Putative zinc-sensing zinc fingers of metal-response element-binding transcription factor-1 stabilize a metal-dependent chromatin complex on the endogenous metallothionein-I promoter

The metalloregulatory functions of metal-response element-binding transcription factor-1 (MTF-1) have been mapped, in part, to its six highly conserved zinc fingers. Here we examined the ability of zinc finger deletion mutants of mouse MTF-1 to regulate the endogenous metallothionein-I (MT-I) gene in cells lacking endogenous MTF-1. MTF-1 knockout mouse embryo fibroblasts were transfected with expression vectors for FLAG-tagged MTF-1 (MTF-1(flag)) or finger deletion mutants of MTF-1(flag) and then assayed for metal induction of MT-I gene expression, nuclear translocation, and in vitro DNA-binding activity of MTF-1 and its stable association with the endogenous chromosomal MT-I promoter. Intact MTF-1(flag) restored metal responsiveness of the MT-I gene, underwent nuclear translocation, displayed increased in vitro DNA binding in response to zinc and less so to cadmium, and rapidly formed a stable complex with the MT-I promoter chromatin in response to both of these metals. In contrast, although deletion of finger 1, fingers 5 and 6, or finger 6 only had variable effects on the nuclear localization and in vitro DNA-binding activity of MTF-1, each of these finger-deletion mutants severely attenuated metal-induced MTF-1 binding to the MT-I promoter chromatin and activation of the endogenous MT-I gene. These results demonstrated that the metal-induced recruitment of MTF-1 to the MT-I promoter is a rate-limiting step in its metalloregulatory function and that an intact zinc finger domain is required for this recruitment. During the course of these studies, it was discovered that mouse MTF-1 is polymorphic. The impact of these polymorphisms on MTF-1 metalloregulatory functions is discussed.