RNAPII transcription occurs in three steps: initiation, elongation, and termination. Initiation
involves the recognition of core promoter elements on DNA by RNAPII and general transcription factors and synthesis of the first few nucleotides of the RNA transcript. The steps in initiation were defined biochemically using purified factors and order-of-addition experiments, but larger assemblies of multiple factors may in fact engage promoters in vivo. A key step in initiation is promoter binding by TFIID, a multi-subunit factor containing TATA-binding protein (TBP) and TBP-associated factors (TAFs). Canonical promoters contain a TATA box (consensus sequence TATA(A/T)A(A/T)(A/G)) around 30 base pairs upstream of the transcription start site. However, TFIID binding, through additional core promoter elements recognized by TAFs, is also essential for transcription at non-TATA promoter sites. The TBP-promoter complex is stabilized by subsequent binding of TFIIA and TFIIB,
with TFIIB playing a key role in subsequent recruitment and positioning of RNAPII. The preinitiation form of RNAPII is thought to bind the promoter in a complex with TFIIF, which stabilizes the complex of RNAPII with the promoter and other initiation factors and helps to ensure that RNAPII initiates transcription at the appropriate location.
[Therapeutic Targeting of the General RNA Polymerase II Transcription Machinery]

Before RNA synthesis commences, RNAPII is also bound by Mediator, a 23-subunit assembly
that further stabilizes the initiating RNAPII complex and acts as a physical link to gene-specific transcriptional activators bound to regulatory sites on DNA. Mediator is a dynamic complex composed of head, middle, tail, and kinase modules [10]. The head and middle modules are essential for cell viability and global RNAPII transcription, and structural studies demonstrate that these modules make key contacts with the initiating RNAPII complex
[Therapeutic Targeting of the General RNA Polymerase II Transcription Machinery]

Mediator bound at the core promoter
[Therapeutic Targeting of the General RNA Polymerase II Transcription Machinery]