Roles of zinc fingers and other regions of the transcription factor human MTF-1 in zinc-regulated DNA binding.

Mammalian metallothionein genes are transcriptionally regulated by heavy metals through cis-acting metal responsive elements (MREs). The MRE-binding transcription factor-1 (MTF-1), a protein containing six C(2)H(2)-type Zn fingers, is essential for MRE-mediated transcriptional activation. DNA binding of MTF-1 is known to be stimulated by Zn in vitro, but the binding was also largely influenced by redox conditions, suggesting that redox signals could modulate MTF-1 activity. To locate the functional domain required for Zn regulation, several deletion mutants of human MTF-1b, a newly cloned transcriptionally active MTF-1 variant, were characterized. This analysis showed that the N-terminal region and Zn-finger domain play roles in metal response. Functional roles of individual Zn fingers were estimated by co-transfection assays by using an MRE-driven reporter gene and vectors that express MTF-1b mutants each carrying one defective finger. Mutations in the N-terminal four fingers dramatically reduced the transcriptional activity, and at least for three of them the transcriptional defect was due to reduced DNA binding. These results indicate that the six Zn fingers are not functionally equivalent, probably sharing distinct roles such as direct DNA recognition and regulatory functions.