section 26.4
Mechanisms of Gene Regulation
in
Eukaryotes
601
There are several different forms of both
a
and
fi
subunits
differing by only one or a few amino acids, and the forms
that are present depend on the stage of development of an
organism (Chapter 28). For example, the following tem-
poral sequence shows the subunit types present in humans
at various times after conception.
Subunit Embryonic
Fetal
Adult
type
(<8w k)
(8-41 wk)
(birth —►
henceforth)
a-like
f 2
—>
fi
a
a
/1-like
€
y G
and
y A
/3
and
S
Both the
a-
and /3-like genes form distinct gene clusters.
A property of each cluster is that the order of the genes is
the order in which they are expressed in development.
26.4 Mechanisms of Gene Regulation
in Eukaryotes
Gene expression in human cells is regulated primarily at
the level of transcription as it is in prokaryotic cells. How-
ever, because transcription is more complex in eukaryotic
cells, gene expression can be regulated at many different
stages:
1. Initiation of transcription,
2. Processing the primary transcript (this includes
capping, splicing out introns, addition of the polyA
tail, and RNA editing in which specific nucleotides
are changed),
3. Transport of the processed mRNA from the nucleus to
the cytoplasm,
4. Translation of the mRNA (synthesis of protein), and
5. Posttranslational modification of the protein(s).
Each of these regulatory steps is crucial to the proper
functioning of the cell and the organism.
Transcription Factors
The synthesis of mRNA molecules by RNA polymerase
II (RNAPII) is a multistep process requiring the interac-
tion of numerous proteins with DNA in the region of the
promoter (Chapter 25). However, before the expression
of a gene can be initiated, the
transcription factor
TFIID
must attach to the promoter. This is followed by the at-
tachment of other proteins, called
general transcription
factors (GTFs),
to DNA in the region of the promoter. A
description of the functions of some human GTFs is given
in Table 26-3. The various GTFs facilitate attachment of
RNAPII to the promoter at the correct nucleotide for ini-
tiation, destabilize the DNA at the promoter, and initiate
transcription; together the GTFs and RNAPII are called
TA BLE 26-3
The Role o f General Transcription Factors in Initiation
o f Gene Expression in Human Cells
Factor
Subunits*
Function
TFIID
13
Recognizes TATA box; recruits TFIIB
(composed of factors TBP and TAFs)
TFIIA
3
Stabilizes TFIID binding
TFIIB
1
Orients RNAPII to start site
TFIIE
2
Recruits TFIIH (helicase)
TFIIF
2
Destabilizes nonspecific
RNAPII-DNA interactions
TFIIH
9
Promotes promoter melting by
helicase activity
*RNAPII contains nine subunits.
the preinitiation complex. Once the first phosphodiester
bond has formed, transcription has been initiated.
The large number of GTFs required to initiate transcrip-
tion does not completely solve the transcription problem.
DNA is organized in chromatin and is wrapped around
histone proteins and tightly packaged into nucleosomes
(Chapter 24). These structures inhibit transcription as well
as DNA replication. A distinct class of transcription fac-
tors has been identified that modifies chromatin structure
so that transcription can occur. A complex of proteins
termed RSF (
remodeling and spacing factor)
facilitates
transcription initiation on chromatin templates
in vitro.
Another protein complex termed FACT (facilitates chro-
matin transcription) promotes elongation of RNA chains
through nucleosomes. Together, RSF and FACT disas-
semble nucleosomes and permit transcription initiation
to occur.
In addition to the GTEs that control the initiation of
transcription at the TATA box and the protein factors that
disassemble nucleosomes, other transcription factors are
required to regulate the expression of particular genes
or families of genes. Each transcription factor binds to
DNA at a specific sequence, which ensures specificity
(Table 26-4). The large number of transcription factors
have certain structural motifs that facilitate their binding
to DNA and, based on structural similarities, fall into four
distinct groups (Figure 26-7).
1.
Zinc finger:
The structure of the zinc finger motif
(discovered in TFIIA) consists of two cysteine
residues and two histidine residues separated by 12
amino acids. The two cysteine residues are separated
by two amino acids and the two histidine residues are
separated by three amino acids. The cysteine and
histidine residues are linked by a zinc ion and this
