Transcript myoD

Section N—Regulation of transcription
in eukaryotes
N1 Eukaryotic transcription factors
N2 Examples of transcriptional
regulation
N1 Eukaryotic transcription factors
Transcription factor domain structure：
Transcription factors other than the general transcription
factors of the basal transcription complex were firt identified
through their affinity for specific motifs in promoters, upstream
regulatory elements of enhancer regions.
These factors have two distinct activities. Firstly, they bind
specifically to their DNA-binding site and, secondly, they
activate transcription. These activities can be assigned to
separate protein domains called activation domains and DNAbinding domains.
In addition, many transcription factor occur as homo- or
heterodimers, held togather by dimerization domains. A few
transcription factors have ligand-binding of an accessory small
molecule.
Transcription factor domain structure
Prokaryotes
• Helix-turn-helix
Eukaryotes
• Zinc finger
• Leucine zipper
• Helix-loop-helix
DNA-binding domains:
The helix-turn-helix domain
Characteristic:
This domain contains a 60amino acid homeodomain
which is encoded by a
sequence called the homebox
This domain consists of
four alpha-helices in which
helices II and III are at right
angles to each other and are
sparated by a beta-turn.
this domain binds so that
one helix, know as the
recognition helix, lies partly
in the major groove and
interacts with the DNA.
DNA-binding domains: the zinc finger domain
Many, many; examples:
estrogen receptor; TFIIIA
Zn++ coordinated by:
2 Cys + 2 His = C2H2-type
X Cys = CysX; X = 4 or 6
N-terminal end
Often motif repeated 2 to 13 times
Bind to DNA major grove
Dimerization domains: leucine zippers
Zipper: every 7th residue
is a Leu  Hydrophobic
interface
Leucine Zipper proteins contain a
hydrophobic leucine residue at every
sventh position in a region that is often
at the C-terminal part of the DNAbinding domain. These leucines lie in
an alpha-helical region and the regular
repeat of these residues forms a
hydrophobic surface on one side of the
alpha-helix with a leucine every
second turn of the helix.
Dimerization domains: leucine zippers
The N-terminal basic domains of each helix form a symmetrical structure
in which each basic domain lies along the DNA in opposite directions,
interacting with a symmetrical DNA recognition site so that the protein in
effect forms a clamp around the DNA.
Transcription activation domains
• acidic activation domains: Comparison of the transactivation domains of
yeast Gcn4 and Gal4, mammalian glucocorticoid receptor and herpes virus
activator VP16 shows that they have a very high proportion of acidic amino
acids. These have been called acidic activation domains or ‘acid blobs’ or
‘negative noodles’ and are characteristic of many transcription activation
domains. It is still uncertain what other features are required for these regions
to function as efficient transcription activation domains.
• Glutamine-rich domains: were first identified in two activatio regions of the
transcription factor SP1. As with acidic domains, the proportion of glutamine
residues seems to be more important than overall structure. Domain swap
experiments between glutamine-rich transcactivation regions from the diverse
transcription factors SP1 and the Drosophila protein Antennapedia showed
that these domains could substitue for each other.
• Proline-rich domains: have been identified in several transcription factors.
As with glutamine, a continuous run of proline residues can activate
transcription. This domain is found, for example, in the c-Jun, AP2 and Oct-2
transcription factors.
Repressor domains
Repression of transcription may occur by indirect
interference with the function of an activator. This
may occur by:
• Blocking the activator DNA-binding site.
• Formation of a non-DNA-binding complex
• Masking of the activation domain without
preventing DNA binding
Targets for transcriptional regulation
The presence of diverse activation domains raises the
question of whether the each have the same target in the basal
transcription complex or different target for the activation of
transcription. They are distinguishable from each other since
the acidic activation domain can activate transcription from a
downstream enhancer site while the proline domain only
activates weakly and the glutamine domain not at all. Proposed
targets of different transcriptional activators include:
• Chromatin structure;
• Interaction with TFIID through specific TAFIIS
• Interaction with TFIIB
• Interaction or modulation of the TFIIH complex activity
leading to differential phosphorylation of the CTD of RNA Pol II
N2 Examples of transcriptional regulation
Constitutive transcription factors：SP1
• SP1 binds to a GC-rich sequence with the consensus
sequence GGGCGC
• It is a constitutive transcription factor whose binding site is
found in the promoter of many housekeeping genes.
•It contains the zinc finger motifs and has been shown to
contain two glutamine-rich transactivation domains.
• SP1 have been shown to interact specifically with TAFII 110,
onw of thw TAFIIs which bind to the TATA binding protein to
make up TFIID.
Hormonal regulation: steroid hormone receptors
Many transcription factors are activated by
hormones which are secreted by one cell type
and transmit a signal to a different cell type.
One class of hormones, the steroid hormones,
are lipid soluble and can diffuse through cell
membranes to interact with transcripton
factors called steroid hormine receptors.
steroid
Inibitor
(HSP90)
Glucocorticoid
receptor
In the absence of the steroid hormone, the
receptors is bound to an inhibit, and located in
the cytoplasm.
The steroid hormone bind to the receptor and
releases the receptor from the inhibitor,
allowing the receptor to dimerize and
translocate to the nucleus. The DNA-binding
domain of the steroid hormone receptor then
interacts with its specific DNA-binding
sequence or response element, and this gives
reise to activation of the target gene.
Dissociation and
dimerization
Nuclear
translocation
Glucocorticoid
response element
Interferon-γ
Regulation phosphorylation
STAT proteins
Many hormones do not diffuse
into the cell. Instead, they bind to
cell-surface receptors and pass a
sgnal to proteins within the cell
through a process called signal
transduction. This process often
involves protein phosphorylation.
Interferon-γ induces phosphorylation
of a transcription factor called
STAT1α through JAK. When
STAT1αbecmes phosphorylated at a
specific tyrosine residue, it is able to
form a homodimer which moves from
the cytoplasm into the nucleus.
INF- γ
receptor
Unphosphorylated
STAT 1αmonoers
Phosphorylation
Dmerization
Phosphorylated
STAT1α dimer
Nuclear translocation
Response element
Transcription elongation：HIV Tat
Activated TFIIH phosphorylates CTD of RNA Pol II
Initiation complex
Cellular factors
Tat
Polymerase
Precious transcript loops
backwards to interact with
the initiation compex
Tat-TAR cellular factor
complex activates TFIIH
TAR stem-loop structure
Human immunodeficiency virus (HIV) encodes an activatir protein called Tat, which is
required for productive HIV gene expression. Tat binds to an RNA stem-loop structuire
called TAR, which is present in the 5’-untranslated region of all HIV RNAs, just after
the HIV transcription start site. The predominant effect of Tat in mammalian cells lies at
the level of transcription elongation.
Cell determination: myoD
Nucleus
myoD
Other muscle-specific genes
DNA
OFF
Embryonic
precursor cell
OFF
1 Determination. Signals from
OFF
myoD is a “master control”
gene: it makes
a
mRNA
transcription factor that can activate other
The cell is now
protein
ireversibly
muscle specific genes. MyoD
(transcription factor)
other cells activate a master
regulatory gene, myoD,
Myoblast
(determined)
2
determined
Differentiation. MyoD
protein activates
other muscle-specific
transcription factors, which
in turn activate genes for
muscle proteins.
The embryonic precursor
cell is still undifferentiated
mRNA
MyoD
Muscle cell
(fully differentiated)
The cell is now fully
differentiated
mRNA
Another
transcription
factor
mRNA
mRNA
Myosin, other
muscle proteins,
and cell-cycle
blocking proteins
Determination and differentiation of muscle cells
Nucleus
Master control gene myoD
Other muscle-specific genes
DNA
OFF
Embryonic
precursor cell
1
Myoblast
(determined)
成肌细胞2
Determination.
Signals from other
cells activate a
master regulatory
gene, myoD,
Differentiation. MyoD
protein activates
other muscle-specific
transcription factors, which
in turn activate genes for
muscle proteins.
OFF
OFF
mRNA
MyoD protein
(transcription factor)
mRNA
MyoD
Muscle cell
(fully differentiated)
Fig 21.10
The cell is now fully
differentiated
mRNA
Another
transcription
factor
The cell is now
ireversibly
determined to
become a
muscle cell.
mRNA
mRNA
Myosin, other
muscle proteins,
and cell-cycle
blocking proteins
Determination and differentiation of muscle cells
Nucleus
Master control gene myoD
Other muscle-specific genes
DNA
OFF
Embryonic
precursor cell
OFF
1 Determination. Signals from
other cells activate a master
regulatory gene, myoD,
OFF
mRNA
MyoD protein
(transcription factor)
Myoblast
(determined)
The cell is now
ireversibly
determined
Differentiation. MyoD
2 protein activates
other muscle-specific
transcription factors, which
in turn activate genes for
muscle proteins.
Muscle cell
(fully differentiated)
mRNA
MyoD
The cell is now fully
differentiated
mRNA
Another
transcription
factor
mRNA
mRNA
Myosin, other
muscle proteins,
and cell-cycle
blocking proteins
Embryonic development: homeodomain proteins
The homeobox is a
conserved DNA
sequence which encodes
the helix-turn-helix DNA
binding protein structure
called the homeodomain.
The homeodomain was
first discovered in the
transcription factors
encoded by homeptic
genes of Drosophila.
Homeotic genes


Regulatory
genes that
control organ
identity
Conserved
from flies to
mammals