Requirement of TRAP/Mediator for Both Activator-Independent and Activator-Dependent Transcription in Conjunction with TFIID-Associated TAF_IIs

Abstract

The multiprotein human TRAP/Mediator complex, which is phylogenetically related to the yeast SRB/Mediator coactivator, facilitates activation through a wide variety of transcriptional activators. However, it remains unclear how TRAP/Mediator functions in the context of other coactivators. Here we have identified a previously uncharacterized integral subunit (TRAP25) of the complex that is apparently metazoan specific. An antibody that is specific for TRAP25 allowed quantitative immunodepletion of essentially all TRAP/Mediator components from HeLa nuclear extract, without detectably affecting levels of RNA polymerase II and corresponding general transcription factors. Surprisingly, the TRAP/Mediator-depleted nuclear extract displayed severely reduced levels of both basal and activator-dependent transcription from DNA templates. Both activities were efficiently restored upon readdition of purified TRAP/Mediator. Moreover, restoration of basal and activator-dependent transcription to extracts that were simultaneously depleted of TRAP/Mediator and TFIID (TBP plus the major TAF_IIs) required addition of both TBP and associated TAF_IIs, as well as TRAP/Mediator. These observations indicate that TAF_IIs and Mediator are jointly required for both basal and activated transcription in the context of a more physiological complement of nuclear proteins. We propose a close mechanistic linkage between these components that most likely operates at the level of combined effects on the general transcription machinery and, in addition, a direct role for Mediator in relaying activation signals to this machinery.

We thank Y. K. Kang for a purified TRAP (Flag-NUT2) preparation.

This work was supported by NIH grant CA42567 to R.G.R.