Amplification of the metallothionein-I gene in cadmium-resistant mouse cells.

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Abstract
Friend leukemia cells resistant to Cd toxicity were selected. More than 70% of total cysteine incorporation in these cells was into the metal-binding protein, metallothionein. cDNA [complementary DNA] and genomic DNA clones containing the metallothionein-I gene were used to measure the concentration of its mRNA, the rate of gene transcription and the number of genes. On a per cell basis, optimally induced, Cd-resistant cells have 14-fold more metallothionein-I mRNA, a 6-fold higher rate of metallothionein-I gene transcription and 6-fold more metallothionein-I genes than do nonresistant cells. Metaphase spreads revealed that the resistant cells are nearly tetraploid and contain, on the average, 3 very small chromosomes that are absent from nonresistant Friend cells.