Mechanism of synergistic transcriptional transactivation by the human glucocorticoid receptor.

Induction of transcription from a promoter with two upstream glucocorticoid response elements is 10- to 20-fold greater than that from a similar promoter with only one response element. We have shown that interactions involving the major transactivation domain of the glucocorticoid receptor (tau 1) are the sole determinant of such synergistic transactivation by the receptor. The other transactivation domain of the receptor (tau 2) did not mediate synergistic transactivation, and therefore the ability to synergize is operationally distinct from the transactivation function per se. The level of synergistic transactivation observed in vivo can be accounted for by the level of cooperative DNA binding seen in vitro for a glucocorticoid receptor derivative containing only the tau 1 and DNA-binding domains. Cooperative DNA binding was also observed using a tau 1-DNA-binding domain protein, which was expressed in Escherichia coli and extensively purified. Therefore, it is likely that direct protein-protein interactions between tau 1 domains mediate the cooperative DNA binding. The role of cooperative DNA binding for synergistic transactivation in vivo is discussed in relation to other possible mechanisms.