Abstract
We have reported previously (J. Koropatnick et al., Nucleic Acids Res 13:5423-5439, 1985) that metallothionein-1 (MT-1) genes in adult mouse liver undergo a two- to three-fold increase in average copy number within 6 hr of treatment of mice with high levels of cadmium salts. The extra copies persist for at least 3 weeks in the absence of subsequent doses of cadmium. We report here that amplified MT-1 genes, which are relatively nuclease resistant early (6 hr) after induction, undergo a change in chromatin structure that renders them nuclease sensitive within 3 days. The change in chromatin structure is accompanied by an increase in the rate of transcription of MT-1 genes to a level approximately twofold higher than that maximally inducible in mouse liver with low MT-1 gene copy number. These data indicate that extra copies of MT-1 genes induced to appear in adult mouse liver cells in situ are, like their counterpart in cultured somatic cells, transcriptionally competent and inducible. However, post-transcriptional events (possibly specific degradation of MT-1 mRNA) have an adverse effect on the level of gene expression at the mRNA level. Two possible mechanisms to explain the appearance of amplified MT-1 genes in organs in situ after heavy metal treatment are discussed. First, de novo amplification of extra MT-1 genes in all, or a subset, of mouse liver cells may be responsible. Alternatively, a portion of mouse liver cells that already possess extra MT-1 genes might be selected for preferential DNA replication.
