For the iron (Fe 2+) transport protein transferrin receptor. A stem loop structure in the mRNA acts as an iron response element and binds a 90 kDa protein in the absence of iron. The RNA and iron binding regions of the protein overlap so in the presence of iron the 90 kDa binding protein can no longer bind to the mRNA iron response element and the stem loop no longer occurs. Since the stem loop is at the 3’ end of the mRNA , the loop is stabilising of the mRNA , protecting it from degradation. In the presence of iron, the loop disappears and the mRNA is degraded by 3’ exonucleases.s
The binding of iron to the 90 kDa protein has opposite effects for ferritin. In this case, the stem loop is at the 5’ end. It inhibits translation by preventing ribosomes getting onto the mRNA and thus its disappearance stimulates transcription. Its removal leads to degradation of the mRNA and thus reduces translation.
mRNA stability. When milk protein synthesis is stimulated in the mammary epithelium at child birth, the rapid increase in casein level arising from the pituitary hormone prolactin results from increased transcription of the casein gene but also from stabilisation of its mRNA. In fact, the stabilisation of mRNA is an essential component of the rapid build up of casein protein and this sort of regulation is evident in many situations in which the production of a particular protein needs to be increased to a high level. The mechanism is not well understood. The poly A tail protects the mRA from 3’ degradation. Histone mRNA (histones are produced during the DNA synthetic phase of the cell cycle) do not have a poly(A) tail and are unstable. The sequence of bases in the 3’ untranslated region, especially runs of As and Us can affect the stability of individual mRNAs. However, the enzzymes that break down the RNA are not well characterised.
Protein stability. Ubiquitin system. 20 kda protein ubiquitin is activated by ATP It is linked by its C terminus to amino group on a lysine side chain in target protein. Enzyme is ubiquitin protein ligase. Up to 50 molecules of ubiquitin / target protein molecule. Ubiquitinylated protein molecule then degraded by proteosome. A large multiprotien complex (2000 kDa) The level of cell cycle regulatory proteins called cyclins are produced at the G1 to S phase boundary of the cell cycle. The proteins stimulate kinases which trigger DNA synthesis. Once this triggering has occurred, there is no further need for the cyclins and they are degraded by the ubiquitin system. This system allows for rapid removal of proteins. It is selective. Another method for degrading proteins, the lysozome is non-selective. It was originally thought that breakdown of proteins occurred in the lysozomes. However, reticulocytes which do not have lysozomes still break down abnormal proteins. The system involves a 76 amino acid residue protein called ubiquitin, because it is widespread in eukaryotic spp. It is also one of the most conserved proteins known, differing in only 3 AA between human and fruit fly. Attachment is to the C terminus of ubiquitin and this is transferred to the amino group of a lysine side chain of the protein. Many ubiquitins per target protein molecule. Proteasome is a 20 s/u multi protein complexcontaining at least 5 different proteolytic activities in the shape of a bi-capped hollow barrel. UBIQUITIN IS NOT DEGRADED.
دانشگاه علوم پزشكي وخدمات بهداش
Dr. Parvin Pasalar
Tehran University of Medical Sciences
To know and explain:
Regulation of Bacterial Gene Expression
Constitutive ( house keeping) vs. Controllable genes
OPERON structure and its role in gene regulation
Regulation of Eukaryotic Gene Expression at different levels:
Histon modifications(Chromatin Remodeling)
Increasing the number of gene copies (gene amplification)
Changing the rate of initiation of transcription
Changing the rate of initiation of translation
Using of Untranslating Region (UTR)
Cross talk between different regulatory pathways
Classification of gene with respect to
Constitutive ( house keeping) genes:
1- Are expressed at a fixed rate, irrespective to the cell
2- Their structure is simpler
1- Are expressed only as needed. Their amount may
increase or decrease with respect to their basal level in
2- Their structure is relatively complicated with some
Different ways for regulation of gene
expression in bacteria
1- Promoter recognition:
2-Transcription elongation( Attenuation)
OPERON in gene regulation of
Definition: a few genes that are controlled collectively by
Its structure: Each Operon is consisted of few structural
genes( cistrons) and some cis-acting element such as
promoter (P) and operator (O).
Its regulation: There are one or more regulatory gene
outside of the Operon that produce trans-acting factors such
as repressor or activators.
1- Catabolic (inducible) such as Lac OPERON
2- Anabolic (repressible) such as ara OPERON
3- Other types
The activity of an Operon in the presence or
the absence of repressor
Lac OPERON an inducible Operon
In the absence
In the presence
CRP or CAP is positive regulator of Lac
and some other catabolic Operons
In the presence of lac +
CRP= Catabolic gene regulatory Protein
CRP= cAMP receptor Protein
CAP= Catabolic gene Activating Protein
Trp OPERON a repressible example
In the absence of Trp
In the presence of Trp
Attenuation by different secondary RNA structure
Eukaryotic gene regulation occurs
at several levels
Control at DNA level by - 1
Heterochromatin is the most tightly packaged form of DNA.
transcriptionally silent, different from cell to cell
Methylation is related to the Heterochromatin formation
Small percentages of newly synthesized DNAs (~3% in
mammals) are chemically modified by methylation.
Methylation occurs most often in symmetrical CG sequences.
Transcriptionally active genes possess significantly lower levels of
methylated DNA than inactive genes.
Methylation results in a human disease called fragile X syndrome;
FMR-1 gene is silenced by methylation.
Control at DNA level by Histon - 2
Acetylation by HATs
and coactivators leads to
HDACs and corepressors
leads to heterochromatin
Control at DNA level by gene -3
Repeated rounds of DNA replication yield multiple
copies of a particular chromosomal region.
Control at transcription - 4
By using different sequences (promoter, enhancer or silencer
sequences) and factors, the rate of transcription of a gene is controlled
gene control region for gene X
Alternative splicing: A Role - 5
in Sexual Behavior in Drosophila
a. In Drosophila courtship, the male behaviors include: Following,
Singing & …
b. Regulatory genes (fruitless= fru) in the sex determination
pathways control these behaviors.
c. Physiologically, the CNS (central nervous system) is
responsible for key steps in male courtship behavior.) (fruitless)
The sex-specific fru mRNAs are synthesized in only a few
neurons in the CNS (500/100,000). The proteins encoded by
these mRNAs regulate transcription of a set of specific genes,
showing that fru is a regulatory gene. Its expression seems to
be confined to neurons involved in male courtship
Control at mRNA stability- 6
• The stem loop at 3’end is an’ iron response element’.
• The stem loop is stabilised by a 90 kDa protein in the
absence of iron and protects the mRNA from
90 kDa iron sensing protein ( aconitase)
Transferrin receptor mRNA
Degraded by 3’ nuclease
In the presence of iron, transferrin receptor
protein synthesis is reduced.
No iron :
Control at mRNA stability- 6
• A stem loop is stabilised by the 90 kDa protein in the absence
• This time, the stem loop is at the 5’ end of the mRNA.
• The presence of the stem loop prevents translation of this mRNA
by blocking the progress of the ribosomes along the mRNA.
• In the presence of iron, the hairpin is lost, the ribosomes can
translate the mRNA and ferritin protein synthesis is increased.
Control at mRNA stability- 6
• Some hormones which enhance the production
of proteins also increase the half life of the
Estrogen : ovalbumin
Prolactin : casein
t 1/2 from 2- 5hr to
Control at initiation of translation - 7
Specific sequences make specific secondary structures
Specific protein factors bind to these secondary structures
8-Regulation by protein stability
•Ubiquitin-dependent proteolysis. Cyclins control of cell cycle.
• Protein molecule is tagged for degradation by attachment of a
20 kDa protein, ubiquitin
• The stability of a protein depends upon its N-terminal
amino acid (the N-end rule).
N-terminal : For example arginine , lysine : protein t1/2 =
N-terminal : For example methionine, alanine, : t1/2 >20
Regulation by water soluble Hormones
Polypeptide hormones bind at the cell surface and activate
transmembrane enzymes to produce second messengers
(such as cAMP) that activate gene transcription.