Mediators:
As stated above, activators regulate transcription by binding to transcription factors or RNAPII (8). However, there are some activators that do not directly bind to the pre-initiation complex, but rather relay their message through other protein factors called mediators.
In yeast, a sub-complex that associates with the CTD of the RNAPII serves as a mediator for the enzyme. This complex is composed of 20 proteins and serves to stimulate the phosphorylation function of TFIIH. As a result, this mediator complex serves to promote the transition from the initiation stage to the elongation stage of transcription.
Holoenzyme
In yeast, RNAPII was isolated in a holoenzyme form (1). This means the enzyme is bound to its cofactors: general transcription factors and the multi-protein mediator sub-complex. In prokaryotes, a similar holoenzyme is comprised of RNAP and its cofactor called σ-factor. It should be noted that there are various forms of eukaryotic holoenzymes depending on which mediator complexes are bound to the enzyme. For example, a common holoenzyme, Srb/mediator holoenzyme, contains many SRB proteins that make up its mediator sub-complex.