Table 1 Mediator modules, subunits and interacting TFs and Pol II CTD
Mediator is a huge multi-protein complex (26 subunits in the human ). It is the primary regulator of the PEC, which includes TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH, Mediator and Pol II [27,79]. Mediator is divided into Head, Middle and Tail modules. The interactions and functions of some subunits have been partly elucidated [24–27,79]. The Table lists some of the known TF AD interactions . Because only scant structural data are available, we have not distinguished between yeast and human. The CDK8 submodule triggers Mediator conformational change , MED5 has acetyltransferase activity, and Tail MED14 was shown recently to bind the N-terminal domain of PPARγ . The partial list in this Table illustrates that a TF can bind multiple Mediator subunits even belonging to different modules (e.g. p53, GCN4); however, a TF can also require different subunits, as in p53 activation of p21 by Nutlin which does not require CDK8, whereas UV light C does . Such cases could relate to communication pathways. Secondly, TFs do not necessarily require the same subunits. For example, while MED1 is not required for PPARγ , it is required by, for example, C/EBPβ , PGC-1α  and ERα . Yeast and human subunit interaction maps have been constructed [35,40,41]. The Pol II CTD has up to 52 heptapeptide repeats (YSPTSPS) . Ser5 and Ser7 are phosphorylated by CDK7 (TFIIH) which is recruited (with TFIIB) by Mediator–Pol II complex before transcription initiation . Ser2 phosphorylation by CDK9 is required for elongation. Ser5 phosphorylation is required for initiation of transcription elongation. Submodule CDK8 phosphorylates CDK7. Key mechanistic questions relate to how TF–Mediator binding activates Pol II and how this relates to the CTD phosphorylation requirement. C/EBPβ, CCAAT/enhancer-binding protein β; E1A, early region 1A; ER, oestrogen receptor; GABP, GA-binding protein; Gli3, GLI family zinc finger 3; GR, glucocorticoid receptor; HNF-4, hepatocyte nuclear factor 4; PGC-1α, PPARγ co-activator-1α; RTA, replication and transcription activator; Sox9, SRY (sex-determining region Y)-box 9; RAR, retinoic acid receptor; TR, thyroid hormone receptor.