The Yeast CUP1 Gene: A Model for Biological Detoxification of Heavy Metal Effluents
In Saccharomyces cerevisiae, CUP1 codes for a small copper-binding, cysteine-rich protein, copper chelatin. We cloned and sequenced the CUP1 gene and find that the protein encoded is similar but not identical to mammalian metallothionein. Strains sensitive to copper contain only a single gene copy, while resistant strains harbor many tandemly organized copies. Resistant laboratory strains contain 12, 13, or 14 copies while industrial strains have 2–16 repeats. Moderate selection for enhanced resistance yields strains with up to 28 copies. These strains retain their high copy numbers in the absence of continued selection. Copper chelatin is pro-duced by cells challenged with copper in direct proportion to the gene copy number. CUP1 genes are transcribed; mRNA levels being induced approximately 20-fold above the basal level by Cu++ ions. A strain’s basal level is directly related to its gene number. Copper uptake is extremely rapid; 50% of the maximal level is achieved in 15 seconds, and Weser (1979) showed that the metal ion is bound covalently.