DNA helicase hydrolyzes ATP, unwinds the DNA, exposing the template strand.
•Next, RNA polymerase II remains at the promoter synthesizing short lengths of RNA until it
undergoes a conformational change, moves away from the promoter and enters the elongation
phase of transcription.
oA key step in this transition is the addition of phosphate groups to the “tail” of the RNA
polymerase (known as the CTD or C-terminal domain).
oIn humans, the CTD consists of 52 tandem repeats of
a seven-amino-acid sequence, which extend from the
RNA polymerase core structure. During transcription
initiation, the serine located at the fifth position in
the repeat sequence (Ser5) is phosphorylated by
•The polymerase can then disengage the factors, and
Polymerase II Also Requires Activator, Mediator, and Chromatin
•Since eukaryotic DNA is packaged into nucleosomes, and further into higher-
order chromatin, it requires other proteins before the GTF attach to
the TATA box
•First, gene regulatory proteins known as transcriptional activators
must bind to specific sequences in DNA and help to attract RNA
polymerase II to the start point of transcription.
•Second, eukaryotic transcription initiation requires the presence of a protein
complex known as Mediator, which allows the activator proteins to
communicate properly with the polymerase II and with the GTFs.
•Finally, transcription initiation in a eukaryotic cell typically requires
the local recruitment of chromatin- modifying enzymes, including chromatin
remodelling complexes and histone-modifying enzymes.
Transcription Elongation Produces Superhelical Tension in DNA
•There is another barrier to elongating polymerases because of DNA supercoiling: a conformation