Constitutive transcription of the gene for metallothionein in a cadmium-resistant yeast.

A cadmium-resistant strain of Saccharomyces cerevisiae produces a cadmium metallothionein encoded by the CUP1 gene as does a copper-resistant strain. The mechanism of expression of the gene is inducible by copper ions in the copper-resistant strain. However, assays of CUP1-specific mRNA revealed that the transcription of the CUP1 gene in the cadmium-resistant strain is constitutive and the rate of transcription is further increased by exposure to cadmium or copper ions. This result was confirmed by the appearance of constitutive-expression segregants from diploid crosses between the cadmium-resistant strain and a strain with a reporter gene having the promoter of CUP1.