Gene Regulation
Constitutive genes
Some genes need to be functioning at all time. Example:
house keeping genes which concern with basic metabolic
processes
Regulated genes
Some genes are function according to need
Turn on → inducible
Turn off → repressible
Inducible systems: gene expression is off
unless there is the presence of some
molecule (called an inducer) that allows it.
Repressible systems: gene expression is
usually turn on except in the presence of
some molecule (called a co-repressor) that
suppresses gene expression
Gene regulation in prokaryotes
simple, single cell organisms·
Genes are grouped together based on
similar functions into functional units called
operons
Operon: genes which regulated as group and
they often adjacent to each other.
Inducible operon
Example: lac operon in E. Coli
Function: To produce enzymes which break
down lactose
when lactose is present, they turn on and
produce enzymes
Lac operon consist of two components
1. repressor genes
2. Functional units
2.1. Promoter (P) - aids in RNA polymerase binding
2.2. Operator (O) - "on/off" switch - binding site for the repressor
protein
2. 3. functional genes: consist of three genes
a. lacZ produces B-galactosidase (break bond between glucose and
galactose)
b. lacY produces permease (allow to lactose to enter through cell
membrane from outer environment)
c. lacA produces B-galactosidase transacetylase.
Repressor gene (lacI): produces repressor protein
with two binding sites, one for the operator and
one for lactose
When lactose is present, an isomer of lactose,
allolactose, will also be present in small amounts.
Allolactose binds and changes the conformation of
the repressor protein so that it is no longer
capable of binding to the operator
Lac operon occur through two control:
1. Negative control
If lactose is not present
The repressor gene produces repressor, which binds to the
operator. This blocks the action of RNA polymerase, thereby
preventing transcription
if lactose is present:
The repressor gene produces repressor, which has a site for binding with
allolactose. The allolactose bind to repressor which become inactive
and incapable of binding with the operator, so the RNA polymerase
is bind to promoter and proceed transcription
2. positive control
Glucose is the sugar of choice of E. coli and
if glucose is in supply, then the bacteria will
preferentially break down glucose over lactose
• If glucose is present, the lac operon will
be repressed
• if the concentrations of glucose and lactose are
high, the concentration of cAMP will be low, CAP
(catabolite activator protein) will not be activated,
RNA polymerase will not be able to bind well to
the promoter, and the operon will be operating at
a very low level (i.e. almost off)
• However, if the concentration of glucose is low and
lactose is high, the concentration of cAMP will be
high, CAP will be activated and bind to the DNA
which will promote RNA polymerase binding and
initiate transcription
Repressible operon
trp Operon
It composed of :
1. Five genes (trpA, trpB, trpC, trpD, and trpE)
involved in the production of the amino acid
tryptophan
2. trpR produces an inactive repressor protein
• Accumulation of the end product (tryptophan)
represses synthesis of the enzymes
• Tryptophan binds to the inactive repressor
protein which changes to active form and binds to
the operator, repressing the operon
Gene Regulation in Eukaryotes
• Eukaryotic gene is monocistronic
• regulation reflects cell type and
developmental timing of gene expression
• eukaryotes use positive control more than
negative control.
I-Transcriptional Control
A. Transcription start site
This is where a molecule of RNA polymerase II binds.
B. The basal promoter
• Basal promoter contains TATA box where TATA-
binding protein bind
• It upstream promoter whose structure and
associated binding factors differ from gene to gene
C. Enhancers
Enhancer-binding protein bind to regions of DNA that are
thousands of base pairs away from the gene they control or
located upstream, downstream, or even within the gene they
control. Binding increases the rate of transcription of the gene
D. Silencers
Silencers are control regions of DNA that, like enhancers, may be
located thousands of base pairs away from the gene they control.
However, when transcription factors bind to them, expression of
the gene they control is repressed.
E. Insulators
• stretches of DNA (as few as 42 base pairs)
• Located between the enhancer(s) and promoter
or silencer(s) and promoter of adjacent genes or
clusters of adjacent genes.
• Their function is to prevent a gene from being
influenced by the activation (or) repression of its
neighbors
Example:
In mice (and humans), only the allele for insulin-like
growth factor 2 (Igf2) inherited from one's father is
active; that inherited from the mother is not - a
phenomenon called imprinting.
• The mechanism: the mother's allele has an insulator
between the Igf2 promoter and enhancer . But in the
father's allele the insulator has been methylated. so
the enhancer is now free to turn on the father's Igf2
promoter .
II- Post-Transcriptional Control
A. Split Genes
Most eukaryotic genes are split into segments
The genes stretched into two types of fragment:
1. Exon (coding sequence) which translated to protein so it transcript and
translate
2. Intron (non coding sequence) which transcript but not translate
Examples:
The gene for one type of collagen found in chickens is split into 52 separate
exons.
Even the genes for rRNA and tRNA are split by ribozyme (self splicing)
• In general, introns much longer than exons.
• An average eukaryotic exon is only 140 nts long, but may intron stretches
for 480,000 nucleotides.
B. Alternative polyadenylation
After splicing specific enzyme synthesize the poly(A) tail
at the RNA's 3' end. In some genes, these proteins may
add a poly(A) tail at any one of several possible sites.
Therefore, polyadenylation can produce more than one
transcript from a single gene (alternative
polyadenylation), similar to alternative splicing
Alternative polyadenylation can also
shorten the coding region, thus making the
mRNA code for a different protein, but this
is much less common than just shortening
the 3' untranslated region.
III- Translational Control
There are regions on the beginning of mRNA
which do not code for proteins. These are the
leaders.
Proteins and other molecules can bind to the
leader which can enhance or restrict ribosome
binding and thus translation
mRNA molecules in cytoplasm may be degraded and
recycled to make more RNA.
Example: poly A regulation in oocyte
After fertilization mRNA in egg shows significant
increase in translation without new mRNA synthesis.
Because stored mRNA has short A tail and it associated
with protein inhibit translation and to activate stored
mRNA, cytoplasmic polyadenylation enzyme adds about
150 A residues to make full length poly A tail and
initiate translation.
IV- Post-Translational Control
Protein Cleavage and/or
Splicing - the newly formed
protein is rarely functional as is.
They typically need to be
modified
Protein function can be
modified by addition of methyl,
phosphoryl or glycosyl groups.
