Constitutive and metal-inducible protein:DNA interactions at the mouse metallothionein I promoter examined by in vivo and in vitro footprinting.

A method of high resolution in vivo footprinting has been developed and used to survey the mouse metallothionein I (MT-I) promoter for protein : DNA interactions associated with basal-level transcription and with high-level metal-induced transcription. This promoter and its associated regulatory region is structurally complex. It contains multiple potential binding sites for metal regulatory factors and for other transcription factors, including SP1 and MLTF. In several cases potential recognition sites overlap, and the experiments reported here provide a view of which sites are utilized in vivo. These data also show how the pattern of protein : DNA contacts changes when cells are shifted from basal-level expression to metal-induced expression. The noninduced footprint pattern consists of interactions at basal elements that are thought to be responsible for the moderate transcription of this gene in the absence of added metals. These interactions remain unchanged upon metal induction. When MT-I expression is increased by exposing cells to zinc or cadmium, a new footprint pattern is observed. It includes the basal interactions and a new set of metal-dependent footprints that are positioned over all five genetically defined metal responsive elements (MREs), MRE-A--MRE-E. In addition, these data identify a sixth probable MRE, MRE-F, which displays a dimethylsulfate (DMS) footprint similar to that at other MREs.