Essential Cell Biology 5th edition

From DNA to RNA
235
Initiation of Eukaryotic Gene Transcription Is a Complex
Process
Many of the principles we just outlined for bacterial transcription also
apply to eukaryotes. However, the initiation of transcription in eukaryotes
differs in several important ways from the process in bacteria:
1. While bacteria use a single type of RNA polymerase for transcription,
eukaryotic cells employ three: RNA polymerase I, RNA polymerase II,
and RNA polymerase III. These polymerases are responsible for
transcribing different types of genes. RNA polymerases I and III
transcribe the genes encoding transfer RNA, ribosomal RNA, and
various other RNAs that play structural and catalytic roles in the
cell (Table 7–2). RNA polymerase II transcribes the rest, including all
those that encode proteins—which constitutes the majority of genes
in eukaryotes (Movie 7.3). Our subsequent discussion will therefore
focus on RNA polymerase II.
2. Whereas the bacterial RNA polymerase (along with its sigma subunit)
is able to initiate transcription on its own, eukaryotic RNA polymerases
require the assistance of a large set of accessory proteins. Principal
among these are the general transcription factors, which must assemble
at each promoter, along with the polymerase, before transcription can
begin.
3. The mechanisms that control the initiation of transcription in
eukaryotes are much more elaborate than those that operate in
prokaryotes—a point we discuss in detail in Chapter 8. In bacteria,
genes tend to lie very close to one another, with only very short
lengths of nontranscribed DNA between them. But in plants and
animals, including humans, individual genes are spread out along
the DNA, with stretches of up to 100,000 nucleotide pairs between
one gene and the next. This architecture allows a single gene to be
controlled by a large variety of regulatory DNA sequences scattered
along the DNA, and it enables eukaryotes to engage in more complex
forms of transcriptional regulation than do bacteria.
4. Eukaryotic transcription initiation must deal with the packing of DNA
into nucleosomes and higher-order forms of chromatin structure, as
we describe in Chapter 8.
To begin our discussion of eukaryotic transcription, we take a look at the
general transcription factors and see how they help RNA polymerase II
initiate the process.
QUESTION 7–3
Could the RNA polymerase used for
transcription also be used to make
the RNA primers required for DNA
replication (discussed in Chapter 6)?
Eukaryotic RNA Polymerase Requires General
Transcription Factors
The initial finding that, unlike bacterial RNA polymerase, purified eukaryotic
RNA polymerase II cannot initiate transcription on its own in a test
tube led to the discovery and purification of the general transcription
TABLE 7–2 THE THREE RNA POLYMERASES IN EUKARYOTIC CELLS
Type of Polymerase
RNA polymerase I
RNA polymerase II
RNA polymerase III
Genes Transcribed
most rRNA genes
all protein-coding genes, miRNA genes, plus genes for
other noncoding RNAs (e.g., those of the spliceosome)
tRNA genes, 5S rRNA gene, genes for many other small
RNAs