We describe a modification of the tetracycline-inducible eukaryotic gene expression system with decreased basal levels of expression in HeLa cells. It employs the tetracycline-inducible transactivator and a tetracycline-regulated repressor fusion acting on the same promoter. To avoid heterodimerization or competition for the same DNA site, each was provided with different DNA recognition and/or protein dimerization specificities. We achieved active silencing in the uninduced state resulting in approximately 6-fold reduced levels of basal transcription and several hundred-fold activation of gene expression upon addition of tetracycline.

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