Essential Cell Biology 5th edition

236 CHAPTER 7 From DNA to Protein: How Cells Read the Genome
(A)
(B)
(C)
TATA box
start of transcription
gene
TBP TFIID
TFIIB
TFIIF other factors
Figure 7–12 To begin transcription, eukaryotic RNA polymerase
II requires a set of general transcription factors. These factors are
designated TFIIB, TFIID, and so on. (A) Most eukaryotic promoters
contain a DNA sequence called the TATA box. (B) The TATA box is
recognized by a subunit of the general transcription factor TFIID,
called the TATA-binding protein (TBP). For simplicity, the DNA
distortion produced by the binding of the TBP (see Figure 7–13) is not
shown. (C) The binding of TFIID enables the adjacent binding of TFIIB.
(D) The rest of the general transcription factors, as well as the RNA
polymerase itself, then assemble at the promoter. (E) TFIIH pries apart
the double helix at the transcription start point, using the energy of
ATP hydrolysis, which exposes the template strand of the gene. TFIIH
also phosphorylates RNA polymerase II, releasing the polymerase from
most of the general transcription factors, so it can begin transcription.
The site of phosphorylation is a long polypeptide “tail” that extends
from the polymerase. Once the polymerase moves away from the
promoter, most of the general transcription factors are released
from the DNA; the exception is TFIID, which remains bound through
multiple rounds of transcription initiation.
TFIIE
TFIIH
RNA polymerase II
factors. These accessory proteins assemble on the promoter, where
they position the RNA polymerase and pull apart the DNA double helix
to expose the template strand, allowing the polymerase to begin transcription.
Thus, the general transcription factors have a similar role in
eukaryotic transcription as sigma factor has in bacterial transcription.
(D)
most of the
general
transcription
factors
(E)
P
ribonucleoside
triphosphates
(UTP, ATP, CTP, GTP)
P
RNA
TRANSCRIPTION
ECB4 e7.12-7.12
N
Figure 7–12 shows the assembly of the general transcription factors at a
promoter used by RNA polymerase II. The process begins with the binding
of the general transcription factor TFIID to a short segment of DNA
double helix composed primarily of T and A nucleotides; because of its
composition, this part of the promoter is known as the TATA box. Upon
binding to DNA, TFIID causes a dramatic local distortion in the DNA double
helix (Figure 7–13); this structure helps to serve as a landmark for the
subsequent assembly of other proteins at the promoter. The TATA box
is a key component of many promoters used by RNA polymerase II, and
it is typically located about 30 nucleotides upstream from the transcription
start site. Once TFIID has bound to the TATA box, the other factors
assemble, along with RNA polymerase II, to form a complete transcription
initiation complex. Although Figure 7–12 shows the general transcription
factors loading onto the promoter in a certain sequence, the actual order
of assembly probably differs somewhat from one promoter to the next.
Like bacterial promoters, eukaryotic promoters are composed of several
distinct DNA sequences; these direct the general transcription factors
where to assemble, and they orient the RNA polymerase so that it will
begin transcription in the correct direction and on the correct DNA template
strand (Figure 7−14).
Once RNA polymerase II has been positioned on the promoter, it must
be released from the complex of general transcription factors to begin
its task of making an RNA molecule. A key step in liberating the RNA
polymerase is the addition of phosphate groups to its “tail” (see Figure
5′
3′
A
A A A
G T
A
T
3′
5′
C
Figure 7–13 TATA-binding protein (TBP) binds to the TATA box
(indicated by letters) and bends the DNA double helix. TBP, a
subunit of TFIID (see Figure 7–12), distorts the DNA when it binds.
TBP is a single polypeptide chain that is folded into two very similar
domains (blue and green). The protein sits atop the DNA double helix
like a saddle on a bucking horse (Movie 7.4).