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.     

Structural and functional studies of the amino terminus of yeast metallothionein

Purified yeast copper-metallothionein lacks 8 amino-terminal residues that are predicted from the DNA sequence of its gene. The removed sequence is unusual for metallothionein in its high content of hydrophobic and aromatic residues and its similarity to mitochondrial leader sequences. To study the significance of this amino-terminal cleavage, several mutations were introduced into the metallothionein coding gene, CUP1. One mutant, which deletes amino acid residues 2-8, had a minor effect on the ability of the molecule to confer copper resistance to yeast but did not affect CUP1 gene regulation. A second mutation, which changes two amino acids adjacent to the cleavage site, blocked removal of the extension peptide but had no effect on copper detoxification or gene regulation. Immunofluorescence studies showed that both the wild-type and these two mutant proteins are predominantly cytoplasmic with no evidence for mitochondrial localization. The cleavage site mutation allowed isolation and structural characterization of a full length metallothionein polypeptide. The copper content and luminescent properties of this molecule were identical to those of the truncated wild-type protein indicating a homologous cluster structure. Moreover, the amino-terminal peptide was selectively removed by various endopeptidases and an exopeptidase suggesting that it does not participate in the tertiary fold. These results argue that the amino-terminal peptide is not required for either the structural integrity or biological function of yeast metallothionein.     

Versatile cassettes designed for the copper inducible expression of proteins in yeast

A series of yeast expression vectors and cassettes utilizing the CUP1 gene of Saccharomyces cerevisiae have been constructed. The cassettes contain multiple cloning sites for gene fusions and were created by inserting a 27-bp polylinker at the +14 position of the CUP1 gene. The cassettes are portable as restriction fragments and enable copper-regulated expression of foreign proteins in S. cerevisiae. In copper sensitive yeast, multiple copies of the CUP1 cassettes confer copper resistance due to the production of the copper metallothionein. Genes cloned into the CUP1 cassettes, however, usually prevent translation of the metallothionein leading to a loss of resistance. This could be useful for one-step cloning into yeast.     

The CUP2 gene product regulates the expression of the CUP1 gene, coding for yeast metallothionein.

The yeast CUP1 gene codes for a copper-binding protein similar to metallothionein. Copper sensitive cup1s strains contain a single copy of the CUP1 locus. Resistant strains (CUP1r) carry 12 or more multiple tandem copies. We isolated 12 ethyl methane sulfonate-induced copper sensitive mutants in a wild-type CUP1r parental strain, X2180-1A. Most mutants reduce the copper resistance phenotype only slightly. However, the mutant cup2 lowers resistance by nearly two orders of magnitude. We cloned CUP2 by molecular complementation. The smallest subcloned fragment conferring function was approximately 2.1 kb. We show that CUP2, which is on chromosome VII, codes for or controls the synthesis or activity of a protein which binds the upstream control region of the CUP1 gene on chromosome VIII. Mutant cup2 cells produced extremely low levels of CUP1-specific mRNA, with or without added copper ions and lacked a factor which binds to the CUP1 promoter. Integrated at the cup2 site, the CUP2 plasmid restored the basal level and inducibility of CUP1 expression and led to reappearance of the CUP1-promoter binding factor. Taken collectively, our data establish CUP2 as a regulatory gene for expression of the CUP1 metallothionein gene product.     

Regulation of the yeast metallothionein gene

To study regulation of the yeast CUP1 gene, we have employed plasmids containing the CUP1 regulatory sequences fused to the Escherichia coli galK gene. A comparison of galK expression from low- and high-copy-number CUP1/galK fusion plasmids demonstrated that both basal and induced levels of galactokinase (GalK) increase proportionately with plasmid copy number. Host strains with an amplified, single or deleted CUP1 locus were compared to look for effects of chromosomal CUP1 gene dosage on expression from the episomal CUP1 promoter. Basal GalK levels are similar in CUP1R and cupls hosts, but can be induced to higher levels in the cup1s than the CUP1R host. In contrast, in a strain deleted for the chromosomal copy of CUP1, synthesis of GalK is constitutive but can be induced to yet higher levels by copper. A hybrid vector, placing the CUP1 coding sequence under the control of a constitutive promoter, was constructed. Introduction of this hybrid CUP1 gene into the deletion host containing the CUP1/galK plasmid restores regulation. Thus, metallothionein, in trans, can effect repression of the CUP1 promoter. The possible roles of metallothionein and free copper in CUP1 regulation are discussed.     

Yeast metallothionein function in metal ion detoxification

A genetic approach was taken to test the function of yeast metallothionein in metal ion detoxification. A yeast strain was constructed in which the metallothionein locus was deleted (cup1 delta). The cup1 delta strain was complemented with normal or mutant metallothionein genes under normal or constitutive regulatory control on high copy episomal plasmids. Metal resistance of the cup1 delta strain with and without the metallothionein-expressing vectors was analyzed. The normally regulated metallothionein gene conferred resistance only to copper (1000-fold); constitutively expressed metallothionein conferred resistance to both copper (500-fold) and cadmium (1000-fold), but not to mercury, zinc, silver, cobalt, nickel, gold, platinum, lanthanum, uranium, or tin. Two mutant versions of the metallothionein gene were constructed and tested for their ability to confer metal resistance in the cup1 delta background. The first had a deletion of a highly conserved amino acid sequence (Lys-Lys-Ser-Cys-Cys-Ser). The second was a hybrid gene consisting of the sequences coding for the first 20 amino acids of the yeast protein fused to the monkey metallothionein gene. Expression of these genes under the CUP1 promoter provided significant protection from copper, but none of the other metals tested. These results demonstrate that there is significant flexibility in the structural requirements for metallothionein to function in copper detoxification and that yeast metallothionein is also capable of detoxifying cadmium under conditions of constitutive expression.     

Copper ions and the regulation ofSaccharomyces cerevisiae metallothionein genes under aerobic and anaerobic conditions

We have previously reported that theSaccharomyces cerevisiae CRS5 metallothionein gene is negatively regulated by oxygen. The mechanism of this repression was the focus of the current study. We observed that the aerobic repression ofCRS5 is rapid and occurs within minutes of exposing anaerobic cultures to air. Furthermore, theCUP1 metallothionein gene ofS. cerevisiae was found to be subject to a similar down-regulation of gene expression. We provide evidence that the aerobic repression of yeast metallothioneins involves copper ions and Ace1, the coppertrans-activator ofCUP1 andCRS5 gene expression. A functional Ace1 binding site was found to be necessary for the aerobic repression ofCRS5. Moreover, the aerobic down-regulation of the metallothioneins was abolished when cells were treated with elevated levels of copper. Our studies show that anaerobic cultures accumulate higher levels of copper than do aerobic cells and that this copper is rapidly lost when cells are exposed to air. In fact, the kinetics of this copper loss closely parallels the kinetics ofCUP1 andCRS5 gene repression. The yeast metallothionein genes, therefore, serve as excellent markers for variations in copper accumulation and homeostasis that occur in response to changes in the oxidative status of the cell.     

Copper ions and the regulation ofSaccharomyces cerevisiae metallothionein genes under aerobic and anaerobic conditions

We have previously reported that theSaccharomyces cerevisiae CRS5 metallothionein gene is negatively regulated by oxygen. The mechanism of this repression was the focus of the current study. We observed that the aerobic repression ofCRS5 is rapid and occurs within minutes of exposing anaerobic cultures to air. Furthermore, theCUP1 metallothionein gene ofS. cerevisiae was found to be subject to a similar down-regulation of gene expression. We provide evidence that the aerobic repression of yeast metallothioneins involves copper ions and Ace1, the coppertrans-activator ofCUP1 andCRS5 gene expression. A functional Ace1 binding site was found to be necessary for the aerobic repression ofCRS5. Moreover, the aerobic down-regulation of the metallothioneins was abolished when cells were treated with elevated levels of copper. Our studies show that anaerobic cultures accumulate higher levels of copper than do aerobic cells and that this copper is rapidly lost when cells are exposed to air. In fact, the kinetics of this copper loss closely parallels the kinetics ofCUP1 andCRS5 gene repression. The yeast metallothionein genes, therefore, serve as excellent markers for variations in copper accumulation and homeostasis that occur in response to changes in the oxidative status of the cell.