Structure and expression of the human metallothionein-IG gene. Differential promoter activity of two linked metallothionein-I genes in response to heavy metals.

The human metallothionein (MT)-IG gene (hMT-IG) is tandemly linked in a head-to-head fashion with the hMT-IF gene. The hMT-IG gene encodes a MT-I polypeptide and has a tripartite structure. The 5'-flanking region of the hMT-IG gene has a TATAA box, four GC motifs, and at least four metal responsive elements. The 3'-untranslated region has a variation of the polyadenylation signal, AATTAA, and the 3'-flanking region a YGTGTTYY RNA processing signal. This gene is expressed in hepatoma-derived cell lines (Hep G2 and Hep3B2) in response to the heavy metals (cadmium, copper, and zinc) but not to the glucocorticoid analogue dexamethasone. In contrast, the lymphoblastoid cell line (Wi-L2) does not express the hMT-IG gene. These results suggest that the hMT-IG gene is regulated differentially and in a cell type-specific manner. Transient expression studies of the chloramphenicol acetyltransferase (CAT) gene under the transcriptional control of either the hMT-IG or hMT-IF promoter in Hep G2 cells has demonstrated that both promoters contain all the necessary cis-acting elements to elicit a similar pattern of heavy metal inducibility. However, the hMT-IG promoter in all instances is five times more active than the hMT-IF promoter. The differences in promoter activity of these genes could possibly be due to inherent differences in their basal level regulatory sequences. The expression of MT-IGcat in transfected Wi-L2 cells demonstrates that the hMT-IG promoter is not cell type-specific.