Modulation of transcription factor activity by a distant steroid modulatory element

Variations in the biological activity of antisteroids, as determined by their percent agonist activity, is a well known but poorly understood phenomenon. For example, in tyrosine aminotransferase (TAT) induction by the antiglucocorticoid dexamethasone 21-mesylate in rat hepatoma tissue culture cells, the percent agonist activity varies with the density of cultured cells. A 21-basepair sequence of the rat TAT gene has now been isolated which confers all of the induction properties of the endogenous TAT gene to homologous and heterologous promoters and genes. We call this 21-basepair sequence, which acts in concert with a trans-acting factor identified by gel shift experiments, a glucocorticoid modulatory element. The changes in induction properties were found to be independent of the fold induction by dexamethasone, thus arguing that the GME does not synergize with the glucocorticoid response element. A model incorporating this new element is advanced which can explain the observed variations of TAT induction and may be generally applicable for the mechanism of action of other steroid hormones.