Essential Cell Biology 5th edition

From DNA to RNA
233
Signals in the DNA Tell RNA Polymerase Where to Start
and Stop Transcription
The initiation of transcription is an especially critical process because it is
the main point at which the cell selects which RNAs are to be produced.
To begin transcription, RNA polymerase must be able to recognize the
start of a gene and bind firmly to the DNA at this site. The way in which
RNA polymerases recognize the transcription start site of a gene differs
somewhat between bacteria and eukaryotes. Because the situation in
bacteria is simpler, we describe it first.
When an RNA polymerase collides randomly with a DNA molecule, the
enzyme sticks weakly to the double helix and then slides rapidly along its
length. RNA polymerase latches on tightly only after it has encountered
a gene region called a promoter, which contains a specific sequence of
nucleotides that lies immediately upstream of the starting point for RNA
synthesis. As it binds tightly to this sequence, the RNA polymerase opens
up the double helix immediately in front of the promoter to expose the
nucleotides on each strand of a short stretch of DNA. One of the two
exposed DNA strands then acts as a template for complementary basepairing
with incoming ribonucleoside triphosphates, two of which are
joined together by the polymerase to begin synthesis of the RNA strand.
Elongation then continues until the enzyme encounters a second signal
in the DNA, the terminator (or stop site), where the polymerase halts
and releases both the DNA template and the newly made RNA transcript
(Figure 7–9). The terminator sequence itself is also transcribed, and it is
the interaction of this 3ʹ segment of RNA with the polymerase that causes
the enzyme to let go of the template DNA.
start
site
gene
stop
site
5′
3′
3′
5′
DNA
RNA polymerase
promoter
RNA SYNTHESIS
BEGINS
template strand
terminator
5′
3′
5′
SIGMA FACTOR RELEASED
3′
5′
POLYMERASE CLAMPS FIRMLY DOWN ON DNA;
RNA SYNTHESIS CONTINUES
5′
3′
5′
3′
5′
growing RNA transcript
5′
TERMINATION AND RELEASE OF
BOTH POLYMERASE AND
COMPLETED RNA TRANSCRIPT
gene
terminator sequence
3′
SIGMA
FACTOR
REBINDS
3′
5′
3′
5′
Figure 7–9 Signals in the nucleotide
sequence of a gene tell bacterial RNA
polymerase where to start and stop
transcription. Bacterial RNA polymerase
(light blue) contains a subunit called sigma
factor (yellow) that recognizes the promoter
of a gene (green). Once transcription has
begun, sigma factor is released, and the
polymerase moves forward and continues
synthesizing the RNA. Elongation continues
until the polymerase encounters a sequence
in the gene called the terminator (red ).
After transcribing this sequence into RNA
(dark blue), the enzyme halts and releases
both the DNA template and the newly
made RNA transcript. The polymerase then
reassociates with a free sigma factor and
searches for another promoter to begin the
process again.