Negative regulation of transcription in vitro by a glucocorticoid response element is mediated by a trans-acting factor.

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RESUMO

In vitro experiments with cell extracts prepared from a mouse mammary epithelial cell line demonstrated that a cis-acting glucocorticoid response element (GRE) of the mouse mammary tumor virus represses transcription from its homologous promoter. Competition transcription experiments, in which a molar excess of a restriction fragment that contains the GRE is added to the cell-free assay, revealed that a nuclear factor mediates in trans the negative regulation of mammary tumor virus transcription in vitro. Gel retention assays indicated that a factor in the extracts specifically recognizes the GRE. One unusual result of the gel retention studies was that heating the GRE probe to 65 degrees C before addition to a binding assay increases the formation of the specific protein-DNA complex 20-fold. Exonuclease III footprinting demonstrated that the sequences recognized by the factor are identical for either untreated or heat-treated probe. The footprinting also demonstrated that this factor recognizes sequences that are distinct from those recognized by the glucocorticoid receptor. A synthetic oligonucleotide based on the sequences identified by the footprinting experiments repressed the activity of a heterologous enhancer-promoter in vivo, as assayed by transient expression assays. We propose that this negative transcription element may control the basal level of expression of some glucocorticoid-modulated genes and may explain the insensitivity of certain tumor cells to steroid hormone action.

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