Connection of core and tail Mediator modules restrains transcription from TFIID-dependent promoters

Abstract

The Mediator coactivator complex is divided into four modules: head, middle, tail, and kinase. Deletion of the architectural subunit Med16 separates core Mediator (cMed), comprising the head, middle, and scaffold (Med14), from the tail. However, the direct global effects of tail/cMed disconnection are unclear. We find that rapid depletion of Med16 downregulates genes that require the SAGA complex for full expression, consistent with their reported tail dependence, but also moderately overactivates TFIID-dependent genes in a manner partly dependent on the separated tail, which remains associated with upstream activating sequences. Suppression of TBP dynamics via removal of the Mot1 ATPase partially restores normal transcriptional activity to Med16-depleted cells, suggesting that cMed/tail separation results in an imbalance in the levels of PIC formation at SAGA-requiring and TFIID-dependent genes. We suggest that the preferential regulation of SAGA-requiring genes by tailed Mediator helps maintain a proper balance of transcription between these genes and those more dependent on TFIID.

Mediator is broadly recruited to gene promoters via a Tail-independent mechanism

“…Our results contrast with earlier studies that used gene deletions and steady state mRNA analysis and observed approximately equal numbers of upregulated and downregulated genes upon long-term Tail inactivation (Ansari et al, 2011;El_Khattabi et al, 2019;Knoll et al, 2018). To compare transcription defects caused by rapid depletion vs gene deletions, we measured changes in levels of newly synthesized mRNA in strains containing med15D or med16D as well as in strains containing degrons fused to other Tail subunits: Med5, Med16, Our Med16 depletion results contrast with a recent report showing that Med16 plays a largely negative role (Saleh et al, 2021). In this earlier study, ~90% and 70% of affected genes were upregulated by rapid Med16 depletion and deletion, respectively.…”

From structure to molecular condensates: emerging mechanisms for Mediator function

“…model exists, is composed of a Med2-Med3-Med15 triad accompanied by two distal subunits, Med5 and Med16 [28,29]. The triad, a stable subcomplex thought to have transcriptional capabilities independent of Mediator [30][31][32][33][34][35][36], is not present in C. thermophilum. Instead, a much larger Med15 subunit has been proposed to be the counterpart of the yeast Tail triad [27].…”

“…Importantly, it has been reported in S. cerevisiae that following disruption of scaffold subunits Med14 and Med17 or of the Tail subunit Med16, genome-wide binding of most Mediator subunits is strongly reduced with the exception of the Tail module triad (Med2-Med3-Med15). This triad, known to form a stable subcomplex able to be recruited to DNA by itself [30][31][32]36], was recently shown to also activate transcription independently of core Mediator [33][34][35]. Under the assumption that this Mediator-independent mechanism could be extended to mammals, a more intricate Tail might possibly act as a more stable and potent coactivator and maintain partial transcriptional levels in MED14-depleted conditions.…”