Heat shock transcription factor activates transcription of the yeast metallothionein gene

Transcriptional Alteration of Gene Biomarkers in Hemocytes of Wild Ostrea edulis with Molecular Evidence of Infections with Bonamia spp. and/or Marteilia refringens Parasites

The European flat Ostrea edulis is highly susceptible to intracellular parasitic infections, particularly bonamiosis and marteiliosis. The defensive response of oyster to both bonamiosis and marteiliosis is typically mediated by hemocytes, which play a pivotal role in immune system homeostasis. In the present study, we first used a DNA-based tool in order to rapidly and specifically detect the presence of parasites in oysters from natural banks in the middle Adriatic Sea. In a second step, we used qRT-PCR to analyze the mRNA levels of a set of genes (i.e., superoxide dismutase (SOD), glutathione S-transferase (GST), metallothionein (MT), heat shock protein (HSP) 70 and 90, inhibitor of apoptosis (IAP), fas ligand (FAS), galectin (GAL) and extracellular superoxide dismutase (Ec-SOD)) expressed by hemocytes of flat oysters infected by the parasites, present singularly or in combination, compared to hemocytes from non-infected specimens. The results indicate that the presence of parasite DNA may be associated to a general upregulation of host genes related to apoptosis, detoxification and oxidative stress protection, with the exception of Ec-SOD, whose trend to a downregulation might reflect a mechanism for parasite escape before internalization.

Novel aspects of heat shock factors: DNA recognition, chromatin modulation and gene expression

Heat shock factor (HSF) is an evolutionarily conserved stress-response regulator that activates the transcription of heat shock protein genes, whose products maintain protein homeostasis under normal physiological conditions, as well as under conditions of stress. The promoter regions of the target genes contain a heat shock element consisting of multiple inverted repeats of the pentanucleotide sequence nGAAn. A single HSF of yeast can bind to heat shock elements that differ in the configuration of the nGAAn units and can regulate the transcription of various genes that function not only in stress resistance, but also in a broad range of biological processes. Mammalian cells have four HSF family members involved in different, but in some cases similar, biological functions, including stress resistance, cell differentiation and development. Mammalian HSF family members exhibit differential specificity for different types of heat shock elements, which, together with cell type-specific expression of HSFs is important in determining the target genes of each HSF. This minireview focuses on the molecular mechanisms of DNA recognition, chromatin modulation and gene expression by yeast and mammalian HSFs.

Transcriptional expression levels of cell stress marker genes in the Pacific oyster Crassostrea gigas exposed to acute thermal stress

During the annual cycle, oysters are exposed to seasonal slow changes in temperature, but during emersion at low tide on sunny summer days, their internal temperature may rise rapidly, resulting in acute heat stress. We experimentally exposed oysters to a 1-h acute thermal stress and investigated the transcriptional expression level of some genes involved in cell stress defence mechanisms, including chaperone proteins (heat shock proteins Hsp70, Hsp72 and Hsp90 (HSP)), regulation of oxidative stress (Cu-Zn superoxide dismutase, metallothionein (MT)), cell detoxification (glutathione S-transferase sigma, cytochrome P450 and multidrug resistance (MDR1)) and regulation of the cell cycle (p53). Gene mRNA levels were quantified by reverse transcription-quantitative polymerase chain reaction and expressed as their ratio to actin mRNA, used as a reference. Of the nine genes studied, HSP, MT and MDR1 mRNA levels increased in response to thermal stress. We compared the responses of oysters exposed to acute heat shock in summer and winter and observed differences in terms of magnitude and kinetics. A larger increase was observed in September, with recovery within 48 h, whereas in March, the increase was smaller and lasted more than 2 days. The results were also compared with data obtained from the natural environment. Though the functional molecule is the protein and information at the mRNA level only has limitations, the potential use of mRNAs coding for cell stress defence proteins as early sensitive biomarkers is discussed.

Different Mechanisms Are Involved in the Transcriptional Activation by Yeast Heat Shock Transcription Factor through Two Different Types of Heat Shock Elements

The hydrophobic repeat is a conserved structural motif of eukaryotic heat shock transcription factor (HSF) that enables HSF to form a homotrimer. Homotrimeric HSF binds to heat shock elements (HSEs) consisting of three inverted repeats of the sequence nGAAn. Sequences consisting of four or more nGAAn units are bound cooperatively by two HSF trimers. We show that in Saccharomyces cerevisiae cells oligomerization-defective Hsf1 is not able to bind HSEs with three units and is not extensively phosphorylated in response to stress; it is therefore unable to activate genes containing this type of HSE. Several lines of evidence indicate that oligomerization is a prerequisite for stress-induced hyperphosphorylation of Hsf1. In contrast, oligomerization and hyperphosphorylation are not necessary for gene activation via HSEs with four units. Intragenic suppressor screening of oligomerization-defective hsf1 showed that an interface between adjacent DNA-binding domains is important for the binding of Hsf1 to the HSE. We suggest that Saccharomyces cerevisiae HSEs with different structures are regulated differently; HSEs with three units require Hsf1 to be both oligomerized and hyperphosphorylated, whereas HSEs with four or more units do not require either.

Interaction between heat shock transcription factors (HSFs) and divergent binding sequences: binding specificities of yeast HSFs and human HSF1

The target genes of the heat shock transcription factor (HSF) contain a cis-acting sequence, the heat shock element (HSE), which consists of multiple inverted repeats of the sequence 5'-nGAAn-3'. Using data acquired in this and a previous study, we have identified the HSEs in 59 of 62 target genes of Saccharomyces cerevisiae Hsf1. The Hsf1 protein recognizes continuous and discontinuous repeats of the nGAAn unit; the nucleotide sequences and configuration of the units diverge slightly among functional HSEs. When Schizosaccharomyces pombe HSF was expressed in S. cerevisiae cells, heat shock induced S. pombe HSF to bind to various HSE types, which properly activated transcription from almost all target genes, suggesting that the S. pombe genome also contains divergent HSEs. Human HSF1 induced the heat shock response via HSEs with continuous units in S. cerevisiae cells but failed to do so via HSEs with discontinuous units. Binding of human HSF1 to the discontinuous type of HSE was observed in vitro but was significantly inhibited in vivo. These results show that human HSF1 recognizes HSEs in a slightly different way than yeast HSFs and suggest that the configuration of the unit is an important determinant for HSF-HSE interactions.

Mutated Yeast Heat Shock Transcription Factor Activates Transcription Independently of Hyperphosphorylation

The homotrimeric heat shock transcription factor (HSF) binds to the heat shock element of target genes and regulates transcription in response to various stresses. The Hsf1 protein of Saccharomyces cerevisiae is extensively phosphorylated upon heat shock; a modification that is under positive regulation by its C-terminal regulatory domain (CTM). Hyperphosphorylation has been implicated in gene-specific transcriptional activation. Here, we surveyed genes whose heat shock response is reduced by a CTM mutation. The CTM is indispensable for transcription via heat shock elements bound by a single Hsf1 trimer but is dispensable for transcription via heat shock elements bound by Hsf1 trimers in a cooperative manner. Intragenic mutations located within or near the wing region of the winged helix-turn-helix DNA-binding domain suppress the temperature-sensitive growth phenotype associated with the CTM mutation and enable Hsf1 to activate transcription independently of hyperphosphorylation. Deletion of the wing partially restores the transcriptional defects of the unphosphorylated Hsf1. These results demonstrate a functional link between hyperphosphorylation and the wing region and suggest that this modification is involved in a conformational change of a single Hsf1 trimer to an active form.

Identification of a novel class of target genes and a novel type of binding sequence of heat shock transcription factor in Saccharomyces cerevisiae

In response to hyperthermia, heat shock transcription factor (HSF) activates transcription of a set of genes encoding heat shock proteins (HSPs). The promoter regions of HSP genes contain the HSF binding sequence called the heat shock element (HSE), which consists of contiguous inverted repeats of the sequence 5'-nGAAn-3' (where n is any nucleotide). We have constructed an hsf1 mutant of Saccharomyces cerevisiae and analyzed genome-wide changes in heat shock response in the mutant cells. The results have revealed that Hsf1 is necessary for heat-induced transcription of not only HSP but also genes encoding proteins involved in diverse cellular processes such as protein degradation, detoxification, energy generation, carbohydrate metabolism, and maintenance of cell wall integrity. Approximately half of the Hsf1-regulated genes lacked the typical HSE in their promoter regions. Instead, several of these genes have a novel Hsf1 binding sequence that contains three direct repeats of nTTCn (or nGAAn) interrupted by 5 bp. The number and spacing of the repeating units are critical determinants for heat-induced transcription as well as for recognition by Hsf1. In the yeast genome, the presence of the sequence is enriched in Hsf1-regulated genes, suggesting that it is generally used as an HSE in the Hsf1 regulon.

Carboxy-terminal region of the yeast heat shock factor contains two domains that make transcription independent of the TFIIH protein kinase

BACKGROUND

Phosphorylation of the carboxy-terminal domain (CTD) of the largest subunit of RNA polymerase II is implicated in transition from initiation to elongation in the transcription cycle. In yeast cells, Kin28, a subunit of the general transcription factor TFIIH, is responsible for the CTD phosphorylation. Although Kin28 is indispensable for transcription of many genes, its requirement is bypassed in certain genes such as SSA4 or CUP1, whose transcription is activated by the heat shock factor Hsf1.

RESULTS

We show that C-terminal region of Hsf1, which consists of an activation domain AR2 and a regulatory domain CTM, mediates the Kin28-independent transcription. The AR2 domain, when fused to the DNA-binding domain of Gal4 and recruited to the GAL7 gene via the Gal4-binding sequence, is sufficient for activating GAL7 in the absence of Kin28. We have further found that AR2 has an ability to recruit TATA box-binding protein-associated factors (TAFs) to the promoter. Consistently, transcription from promoters occupied naturally or artificially with TAFs is sustained in the absence of Kin28 function.

CONCLUSIONS

These results show that CTM modulates activation function of AR2 in the Hsf1 molecule. We also suggest that recruitment of TAFs to a promoter is involved in the Kin28-independent transcription.

Artificial recruitment of certain Mediator components affects requirement of basal transcription factor IIE

BACKGROUND

Basal transcription factors are essential for RNA polymerase II (RNAPII)-catalysed transcription of many but not all the mRNA-encoding genes in vivo as well as in vitro. For example, copper-inducible transcription of the copper metallothionein gene CUP1 occurs independently of basal factor TFIIE in budding yeast. To gain insight into the mechanism by which the requirement for TFIIE is bypassed, we artificially recruited certain constituents of Mediator, a large protein complex transmitting signals from various activators to the RNAPII machinery, to the CUP1 promoter by protein fusions with Ace1, the copper-inducible activator.

RESULTS

Fusions with Med2 or Pgd1 activated CUP1 independently of TFIIE. Surprisingly, fusions with neither Srb5 nor Med9 circumvented TFIIE requirement for the CUP1 activation. Components of TFIID were similarly recruited to the CUP1 promoter without activation. By using a chromatin immunoprecipitation technique, we found that TFIIE is necessary for stable binding of TFIIH and RNAPII to the ADH1 promoter, whose activation requires TFIIE. However, binding of TFIIH and RNAPII to CUP1 upon its activation did not require TFIIE.

CONCLUSIONS

Our results strongly suggest that the TFIIE requirement of a gene is determined by a target(s) in Mediator through which the signal of the cognate activator is transmitted.

A novel non-conventional heat shock element regulates expression of MDJ1 encoding a DnaJ homolog in Saccharomyces cerevisiae

The heat shock factor (HSF) is a pivotal transcriptional factor that regulates the expression of genes encoding heat shock proteins (HSPs) via heat shock elements (HSEs). nGAAnnTTCnnGAAn functions as the minimum consensus HSE (cHSE) in vivo. Here we show that the expression of Saccharomyces cerevisiae MDJ1 encoding a mitochondrial DnaJ homolog is regulated by HSF via a novel non-consensus HSE (ncHSE(MDJ1)), which consists of three separated pentameric nGAAn motifs, nTTCn-(11 bp)-nGAAn-(5 bp)-nGAAn. This is the first evidence to show that the immediate contact of nGAAn motifs is dispensable for regulation by HSF in vivo. ncHSE(MDJ1) confers different heat shock responses versus cHSE and, unlike cHSE, definitively requires a carboxyl-terminal activation domain of HSF in the expression. ncHSE(MDJ1)-like elements are found in promoter regions of some other DnaJ-related genes. The highly conserved HSF/HSE system suggests that similar ncHSEs may be used for the expression of HSP genes in other eukaryotes including humans.

Transcription factors regulating the response to oxidative stress in yeast

A main avenue of defense against fungal infection uses oxidative killing of these and other microorganisms. Consequently, the ability of fungi to withstand an oxidative challenge has important implications for their ultimate pathogenicity in a host organism. Fungi also serve as an excellent model system for handling of reactive oxygen species in eukaryotic cells. For these reasons, a great deal of work has been invested in analyzing pathways involved in and the mechanisms regulating oxidative stress tolerance in fungi. The goal of this review is to discuss the current state of knowledge underlying the ability of fungal cells to mount a response to oxidative stress via activation of transcription factors. Studies in Saccharomyces cerevisiae have identified multiple transcriptional regulatory proteins that mediate tolerance to oxidative stress. Experiments focused on the fission yeast Schizosaccharomyces pombe have led to the discovery of protein kinase cascades highly related to mammalian stress-activated protein kinases. Recent studies on the pathogenic yeast Candida albicans have allowed analysis of the role of a critical oxidant-regulated transcription factor in this important human pathogen. Further understanding of oxidative stress resistance pathways in fungi is an important step toward understanding the molecular pathogenesis of these microorganisms.

Killifish metallothionein messenger RNA expression following temperature perturbation and cadmium exposure

Metallothionein (MT), a cysteine-rich metal binding protein, is considered to play an essential role in the regulation of intracellular metals. Induction of MT in mammalian and nonmammalian tissues following heavy metal exposure may serve as a defense mechanism and a biomarker of environmental exposure to chemical stressors such as toxic metals. In this study, MT messenger RNA (mRNA) expression was characterized in male and female nonspawning and spawning killifish (Fundulus heteroclitus) following an 8-day exposure to specific sublethal stressors, which included temperature perturbation (26 degrees C or 10 degrees C) and/or 6 ppb of waterborne cadmium chloride (CdCl2). Hepatic, gill, and intestinal MT mRNA, expressed as copy number per microgram of total RNA, was assessed by reverse transcriptase-polymerase chain reaction and electrochemiluminescence using winter flounder (Pleuronectes americanus) MT complementary DNA primers. Liver, gill, and intestine MT mRNA expression was significantly (P < 0.05) increased in nonspawning killifish exposed to 26 degrees C compared with those exposed to 19 degrees C (control). In addition, a significant (P < 0.05) increase in gill MT mRNA induction was observed in nonspawning killifish exposed to 6 ppb of waterborne CdCl2 compared with controls. The results of this study demonstrate significant MT mRNA induction in nonspawning killifish following short-term exposure to physiological and chemical stressors. Thus, further research may be necessary before the use of killifish MT mRNA induction as a biomarker of environmental chemical stress exposure alone.

Proline in alpha-helical kink is required for folding kinetics but not for kinked structure, function, or stability of heat shock transcription factor

The DNA-binding domain of the yeast heat shock transcription factor (HSF) contains a strictly conserved proline that is at the center of a kink. To define the role of this conserved proline-centered kink, we replaced the proline with a number of other residues. These substitutions did not diminish the ability of the full-length protein to support growth of yeast or to activate transcription, suggesting that the proline at the center of the kink is not conserved for function. The stability of the isolated mutant DNA-binding domains was unaltered from the wild-type, so the proline is not conserved to maintain the stability of the protein. The crystal structures of two of the mutant DNA-binding domains revealed that the helices in the mutant proteins were still kinked after substitution of the proline, suggesting that the proline does not cause the alpha-helical kink. So why are prolines conserved in this and the majority of other kinked alpha-helices if not for structure, function, or stability? The mutant DNA-binding domains are less soluble than wild-type when overexpressed. In addition, the folding kinetics, as measured by stopped-flow fluorescence, is faster for the mutant proteins. These two results support the premise that the presence of the proline is critical for the folding pathway of HSF's DNA-binding domain. The finding may also be more general and explain why kinked helices maintain their prolines.

Structural analysis of yeast HSF by site-specific crosslinking

We have introduced cysteine substitutions into the yeast HSF1 gene at a variety of locations. Most have no phenotypic effect, and therefore provide site-specific probes for thiol-specific reagents. Crosslinking of single mutants identifies locations where equivalent regions of individual monomers can approach each other in the HSF trimer. Crosslinking of double mutants indicates regions that can approach closely within a single subunit. Results for the DNA binding domain and trimerization domain are consistent with known structural information, and provide essential controls on the validity of the technique. In contrast to these two domains, the N-terminal and C-terminal domains, wherein lie the transcriptional activators, are highly flexible, and do not appear to be in stable contact with any other portions of the protein. None of these patterns are affected by the conformational change that is induced by superoxide or heat shock. We suggest a new model for the mechanism of HSF regulation that accomodates the structural information provided by these studies.

The Skn7 response regulator of Saccharomyces cerevisiae interacts with Hsf1 in vivo and is required for the induction of heat shock genes by oxidative stress

The Skn7 response regulator has previously been shown to play a role in the induction of stress-responsive genes in yeast, e.g., in the induction of the thioredoxin gene in response to hydrogen peroxide. The yeast Heat Shock Factor, Hsf1, is central to the induction of another set of stress-inducible genes, namely the heat shock genes. These two regulatory trans-activators, Hsf1 and Skn7, share certain structural homologies, particularly in their DNA-binding domains and the presence of adjacent regions of coiled-coil structure, which are known to mediate protein-protein interactions. Here, we provide evidence that Hsf1 and Skn7 interact in vitro and in vivo and we show that Skn7 can bind to the same regulatory sequences as Hsf1, namely heat shock elements. Furthermore, we demonstrate that a strain deleted for the SKN7 gene and containing a temperature-sensitive mutation in Hsf1 is hypersensitive to oxidative stress. Our data suggest that Skn7 and Hsf1 cooperate to achieve maximal induction of heat shock genes in response specifically to oxidative stress. We further show that, like Hsf1, Skn7 can interact with itself and is localized to the nucleus under normal growth conditions as well as during oxidative stress.

Role of an alpha-helical bulge in the yeast heat shock transcription factor

The heat shock transcription factor (HSF) is the master transcriptional regulator of the heat shock response. The identity of a majority of the genes controlled by HSF and the circumstances under which HSF becomes induced are known, but the details of the mechanism by which HSF is able to sense and respond to heat remains an enigma. For example, it is unclear whether HSF senses the heat shock directly or requires ancillary interactions from a heat-induced signaling pathway. We present the analysis of a series of mutations in an alpha-helical bulge in the DNA-binding domain of HSF. Deletion of residues in this bulged region increases the overall activity of the protein. Yeast containing the deletion mutant HSF are able to survive growth temperatures that are lethal to yeast containing wild-type HSF, and they are also constitutively thermotolerant. The increase in activity can be measured as an increase in both constitutive and induced transcriptional activity. The mutant proteins bind DNA more tightly than the wild-type protein does, but this is unlikely to account fully for the increase in transcriptional activity as yeast HSF is constitutively bound to its binding site in vivo. The stability of the mutant proteins to thermal denaturation is lower than wild-type, though their native-state structures are still well-folded. Therefore, the mutants may be structurally analogous to the heat-induced state of HSF, and suggest that the DNA-binding domain of HSF may be capable of sensing heat shock directly.

Activator-specific requirement for the general transcription factor IIE in yeast

The general transcription factor (TF) IIE is required for mRNA synthesis of many, but not all, genes in yeast. In the transcription process, TFIIE regulates TFIIH kinase activity that phosphorylates the carboxy-terminal domain (CTD) of the largest subunit of RNA polymerase II. The CTD and the CTD kinase Kin28, a subunit of TFIIH, have been shown to be dispensable for activation of several heat shock genes and the copper metallothionein gene CUP1. Here we analyzed requirement of TFIIE for transcription of these genes and found that TFIIE is necessary for activation of the heat shock genes by heat shock transcription factor Hsf1. By contrast, transcription of CUP1 mediated by both Hsf1 and copper-activated transcription factor Ace1 was inducible after inactivating TFIIE. These results show that both TFIIE and the CTD/the CTD kinase exhibit "gene specificities" which are overlapping, but not identical to each other, and thereby suggest that TFIIE functions with or without involvement of the CTD/the CTD kinase depending on the gene to be transcribed.

Modulation of human heat shock factor trimerization by the linker domain

Heat shock transcription factors (HSFs) are stress-responsive proteins that activate the expression of heat shock genes and are highly conserved from bakers' yeast to humans. Under basal conditions, the human HSF1 protein is maintained as an inactive monomer through intramolecular interactions between two coiled-coil domains and interactions with heat shock proteins; upon environmental, pharmacological, or physiological stress, HSF1 is converted to a homotrimer that binds to its cognate DNA binding site with high affinity. To dissect regions of HSF1 that make important contributions to the stability of the monomer under unstressed conditions, we have used functional complementation in bakers' yeast as a facile assay system. Whereas wild-type human HSF1 is restrained as an inactive monomer in yeast that is unable to substitute for the essential yeast HSF protein, mutations in the linker region between the DNA binding domain and the first coiled-coil allow HSF1 to homotrimerize and rescue the viability defect of a hsfDelta strain. Fine mapping by functional analysis of HSF1-HSF2 chimeras and point mutagenesis revealed that a small region in the amino-terminal portion of the HSF1 linker is required for maintenance of HSF1 in the monomeric state in both yeast and in transfected human 293 cells. Although linker regions in transcription factors are known to modulate DNA binding specificity, our studies suggest that the human HSF1 linker plays no role in determining HSF1 binding preferences in vivo but is a critical determinant in regulating the HSF1 monomer-trimer equilibrium.

Protein chaperones and the heat shock response in Saccharomyces cerevisiae

Recent studies have shed new light on the complexities of the heat shock response in yeast. Multiple pathways for transcriptional induction of both classic and novel heat shock proteins are emerging together with a more detailed understanding of the interactions between protein chaperones and their physiological targets. New roles for heat shock proteins in defense and recovery from the impacts of thermal stress on critical cellular processes have expanded our understanding of these elaborate and ubiquitous proteins.

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.

Comparison of metallothionein gene expression and nonprotein thiols in ten Arabidopsis ecotypes. Correlation with copper tolerance

Seedlings of 10 Arabidopsis ecotypes were compared with respect to copper tolerance, expression of two metallothionein genes (MT1 and MT2), and nonprotein thiol levels. MT1 was uniformly expressed in all treatments, and MT2 was copper inducible in all 10 ecotypes. MT1 and MT2 mRNA levels were compared with various growth parameters for the 10 ecotypes in the presence of 40 microM Cu2+. The best correlation (R = 0.99) was obtained between MT2 mRNA and the rate of root extension. MT2 mRNA levels also paralleled the recovery phase following inhibition by copper. Induction of MT2 mRNA was initiated at copper concentrations below the threshold for growth inhibition. In cross-induction experiments, Ag+, Cd2+, Zn2+, Ni2+, and heat shock all induced significant levels of MT2 gene expression, whereas Al3+ and salicylic acid did not. The correlation between copper tolerance and nonprotein thiol levels in the 10 ecotypes was not statistically significant. However, 2 ecotypes, Ws and Enkheim, previously shown to exhibit an acclimation response, had the highest levels of nonprotein thiols. We conclude that MT2 gene expression may be the primary determinant of ecotypic differences in the copper tolerance of nonpretreated Arabidopsis seedlings.

Mutated yeast heat shock transcription factor exhibits elevated basal transcriptional activation and confers metal resistance

Cadmium-resistant Saccharomyces cerevisiae strain 301N exhibits high basal as well as cadmium-induced expression of the CUP1 metallothionein gene. Since regulation of CUP1 is usually restricted to copper ions, our goal was to identify the factor responsible for the high metallothionein levels in strain 301N. The gene responsible for the observed phenotype is a spontaneously mutated heat shock transcription factor gene (HSF1). A double, semidominant HSF1 mutant with substitutions at codons 206 and 256 within the DNA-binding domain of the heat shock factor (HSF) confers two phenotypes. The first phenotype is elevated transcriptional activity of the HSF mutant (HSF301), which results in constitutive thermotolerance. A second HSF301 phenotype is enhanced binding affinity for the heat shock element (HSE) within the CUP1 5'-sequences, resulting in high basal transcription of metallothionein. The CUP1 HSE is a minimal heat shock element containing only two perfectly spaced inverted repeats of the basic nGAAn block. Cells containing HSF301 are resistant to cadmium salts. The single R206S mutation is responsible for the high affinity binding to the CUP1 HSE. In addition, the R206S HSF substitution exhibits constitutive transcriptional activation from a consensus HSE (HSE2). The F256Y substitution in HSF attenuates the effects of R206S on the consensus HSE2, but not on the CUP1 HSE.

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.

Mutational analysis of the DNA-binding domain of yeast heat shock transcription factor

Both randomized oligonucleotide cassette mutagenesis and site-directed mutagenesis have been used in combination with a yeast genetic screen to identify critical residues in the DNA-binding domain of heat shock transcription factor from Saccharomyces cerevisiae. Most of the surface residues in this highly conserved domain can be changed to alanine with no observable effect on function. Of nine critical residues identified in this screen, five are within helix alpha 3, previously designated as the probable DNA recognition helix in the crystal structure of the Kluyveromyces lactis protein. The other four residues may be involved in DNA-binding or protein-protein interactions.

Kidney trace metal response to combined cisplatin (CDDP) and hyperthermia

The effects of hyperthermia (HY) on cisplatin (CDDP) nephrotoxicity and kidney metal concentrations were evaluated in female F344 rats. Rats were anaesthetized with a xylazine/ketamine mixture, heated on a water-bed for 1 h to an intraperitoneal temperature of 41.1 +/- 0.2 degrees C, and maintained at hyperthermia for an additional 30 min (HY plateau). CDDP (5 mg/kg body weight) was administered at the start of the heating or at the HY plateau. Neither HY alone nor CDDP (5 mg/kg) at normothermia produced a significant effect on weight loss or nephrotoxicity. CDDP administered at the start of heating produced a moderate 1.5-fold increase in serum urea nitrogen (SUN) and serum creatinine concentrations. CDDP administered at the HY plateau produced a significant six-fold increase in SUN and a four-fold increase in creatinine concentration. Weight loss increased two- to three-fold from the combined regimen, but only the rats given CDDP at the HY plateau continued to lose weight through day 7. A loss of kidney copper (50-60%) resulted from the combined regimen, similar to losses observed with higher doses of CDDP at normothermia. HY alone had little effect on concentrations of kidney copper or zinc up to 4 days post-treatment. The results demonstrate that systemic hyperthermia significantly increases CDDP nephrotoxicity in F344 rats and that kidney copper loss from CDDP exposure at hyperthermia is similar to the loss observed from CDDP at normothermia.

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.