Ligand-specific transactivation of gene expression by a derivative of the human glucocorticoid receptor expressed in yeast

In this study we have reconstituted transactivation of gene expression by the human glucocorticoid receptor in the yeast, Saccharomyces cerevisiae. We have expressed the C-terminal half of the human glucocorticoid receptor (residues 415-777), the smallest derivative that can be expected to function as a ligand-dependent activator of transcription, in yeast cells. The function of the expressed protein has been assayed using a reporter gene consisting of the β-galactosidase gene from Escherichia coli fused to the yeast iso-1-cytochrome c promoter with a glucocorticoid-responsive element from the rat tyrosine aminotransferase gene upstream. Transactivation of expression from the reporter gene by the expressed receptor is seen only in the presence of steroid hormones with glucocorticoid activity and occurs via specific interaction of receptor with the glucocorticoid-responsive element upstream of the reporter gene. This result is different from those obtained for the estrogen receptor in which a similar derivative was not functional in yeast. This suggests that the well documented conservation of structure and function between steroid receptors may not extend to the transactivation domains. Our results also suggest that the mechanism by which receptors are sequestered in an inactive, non-DNA binding state in the absence of ligand may be functionally conserved in yeast. In support of this we show evidence that the expressed receptor is associated with the yeast molecular weight 90,000 heat shock protein as seen in mammalian cells.