Tandem gene amplification mediates copper resistance in yeast

ACE1 regulates expression of the Saccharomyces cerevisiae metallothionein gene

Copper resistance in Saccharomyces cerevisiae is mediated, in large part, by the CUP1 locus, which encodes a low-molecular-weight, cysteine-rich metal-binding protein. Expression of the CUP1 gene is regulated at the level of transcriptional induction in response to high environmental copper levels. This report describes the isolation of a yeast mutant, ace1-1, which is defective in the activation of CUP1 expression upon exposure to exogenous copper. The ace1-1 mutation is recessive and lies in a genetic element that encodes a trans-acting CUP1 regulatory factor. The wild-type ACE1 gene was isolated by in vivo complementation and restores copper inducibility of CUP1 expression and copper resistance to the otherwise copper-sensitive ace1-1 mutant. Linkage analysis and gene deletion experiments verified that this gene represents the authentic ACE1 locus. ACE1 maps to the left arm of chromosome VII, 9 centimorgans centromere distal to lys5. The ACE1 gene appears to play a direct or indirect positive role in activation of CUP1 expression in response to elevated copper concentrations.

ACE1 regulates expression of the Saccharomyces cerevisiae metallothionein gene

Copper resistance in Saccharomyces cerevisiae is mediated, in large part, by the CUP1 locus, which encodes a low-molecular-weight, cysteine-rich metal-binding protein. Expression of the CUP1 gene is regulated at the level of transcriptional induction in response to high environmental copper levels. This report describes the isolation of a yeast mutant, ace1-1, which is defective in the activation of CUP1 expression upon exposure to exogenous copper. The ace1-1 mutation is recessive and lies in a genetic element that encodes a trans-acting CUP1 regulatory factor. The wild-type ACE1 gene was isolated by in vivo complementation and restores copper inducibility of CUP1 expression and copper resistance to the otherwise copper-sensitive ace1-1 mutant. Linkage analysis and gene deletion experiments verified that this gene represents the authentic ACE1 locus. ACE1 maps to the left arm of chromosome VII, 9 centimorgans centromere distal to lys5. The ACE1 gene appears to play a direct or indirect positive role in activation of CUP1 expression in response to elevated copper concentrations.

DIS1: a yeast gene required for proper meiotic chromosome disjunction

Mutants at a newly identified locus, DIS1 (disjunction), were detected by screening for mutants that generate aneuploid spores (chromosome VIII disomes) at an increased frequency. Strains carrying the partially dominant alleles, DIS1-1 or DIS1-2, generate disomes at rates up to 100 times the background level. Mitotic nondisjunction is also increased 10- to 50-fold over background. Half-tetrad analysis of disomes for a marked interval on chromosome VIII yields wild-type map distances, indicating that a general recombination deficiency is not the cause of nondisjunction. Meiotic nondisjunction in DIS1 mutants is not chromosome specific; 5% of the spores disomic for chromosome VIII are also disomic for chromosome III. Although only one disomic spore is found per exceptional ascus most of the disomes appear to be generated in the first meiotic division because recovered chromosome VIII disomes contain mostly nonsister chromosomes. We propose that disome generation in the DIS1 mutants results from precocious separation of sister centromeres.

Changes induced in the permeability barrier of the yeast plasma membrane by cupric ion

A specific effect of Cu2+ eliciting selective changes in the permeability of intact Saccharomyces cerevisiae cells is described. When 100 microM CuCl2 was added to a cell suspension in a buffer of low ionic strength, the permeability barrier of the plasma membranes of the cells was lost within 2 min at 25 degrees C. The release of amino acids was partial, and the composition of the amino acids released was different from that of those retained in the cells. Mostly glutamate was released, but arginine was mainly retained in the cells. Cellular K+ was released rapidly after CuCl2 addition, but 30% of the total K+ was retained in the cells. These and other observations suggested that Cu2+ caused selective lesions of the permeability barrier of the plasma membrane but did not affect the permeability of the vacuolar membrane. These selective changes were not induced by the other divalent cations tested. A novel and simple method for differential extraction of vacuolar and cytosolic amino acid pools by Cu2+ treatment was established. When Ca2+ was added to Cu2+-treated cells, a large amount of Ca2+ was sequestered into vacuoles, with formation of an inclusion of a Ca2+-polyphosphate complex in the vacuoles. Cu2+-treated cells also showed enhanced uptake of basic amino acids and S-adenosylmethionine. The transport of these substrates showed saturable kinetics with low affinities, reflecting the vacuolar transport process in situ. With Cu2+ treatment, selective leakage of K+ from the cytosolic compartment appears to create a large concentration gradient of K+ across the vacuolar membrane and generates an inside-negative membrane potential, which may provide a driving force of uptake of positively charged substances into vacuoles. Cu2+ treatment provides a useful in situ method for investigating the mechanisms of differential solute pool formation and specific transport phenomena across the vacuolar membrane.

Regulated secretion of MuGM-CSF in Saccharomyces cerevisiae via GAL1:MF alpha 1 prepro sequences

Murine granulocyte-macrophage colony-stimulating factor (GM-CSF) was expressed in Saccharomyces cerevisiae using a novel regulated secretion system. This system involves the fusion of the GAL1 upstream regulatory region to the signal sequence of the alpha mating pheromone, and the integration of this GAL1:MF alpha 1 prepro:MuGM-CSF construct into the yeast chromosome. These constructs were very stable under both selective and nonselective conditions: after 30 generations of growth no plasmid loss was observed. The expression and secretion of MuGM-CSF were analyzed by biological assays and Western blots of yeast culture medium and yeast cell extracts. Expression of MuGM-CSF was regulated by galactose induction. In addition, expression levels were proportional to the number of tandem copies of the gene inserted into the yeast chromosome.

Enhancement of copper resistance and CupI amplification in carcinogen-treated yeast cells

Carcinogen-induced amplification at the CupI locus, coding for a metallothionein protein, was studied in the yeast Saccharomyces cerevisiae. Exposure of cells from three different haploid strains, 4939, DBY746 and 320, to chemical carcinogens such as N-methyl-N'-nitro-N-nitroso-guanidine (MNNG), ethylmethanesulfonate (EMS) and 4-nitroquinoline-N-oxide (4NQO) enhanced the frequency of copper-resistant colonies up to several hundred fold. Copper-resistant clones obtained from strains DBY746 and 320, which contain more than one copy of the CupI locus, displayed a four- to eightfold amplification of the CupI sequences. In these clones the amplified CupI sequences were organized in a tandem array. Carcinogen treatment of strain 4939 in which only one copy of the CupI gene is present produced resistant colonies without CupI amplification. The possible use of the yeast system to study gene duplication and amplification is discussed.

The molecular genetics of copper resistance in Saccharomyces cerevisiae--a paradigm for non-conventional yeasts

After a short introduction on the historical background of the development of Saccharomyces cerevisiae as a model eukaryote, a review is given on the present state of genetics and molecular biology of copper resistance in S. cerevisiae. The gene CUP1 encodes a protein (copper metallothionein) of a molecular weight of 6570 dalton. The synthesis of this copper chelatin is induced by copper and is regulated at the level of transcription. Copper resistance (CUPr) is correlated with amplification of CUP1 and resulted in a higher copy number of this gene on chromosome VIII. Spontaneous meiotic alterations of the gene copy number have been studied.

Efficient expression of the yeast metallothionein gene in Escherichia coli

The yeast metallothionein gene CUP1 was cloned into a bacterial expression system to achieve efficient, controlled expression of the stable, unprocessed protein product. The Escherichia coli-synthesized yeast metallothionein bound copper, cadmium, and zinc, indicating that the protein was functional. Furthermore, E. coli cells expressing CUP1 acquired a new, inducible ability to selectively sequester heavy metal ions from the growth medium.

Replication and recombination in gene establishment in non-Saccharomyces yeasts

A brief review is given on the establishment of recombinant DNA technology for non-conventional yeasts. The availability of DNA delivery systems, selectable markers for identification of transformants, and the means of replicating and amplifying the recombinant DNA are discussed. Some of the existing transformation systems among non-conventional yeasts are explained.

Metallothionein genes in Drosophila melanogaster constitute a dual system

We have selected a metallothionein (MT) cDNA clone from a cadmium-resistant Drosophila melanogaster cell line. This clone includes an open reading frame coding for a 43-amino acid protein whose characteristics are a high cysteine content (12 cysteines, 28% of all residues) and a lack of aromatic amino acids. This protein differs markedly from the Drosophila MT (Mtn gene) previously reported [Lastowski-Perry, D., Otto, E. & Maroni, G. (1985) J. Biol. Chem. 260, 1527-1530). The MT system of Drosophila thus consists of at least two distantly related genes, in sharp contrast with vertebrate MT systems, in which the different members of MT gene families display high similarity. The gene corresponding to our MT cDNA (Mto) is inducible in Drosophila cell lines and in both larval and adult flies.

Amino-terminal fragments of delta 1-pyrroline-5-carboxylate dehydrogenase direct beta-galactosidase to the mitochondrial matrix in Saccharomyces cerevisiae

delta 1-Pyrroline-5-carboxylate (P5C) dehydrogenase, the second enzyme in the proline utilization (Put) pathway of Saccharomyces cerevisiae and the product of the PUT2 gene, was localized to the matrix compartment by a mitochondrial fractionation procedure. This result was confirmed by demonstrating that the enzyme had limited activity toward an externally added substrate that could not penetrate the inner mitochondrial membrane (latency). To learn more about the nature of the import of this enzyme, three gene fusions were constructed that carried 5'-regulatory sequences through codons 14, 124, or 366 of the PUT2 gene ligated to the lacZ gene of Escherichia coli. When these fusions were introduced into S. cerevisiae either on multicopy plasmids or stably integrated into the genome, proline-inducible beta-galactosidase was made. The shortest gene fusion, PUT2-lacZ14, caused the production of a high level of beta-galactosidase that was found exclusively in the cytoplasm. The PUT2-lacZ124 and PUT2-lacZ366 fusions made lower levels of beta-galactosidases that were mitochondrially localized. Mitochondrial fractionation and protease-protection experiments showed that the PUT2-lacZ124 hybrid protein was located exclusively in the matrix, while the PUT2-lacZ366 hybrid was found in the matrix as well as the inner membrane. Thus, the amino-terminal 124 amino acids of P5C dehydrogenase carries sufficient information to target and deliver beta-galactosidase to the matrix compartment. The expression of the longer hybrids had deleterious effects on cell growth; PUT2-lacZ366-containing strains failed to grow on proline as the sole source of nitrogen. In the presence of the longest hybrid beta-galactosidase, the wild-type P5C dehydrogenase was still properly localized in the matrix compartment, but its activity was reduced. The nature of the effects of these hybrid proteins on cell growth is discussed.

Tandemly duplicated upstream control sequences mediate copper-induced transcription of the Saccharomyces cerevisiae copper-metallothionein gene

Transcription of the Saccharomyces cerevisiae copper-metallothionein gene, CUP1, inducible by copper. By analyzing deletion and fusion mutants in the CUP1 5'-flanking region, we identified two closely related, tandemly arranged copper regulatory elements. A synthetic version of one of these elements conferred efficient copper induction on a heterologous promoter when present in two tandem copies.

Elevated amounts of methotrexate-binding protein, different from normal dihydrofolate reductase, in a petunia MTX(R)-cell line

A petunia cell line, 1ECB, was previously isolated by the stepwise selection procedure, for resistance to methotrexate (MTX), an antimetabolite for the enzyme dihydrofolate reductase (DHFR). Using ammonium sulfate precipitates of cell lysates of cell line 1ECB and its parental cell line (WT), it was found that the mutant has an increase of 400 fold in (3)H-MTX binding capacity and a decrease in the affinity for MTX binding, at two orders of magnitude, in comparison with the WT. In addition, the DHFR specific activity in the mutant increased only moderately (5- to 10-fold), this activity is extremely sensitive to MTX inhibition as compared to the WT. It is evident that the MTX resistance of line 1ECB results mainly from overproduction of an MTX-binding protein which differs from the WT DHFR by four biochemical criteria. This protein may serve as a trap for the excess amounts of MTX to which the cells are exposed.

Amino-terminal fragments of delta 1-pyrroline-5-carboxylate dehydrogenase direct beta-galactosidase to the mitochondrial matrix in Saccharomyces cerevisiae

delta 1-Pyrroline-5-carboxylate (P5C) dehydrogenase, the second enzyme in the proline utilization (Put) pathway of Saccharomyces cerevisiae and the product of the PUT2 gene, was localized to the matrix compartment by a mitochondrial fractionation procedure. This result was confirmed by demonstrating that the enzyme had limited activity toward an externally added substrate that could not penetrate the inner mitochondrial membrane (latency). To learn more about the nature of the import of this enzyme, three gene fusions were constructed that carried 5'-regulatory sequences through codons 14, 124, or 366 of the PUT2 gene ligated to the lacZ gene of Escherichia coli. When these fusions were introduced into S. cerevisiae either on multicopy plasmids or stably integrated into the genome, proline-inducible beta-galactosidase was made. The shortest gene fusion, PUT2-lacZ14, caused the production of a high level of beta-galactosidase that was found exclusively in the cytoplasm. The PUT2-lacZ124 and PUT2-lacZ366 fusions made lower levels of beta-galactosidases that were mitochondrially localized. Mitochondrial fractionation and protease-protection experiments showed that the PUT2-lacZ124 hybrid protein was located exclusively in the matrix, while the PUT2-lacZ366 hybrid was found in the matrix as well as the inner membrane. Thus, the amino-terminal 124 amino acids of P5C dehydrogenase carries sufficient information to target and deliver beta-galactosidase to the matrix compartment. The expression of the longer hybrids had deleterious effects on cell growth; PUT2-lacZ366-containing strains failed to grow on proline as the sole source of nitrogen. In the presence of the longest hybrid beta-galactosidase, the wild-type P5C dehydrogenase was still properly localized in the matrix compartment, but its activity was reduced. The nature of the effects of these hybrid proteins on cell growth is discussed.

Structure and expression of a tandem duplication of the Drosophila metallothionein gene

A strain of cadmium-resistant Drosophila containing a chromosomal duplication of the metallothionein gene was isolated. This duplication is stably inherited in the absence of selective pressure, and larvae homozygous for it can produce approximately twice as much metallothionein RNA as wild-type larvae. The entire duplication was cloned within a 5.7-kilobase fragment; this fragment contained a direct, tandem repeat of 2.2 kilobases of DNA: 228 bases of 5' flanking DNA, the entire transcription unit, and 1.4 kilobases of 3' flanking sequences. The 3' region of the first repeated unit is joined to the 5' region of the second unit by a 6-base-pair segment we define as the novel joint. This joint forms part of a 10-base-pair inverted repeat of a segment within the 3' region of the first unit. Comparison of the sequences of the 5' and 3' boundaries revealed no extensive regions of similarity at a position corresponding to the novel joint, thus suggesting that a mechanism other than homologous recombination was involved in the origin of this duplication.

Tandemly duplicated upstream control sequences mediate copper-induced transcription of the Saccharomyces cerevisiae copper-metallothionein gene

Transcription of the Saccharomyces cerevisiae copper-metallothionein gene, CUP1, inducible by copper. By analyzing deletion and fusion mutants in the CUP1 5'-flanking region, we identified two closely related, tandemly arranged copper regulatory elements. A synthetic version of one of these elements conferred efficient copper induction on a heterologous promoter when present in two tandem copies.

Indigenous plasmids in Pseudomonas syringae pv. tomato: conjugative transfer and role in copper resistance

Twenty strains of Pseudomonas syringae pv. tomato were examined for the presence of plasmid DNA. P. syringae pv. tomato plasmids were grouped into five size classes: class A ranged from 95 to 103 kilobases (kb); class B ranged from 71 to 83 kb; class C ranged from 59 to 67 kb; class D ranged from 37 to 39 kb; and class E was 29 kb. All strains contained at least two plasmids in classes A and B. The conjugative ability of P. syringae pv. tomato plasmids in three strains was demonstrated by mobilization of the nonconjugative plasmid RSF1010 into Pseudomonas syringae pv. syringae recipients. Plasmids from the three conjugative strains were labeled with Tn5. Four conjugative plasmids were identified by their repeated transfer to P. syringae pv. syringae recipients. P. syringae pv. tomato strains varied in sensitivity to copper sulfate (CuSO4): MICs were 0.4 to 0.6 mM for sensitive strains, 1.2 mM for moderately resistant strains, and 1.6 to 2.0 mM for very resistant strains. One very resistant strain, PT23, functioned as a donor of copper resistance. Recipient P. syringae pv. syringae strains PS51 and PS61 were inhibited by 0.1 mM CuSO4, whereas the CuSO4 MICs for transconjugant strains PS51(pPT23A) and PS61(pPT23C) were 1.8 and 2.6 mM, respectively. P. syringae pv. tomato strains PT12.2 and PT17.2 were inhibited by 0.6 mM copper sulfate, but their copper sulfate MICs were 2.6 and 1.8 mM, respectively, when they acquired pPT23C. Therefore, copper resistance in PT23 was controlled by two conjugative plasmids, designated pPT23A (101 kb) and pPT23C (67 kb).

Meiotic gene conversion mutants in Saccharomyces cerevisiae. I. Isolation and characterization of pms1-1 and pms1-2

The pms1 mutants, isolated on the basis of sharply elevated meiotic prototroph frequencies for two closely linked his4 alleles, display pleiotropic phenotypes in meiotic and mitotic cells. Two isolates carrying recessive mutations in PMS1 were characterized. They identify a function required to maintain low postmeiotic segregation (PMS) frequencies at many heterozygous sites. In addition, they are mitotic mutators. In mutant diploids, spore viability is reduced, and among survivors, gene conversion and postmeiotic segregation frequencies are increased, but reciprocal exchange frequencies are not affected. The conversion event pattern is also dramatically changed in multiply marked regions in pms1 homozygotes. The PMS1 locus maps near MET4 on chromosome XIV. The PMS1 gene may identify an excision-resynthesis long patch mismatch correction function or a function that facilitates correction tract elongation. The PMS1 gene product may also play an important role in spontaneous mitotic mutation avoidance and correction of mismatches in heteroduplex DNA formed during spontaneous and UV-induced mitotic recombination. Based on meiotic recombination models emphasizing mismatch correction in heteroduplex DNA intermediates, this interpretation is favored, but alternative interpretations involving longer recombination intermediates in the mutants are also considered.

Selection and characterization of cadmium-resistant suspension cultures of the wild tomato Lycopersicon peruvianum

Suspension cultures of Lycopersicon peruvianum were selected for resistance to cadmium by stepwise exposure to increasing concentrations of cadmium sulfate. Resistant cells grow in 1500 micromolar Cd(++). This resistance was retained for thirty generations without selection. Both resistant and parental sensitive cultures take up Cd(++) at similar rates and to the same final levels. Exposure of sensitive or resistant cultures to Cd(++), Cu(++), or Zn(++) leads to the intracellular accumulation of a low molecular weight, cysteine-rich, cadmium-binding protein. This metallothionein is induced over fifteen fold by 100 μM cadmium and builds up to about five fold higher levels in the resistant cultures.

The possible role of metallothioneins in copper tolerance of Silene cucubalus

Growth and copper-binding of a copper-tolerant and a copper-sensitive population of Silene cucubalus (L.) Wib. have been studied. The copper-tolerant plants showed a much lower uptake and a proportionally higher transport of copper from root to shoot. A copper-binding protein with an apparent Mr of 8500 resembling metallothionein has been isolated from the roots of copper-treated plants of the tolerant population. After 20 d, the protein was observed to be inducible upon copper supply in the copper-tolerant plants, but not yet in the sensitive ones. This could be an indication of a difference in metalregulated synthesis of the protein. Ion-exchange chromatography of the 8500 protein yielded a major copper-containing fraction eluting at high ionic strength. Other characteristics such as UV absorption and amino-acid composition resembled strongly those of metallothioneins. The involvement of metallothioneins in the detoxification of copper within Cu-tolerant plants is discussed in relation to other mechanisms.

Copper metallothionein of yeast, structure of the gene, and regulation of expression

Addition of copper to yeast cells leads to the induction of a low molecular weight, cysteine-rich protein that binds copper. This protein, termed copper chelatin or thionein, is related to the metallothionein family of proteins that are induced in response to cadmium and zinc in vertebrate cells. We have determined the structure of the yeast copper-binding protein by DNA sequence analysis of the gene. Although the 6573-dalton yeast protein is substantially divergent from vertebrate metallothioneins, the arrangement of 12 cysteine residues, which is a hallmark of metal-binding proteins, is partially conserved. We analyzed the regulatory DNA sequence of the gene by fusing it with the Escherichia coli galactokinase gene and assaying the levels of enzyme activity in yeast in response to copper. The transcriptional activation has a specific requirement for copper. Zinc, cadmium, and gold were unable to regulate the galactokinase activity. The yeast copper metallothionein regulatory sequences represent a previously unreported class of yeast promoter that is regulated by copper.

Proline utilization in Saccharomyces cerevisiae: analysis of the cloned PUT2 gene

The PUT2 gene was isolated on a 6.5-kilobase insert of a recombinant DNA plasmid by functional complementation of a put2 (delta 1-pyrroline-5-carboxylate dehydrogenase-deficient) mutation in Saccharomyces cerevisiae. Its identity was confirmed by a gene disruption technique in which the chromosomal PUT2+ gene was replaced by plasmid DNA carrying the put2 gene into which the S. cerevisiae HIS3+ gene had been inserted. The cloned PUT2 gene was used to probe specific mRNA levels: full induction of the PUT2 gene resulted in a 15-fold increase over the uninduced level. The PUT2-specific mRNA was approximately 2 kilobases in length and was used in S1 nuclease protection experiments to locate the gene to a 3-kilobase HindIII fragment. When delta 1-pyrroline-5-carboxylate dehydrogenase activity levels were measured in strains carrying the original plasmid, as well as in subclones, similar induction ratios were found as compared with enzyme levels in haploid yeast strains. Effects due to increased copy number or position were also seen. The cloned gene on a 2 mu-containing vector was used to map the PUT2 gene to chromosome VIII.

Primary structure and transcription of an amplified genetic locus: the CUP1 locus of yeast

Copper resistance in yeast is controlled by the CUP1 locus. The level of resistance is proportional to the copy number of this locus, which can be found in up to 15 tandemly iterated copies. To elucidate the molecular mechanisms controlling the amplification and expression of the CUP1, locus, we determined its full nucleotide sequence. We have also identified and mapped two transcription units within the basic amplification unit of CUP1 in laboratory yeast strains. One of those transcription units is inducible by copper and encodes a low molecular weight copper binding protein--copper chelatin. The increased production of chelatin, due to both gene amplification and induction of transcription, leads to increased resistance of yeast cells to copper ions.

Industrial yeasts display tandem gene iteration at the CUP1 region

The gene copy number at the CUP1 locus and the resistance level to external copper was directly correlated in five wild-type commercial Saccharomyces strains. An increased copy number of the CUP1 gene leads to increased accumulation of chelatin mRNA, which codes for a low-molecular-weight, copper-binding protein. The enhanced production of this rapidly inducible protein mediates resistance of the cell to copper. Industrial yeasts exhibit homologies to the amplified copper resistance repeat unit found in laboratory strains. However, the extent of tandem iteration is strain dependent, and the repetitious unit is either 1.7 or 1.5 kilobases in length compared with the 2.0-kilobase unit in laboratory strains. Strain 522 (Montrachet) contains two chromosome VIII segments distinguishable by their numbers of repeat units (2 and 11) and the size of the units (1.5 and 1.7 kilobases). Distillers yeast 513 carries a 1.5-kilobase repeat unit on each homologous chromosome, although they contain nine and five iterations, respectively.

Proline utilization in Saccharomyces cerevisiae: analysis of the cloned PUT2 gene

The PUT2 gene was isolated on a 6.5-kilobase insert of a recombinant DNA plasmid by functional complementation of a put2 (delta 1-pyrroline-5-carboxylate dehydrogenase-deficient) mutation in Saccharomyces cerevisiae. Its identity was confirmed by a gene disruption technique in which the chromosomal PUT2+ gene was replaced by plasmid DNA carrying the put2 gene into which the S. cerevisiae HIS3+ gene had been inserted. The cloned PUT2 gene was used to probe specific mRNA levels: full induction of the PUT2 gene resulted in a 15-fold increase over the uninduced level. The PUT2-specific mRNA was approximately 2 kilobases in length and was used in S1 nuclease protection experiments to locate the gene to a 3-kilobase HindIII fragment. When delta 1-pyrroline-5-carboxylate dehydrogenase activity levels were measured in strains carrying the original plasmid, as well as in subclones, similar induction ratios were found as compared with enzyme levels in haploid yeast strains. Effects due to increased copy number or position were also seen. The cloned gene on a 2 mu-containing vector was used to map the PUT2 gene to chromosome VIII.

Industrial yeasts display tandem gene iteration at the CUP1 region

The gene copy number at the CUP1 locus and the resistance level to external copper was directly correlated in five wild-type commercial Saccharomyces strains. An increased copy number of the CUP1 gene leads to increased accumulation of chelatin mRNA, which codes for a low-molecular-weight, copper-binding protein. The enhanced production of this rapidly inducible protein mediates resistance of the cell to copper. Industrial yeasts exhibit homologies to the amplified copper resistance repeat unit found in laboratory strains. However, the extent of tandem iteration is strain dependent, and the repetitious unit is either 1.7 or 1.5 kilobases in length compared with the 2.0-kilobase unit in laboratory strains. Strain 522 (Montrachet) contains two chromosome VIII segments distinguishable by their numbers of repeat units (2 and 11) and the size of the units (1.5 and 1.7 kilobases). Distillers yeast 513 carries a 1.5-kilobase repeat unit on each homologous chromosome, although they contain nine and five iterations, respectively.