Amplification of the metallothionein-I gene in cadmium- and zinc-resistant Chinese hamster ovary cells.

A subclone of Chinese hamster ovary cells, R40F, has been selected for its unusually high resistance to lethal concentrations of Cd and Zn. Although there is a 33% loss in Cd resistance when R40F cells are cultured in the absence of exogenous metals, the Zn resistance remains unaltered. These cells are 80% tetraploid and demonstrate an increased capacity for metallothionein protein synthesis. When compared to wild type cells cultured in the absence of exogenous metals, R40F cells exposed to 200 M Cd for 48 h exhibited an approximate 200-fold increase in metallothionein-I (MT-I) protein. A 32P-labeled mouse MT-I cDNA was employed in solution hybridization studies to measure the level of MT-I mRNA in wild type and R40F cells. Cd (0.5 M) induces MT-I mRNA about 2.5- and 5-fold in wild type and resistant Chinese hamster ovary cells, respectively. When optimally induced, the resistant cells have about 80-fold more MT-I mRNA than the sensitive cells. Southern blot analysis of HincII-cleaved DNA indicates that the MT-I gene is amplified approximately 60- to 75-fold in R40F cells.