Nucleotide sequence requirements for transient expression of human metallothionein-IIA-thymidine kinase fusion 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.