Facilitated Assembly of the Preinitiation Complex by Separated Tail and Head/Middle Modules of the Mediator

Abstract

Mediator is a general coactivator of RNA polymerase II (RNA pol II) bridging enhancer-bound transcriptional factors with RNA pol II. Mediator is organized in three distinct subcomplexes: head, middle, and tail modules. The head and middle modules interact with RNA pol II and the tail module interacts with transcriptional activators. Deletion of one of the tail subunits SIN4 results in derepression of subset of genes, including FLR1, by a largely unknown mechanism. Here we show that derepression of FLR1 transcription in sin4Δ cells occurs by enhanced recruitment of the mediator, as well as Swi/Snf and SAGA complexes. The tail and head/middle modules of the mediator behave as separate complexes at the induced FLR1 promoter. While the tail module remains anchored to the promoter, the head/middle modules are also found in the coding region. The separation of the tail and head/middle modules in sin4Δ cells is also supported by the altered stoichiometry of the tail and head/middle modules at several tested promoters. Deletion of another subunit of the tail module MED2 in sin4Δ cells results in significantly decreased transcription of FLR1, pointing to the importance of the integrity of the separated tail module in derepression. All tested genes exhibited increased recruitment of the tail domain; however, only genes with increased occupancy of the head/middle modules displayed also increased transcription. The separated tail module thus represents a promiscuous transcriptional factor that binds to many different promoters and is necessary for derepression of FLR1 in sin4Δ cells.

The Arabidopsis Mediator Complex Subunits MED16, MED14, and MED2 Regulate Mediator and RNA Polymerase II Recruitment to CBF-Responsive Cold-Regulated Genes

“…Med2p, Med3p, and Med15p in yeast make up the socalled "triad" of subunits that are attached to the remainder of Mediator via the MED16 subunit Sin4p (Li et al, 1995;Kang et al, 2001). The triad can function independently, though less effectively, when released from the Mediator complex by loss of MED16 (Galdieri et al, 2012). Responses that depend specifically on triad function might therefore be expected to be reduced by loss of MED16/SFR6 but abolished in sfr6 med2 double mutants, similar to that seen in yeast med15D med16D deletions (Kim and Gross, 2013).…”

Selective role of Mediator tail module in the transcription of highly regulated genes in yeast

“…This would be consistent with a recent study showing that loss of Med16/Sin4 causes reduced recruitment of the head module compared with the tail module at the induced FLR1 gene. 42 Further work is needed to determine whether and how the Med15/Med3/Med2 triad can function as an independent entity in transcriptional activation. , the tail module is still recruited to normally active genes.…”

Characterization of the Influence of Mediator Complex in HIV-1 Transcription

“…In humans, MED is composed of 28 proteins assembled in four distinct modules as follows: the "Head," which contacts the RNAP II; the "Middle" and "Tail" that transfer regulatory signals after interacting with transcription factors to the Head; and the "CDK8 module," including MED12 and MED13 that associate less stably to the whole complex and have activating and repressing functions (12)(13)(14). These different subunits can contact various transcriptional regulators and act together as an adaptor to convey transcription signals from activators to the general transcription machinery, helping the initiation of transcription by RNAP II (15)(16)(17).…”