The mouse metallothionein-I (mMT-I) promoter is activated by the metal response element-binding transcription factor (MTF), which binds metal response elements (MREs) when stimulated with heavy metals. We analyzed eight K562 erythroleukemia cell clones, each carrying a single integrated copy of an mMT-I/β-geo construct, using a system that can independently assess the level of β-geo expression and the rate at which it is silenced. In these clones, basal expression and rate of silencing vary widely and independently with integration site. This implies that the rates of transcription and of silencing are separate properties determined by interaction of the regulatory elements of the transgene with the site of integration. Induction of the mMT-I promoter with zinc both increases expression level and strongly retards silencing of β-geo expression. At a given integration site, expression level and silencing are affected coordinately by induction. Taken together with earlier studies of distant metal-responsive elements, these results suggest that distance from the promoter may determine whether a factor can increase transcription rate. Stimulation of an MRE can both increase transcription and overcome repressive effects of chromatin; we suggest that these functions are linked
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