Department of Cell Biology, Baylor College of Medicine, Houston, TX 77030.
The progesterone receptor (PR), like other members of the steroid receptor family, is a ligand-induced transcription factor. We have demonstrated previously that progesterone-induced binding of PR to a progesterone response element (PRE)-linked promoter stimulates RNA synthesis from that promoter in a cell-free transcription extract. It has been established that a hormone-mediated activation of PR beyond the removal of associated heat shock proteins is essential for efficient transactivation of the target gene. We now report that treatment with hormone leads rapidly to multiple phosphorylations of both the A and B forms of human PR in a HeLa nuclear extract. The putative kinase is present in the transcriptional extract but fails to phosphorylate the receptor significantly in the absence of specific hormone or DNA. Efficient phosphorylation of the PR occurs only in the presence of PREs, indicating that ligand-induced binding of PR to its cognate DNA response element makes it a preferred substrate for the kinase. The kinetics of the phosphorylation reaction overlap the kinetics of hormone-dependent RNA synthesis from a PRE-containing target promoter in vitro. We postulate that ligand and DNA-dependent phosphorylation of PR is an important functional event in the process leading to receptor-mediated transactivation of target genes.