Deletion experiments have defined two stretches of DNA (genetic elements), lying close to the promoter for a human gene for metallothionein, that separately mediate the induction of the gene by heavy metal ions, particularly cadmium, and by glucocorticoid hormones. The element responsible for induction by cadmium is duplicated, yet a single copy is fully functional; the element responsible for induction by glucocorticoid hormones is coincident with the DNA-binding site for the glucocorticoid hormone receptor.
The human metallothionein-IIA (hMT-IIA) gene contains an enhancer element within its 5' regulatory region. This enhancer element can compete with the SV40 enhancer for one or more cellular factors in vivo. The competition between the two elements is modulated by cadmium, an inducer of hMT-IIA transcription. The data presented are consistent with a model in which heavy metal ions control the ability of the hMT-IIA enhancer to bind a positive factor, leading to increased transcription. The same factor is required for maximal activity of the SV40 enhancer, which suggests that viruses utilize factors that have a normal role in cellular gene expression to control their own genes.
The transient expression of the human metallothionein-IIA (hMT-IIA) gene was examined after introduction into NIH-3T3 cells. A series of deletion mutants within the 5' flanking region of the hMT-IIA gene was constructed and fused to the structural sequences of the Herpes simplex thymidine kinase (TK) gene to generate hMTK chimeric genes. During transient expression, the hMTK gene is responsive to both ligands. In addition we find at least 92 bp of 5' flanking DNA are required for both transient expression and induction by Cd2+, in contrast to only 50 bp required for induction by the heavy metal ions of the same gene in stable transformants. This difference is due to a deletion of a regulatory element, located between nucleotides -70 to -90 of the hMT-IIA gene, which functions to maintain basal expression in the absence of inducers. This element is absolutely required for transient expression of the hMT-IIA gene.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.