Assembly of a mediator/TFIID/TFIIA complex bypasses the need for an activator.

Transcription in eukaryotic cells requires the remodeling of chromatin and the assembly of functional preinitiation complexes (PICs), which contain the general transcription factors (GTFs), RNA polymerase II (Pol II), and coactivators. Genetic and biochemical studies have implicated the multisubunit Mediator coactivator complex (Med) as a critical component of the PIC, a direct target of activators, and a checkpoint for regulated gene expression during differentiation, development (reviewed in ), signaling, and oncogenesis. In this report, we show that a complex containing the activator GAL4-VP16, Med, and TFIID/TFIIA (DA) recruits pol II and the remaining GTFs to a model promoter in vitro. A preassembled DAMed complex bypasses the requirement for an activator. We also demonstrate that coordinated assembly of DAMed is essential to establishing a functional PIC. We conclude that the DAMed complex generates a platform that supports activated levels of PIC assembly and transcription.