ZINC RAPIDLY INDUCES A METAL RESPONSE ELEMENT-BINDING FACTOR

Metal activation of metallothionein gene transcription is mediated by specific promoter sequences, termed metal regulatory elements (MREs). Nuclear extracts prepared from various human cell lines were assayed for their capacity to bind to a synthetic human MREa (hMREa) oligomer. Electrophoretic mobility-shift assays with extracts from control cells detected a single hMREa-containing complex. Addition to the growth medium of zinc, cadmium, or copper-metals known to induce MT biosynthesis in vivo-resulted in the rapid but reversible appearance of a second distinct hMREa-protein complex in all cell lines studied. This result was not seen when the metals were added directly to the extracts from control cells. DNA-binding protein blotting, UV crosslinking, and electroelution experiments were used to characterize the two hMREa-binding factors, termed BF1 and BF2. MRE-BF1 has an apparent molecular mass of almost-equal-to 86 kDa and binds to the hMREa in control cells, whereas MRE-BF2 consists of two molecules of almost-equal-to 28 kDa and binds to the hMREa in metal-treated cells. EDTA and o-phenanthroline inhibited binding of both factors to hMREa in a dose-dependent manner, indicating that a metal atom or atoms are essential for interaction of the factors with DNA.