Mediator requirement for both recruitment and postrecruitment steps in transcription initiation

Mediator complexes are required for activators to stimulate Pol II preinitiation complex assembly on an associated promoter. We show here that for the mouse Egr1 gene, controlled largely by MAP kinase phosphorylation of the ELK1 transcription factor, the MED23 Mediator subunit that interacts with phosphoELK1 is also required to stimulate Pol 11 initiation at a step subsequent to preinitiation complex assembly. In Med23(-/-) cells, histone acetylation, methylation, and chromatin remodeling complex association at the Egr1 promoter were equivalent to that of wild-type cells, yet Egrl induction was greatly reduced. MAP kinase activation stimulated Pol II and GTF promoter binding. However, the difference in factor binding between wild-type and mutant cells was much less than the difference in transcription, and Pol II remained localized to the promoter in mutant cells. These results indicate that an interaction with MED23 stimulates initiation by promoter bound Pol II in addition to Pol II and GTF recruitment.