Assembly of general transcription factors to initiate transcription

The activation of polymerase II (Pol II) to transcribemost
eukaryotic genes (1) requires an initiation
complex assembled at the promoter. It
consists of general transcription factors (TF)
that associate in an ordered sequence. In the
first step (2), TFIID (transcription factor D for
polymerase II) binds to the TATA region. The
TATA box is recognized by a small, 30-kDa
TATA-binding protein (TBP), which is part of
one of the many subunits of TFIID (the bending
of the DNA by TBP is not shown here).
Following this, TFIIB can bind to the complex
(3). Subsequently, other transcription factors
(TFIIH, followed by TFIIE) and Pol II escorted by
TFIIF join the complex and assure that Pol II is
attached to the promoter (4). The binding of
TFIIE extends the polymerase binding sites
further downstream in the 3! direction. Pol II is
then released from the complex and transcription
can begin. A key step is phosphorylation of
Pol II by a subunit of TFIIH,which is a protein kinase.
Other activities of TFIIH involve a helicase
and an ATPase. The site of phosphorylation is a
polypeptide tail, composed in mammals of 52
repeats of the amino acid sequence YSPTSPS in
which the serine (S) and threonine (T) side
chains are phosphorylated.