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We present evidence that two distinct regions of the DNA upstream from the mouse metallothionein-I gene contain metal-responsive regulatory sites. This result was obtained by analyzing a systematic series of deletion, insertion, duplication, and clustered point mutations introduced into cultured cells on a simian virus 40 plasmid vector. The two upstream regions contain a duplicated evolutionarily conserved DNA sequence. While either upstream region is sufficient to confer heavy metal responsiveness, both are required to give maximal levels of induced transcription.
Carter et al. (Sat,) studied this question.