Transcript Chapter 8 Expression of Human Genes

Chapter 8
Expression of Human Genes
Regulation of gene expression occurs at three levels:
A. At the transcriptional level
B. Post-transcriptional
C. Epigenetic and long range control
A. Control of gene expression by binding of trans-acting protein
factors to cis-acting regulatory sequences in DNA and RNA:
• Control by DNA-binding proteins
• Control by RNA-binding proteins
1. Ubiquitous transcription factors are required for transcription
by RNA polymerase I and III
• Requirements for transcription by RNA polymerase I (18S, 5.8S
and 28S RNA)
• Requirements for transcription by RNA polymerase III (tRNA
and 5S RNA)
2. Transcription of polypetide-coding genes require a set of cisacting elements and tissue- and developmental-specific transcription
factors
• RNA polymerase II is responsible for transcribing protein-coding
genes and certain snRNA genes.
3. Transcription factors contain conserved structural motifs that
permit DNA binding:
Transcription factors for humans have two distinct functions
• An activation domain
• A DNA-binding domain
Four types of transcription factors:
1. The leucine zipper motif
2. The helix-loop-helix motif
3. The helix-turn-helix motif
4. The zinc finger motif
4. Several mechanisms permit transcriptional regulation of gene
expression in reponse to external stimuli:
(a) Ligand-inducible transcription factors (for small hydrophobic
hormones such as steroid which diffuse through plasma membrane)
Transcription factors (often known as hormone nuclear receptors)
are activated by binding to a ligand then bind to a response element
located in the promoter regions of about 50-100 target genes and
activates their transcription.
The hormone nuclear receptors are characterized by two conserved
domains:
- a DNA binding domain (68 amino acid long). It contains zinc
fingers and binds to DNA as a dimer.
- a ligand binding domain (240 amino acid)
(b) Activation of transcription factors by signal transduction
For hydrophilic signaling molecules that cannot diffuse through
plasma membrane. Instead they bind to a specific receptor. Two
genral mechanisms permit transmission of signals from cellsurface receptors to the nucleus:
-i- Protein kinases are activated then translocated from the
cytoplasm to the nucleus where they phosphorylate target
transcription factors.
E.g. hormonal signaling through cyclic AMP pathway
-ii- Inactive transcription factors stored in the cytoplasm are
activated by phosphorylation and translocated into the nucleus.
E.g. activation of NF-kB via protein kinase C signaling
5. Translational control of gene expression can involve specific
recognition by RNA-binding proteins of regulatory sequences within
the untranslated sequences of RNA:
Several cis-acting regulatory elements, mostly are bound by transacting RNA-binding proteins and regulate at the translational level in
three ways:
-i- intracellular RNA localization
-ii- translational control of gene expression in response to external
stimuli. E.g. The IRE-binding protein regulates the production of
ferritin heavy chain and transferrin receptor by binding to ironresponse elements (IREs) in the 5’- or 3’-untranslated regions.
-iii- translational control of gene expression of gene expression
during early development. Upon fertilization no new mRNA is
transcribed until the 4-8 cell stage and regulation occurs at the
translational level for maternal mRNA synthesized during oogenesis.
B. Alternative transcription and processing of individual genes
1. Transcription of a single human gene can be initiated from a
variety of alternative promoters and can result in a variety of tissuespecific isoforms. This results in
• Tissue-specificity e.g. dystrophin gene
• Developmental stage-specificity e.g. the insulin-like growth
factor II gene
• Differential subcellular localization e.g. soluble and mebranebound isoforms
• Differential functional capacity e.g. the progesterone receptor
• Sex-specific gene regulation the Dnmt1 methyltransferase
gene
2. Human genes often encode more than one product as a result of
alternative splicing and alternative polyadenylation events.
E.g.
-i- Differential splicing in the WT1 Wilm’s tumor gene
-ii- Alternative polyadenylation of the calcitonin gene results
in tissue-specific products
3. RNA editing is a rare form of post-transcriptional processing
whereby base-specific changes are enzymatically introduced at the
RNA level. Types of RNA editing in humans:
(i) C---> U, occurs in humans by a specific cytosine deaminase
e.g. The expression of the human apolipoprotein B gene in the
intestine involves tissue-specific RNA editing
(ii) A ---> I, the amino group in in carbon 6 of adenine is replaced
by a carbonyl group. I then acts as a G. Occurs in some ligandgated ion channels.
(iii) U ---> C, in mRNA of the WT1 Wilms’ tumor gene
(iv) U ---> A, in alpha-galactosidase mRNA