain contains only a single copy of the CUP1. Although transcription of CUP1 is inducible by metals, the ACE1 protein serves a dual function as a sensor for copper and an inducer for CUP1 transcription in the copper-resistant strain. In the cadmium-resistant strain, the heat-shock factor having a point mutation may be the regulator for CUP1 transcription. Therefore, it has been clarified that production of MT in yeast is controlled by two systems, the amplification of CUP1 and its transcriptional regulation.

A system for gene cloning and manipulation in the yeast Candida glabrata

The opportunistic pathogenic yeast, Candida (Torulopsis) glabrata, is an asexual imperfect fungus that exists largely as a haploid. Besides being a clinically important pathogen, this yeast also provides a model system for understanding basic biological mechanisms such as metal-activated metallothionein-encoding gene transcription. To facilitate molecular genetic studies in C. glabrata, we isolated a strain auxotrophic for uracil biosynthesis. The ura- mutation could be functionally complemented by the URA3 gene of Saccharomyces cerevisiae, consistent with a defect in the C. glabrata URA3 gene in this strain. We also found that the centromere-based S. cerevisiae plasmid pRS316 could stably transform and replicate in multiple copies in C. glabrata. In contrast, high-copy-number S. cerevisiae plasmids containing the 2 mu circle autonomous replication sequence were not able to replicate productively in C. glabrata. We cloned the C. glabrata URA3 gene, encoding orotidine-5'-phosphate decarboxylase, by complementation of a ura3- strain of S. cerevisiae. The deduced amino-acid sequence is highly similar to that of the URA3 protein from S. cerevisiae. C. glabrata URA3 provides a genetic locus for targeted gene integration in C. glabrata. Integrative plasmids were constructed based on the cloned C. glabrata URA3 and are applicable for directed insertions of genes of interest at the ura3 locus through homologous recombination.