The RNA polymerase II holoenzyme

In intact cells, about 20% of RNA polymerase II and
GTFs other than TFIID and TFIIA form a megadalton-
sized conglomerate known as the RNA polymerase
II holoenzyme. In addition to the polymerase itself
(the core polymerase) and general transcription factors,
the holoenzyme also contains Srb proteins and
(debatably) Swi/Snf proteins, as well as other known
and unknown proteins. A multisubunit complex containing
the nine known Srb proteins and TFIIF among
other proteins can be dissociated from a preparation
of the holoenzyme. This complex, known as the Srb/
mediator, participates in the promoter’s response to
transcriptional activators.
Several lines of evidence indicate that the holoenzyme
is the form of RNA polymerase II that initiates
transcription at most, if not all, promoters in vivo
(Koleske & Young, 1995). Biochemical data support a
model in which the assembled holoenzyme is recruited
to promoters at which TFIID is already bound.