Transcript control of gene expression

CONTROL OF GENE EXPRESSION
The development of an
organism must involve the
switching on and off of genes
in an orderly manner.
This is not fully understood in
multi-cellular organisms, but
has been researched in
bacteria….
Genetic Switches in Bacteria
• Francois Jacob and Jacques
Monod worked with E.coli bacteria
in 1960
• E.coli grown in a medium
containing lactose produces the
enzyme lactase (β-galactosidase)
which hydrolyses lactose to glucose
and galactose
• lactose + lactase
glucose +
galactose
• Glucose and galactose can then be
used to supply energy to the
bacterium
• If there is no lactose in the medium,
the bacteria produce no lactase
• However, once lactose is added,
production of the enzyme (lactase)
begins
• By timing enzyme production to suit
the circumstances, energy is saved
• In order for lactose to be used it
must enter the cell first
• Lactose permease is a membrane
protein that enables lactose to enter
the cell
• Lactose permease production is
also induced by lactose
Jacob and Monod set out to
investigate the process of enzyme
induction
• This involved looking at mutant
bacteria which produced lactase
constantly (even in the absence of
lactose)
• The lactase produced by the
mutants was indistinguishable from
the ‘normal’ lactase
• This indicated that the gene
specifying the structure of the
enzyme was quite different from the
gene controlling its rate of
production
• i.e. there were 2 genes, one coding
for the structure and one coding for
the production of the protein (the
mutation was in the one coding for
the production)
Jacob and Monod distinguished
between 2 types of gene
• STRUCTURAL GENES
– Specify the amino acid sequence of
the polypeptide
– The genes coding for lactase and
lactose permease are structural
• REGULATOR GENES
– Which control the level of activity of
the structural genes
• The mutants produced lactase and
lactose permease constantly, which
showed they were controlled by the
same switch
• After mapping the genes, they
found that they were next to each
other on the bacterial chromosome
• These 2 genes and their ‘switch’
were acting as one unit – an operon
• This is the proposed mechanism
• http://www.sumanasinc.com/webco
ntent/anisamples/majorsbiology/lac
operon.html
The lac operon
• The lac operon is made up of regulator
genes, structural genes and a promoter
and operator region
• The regulator gene codes for a repressor
protein
• When lactose is absent, the repressor
protein binds to the operator region and
prevents RNA polymerase from
transcribing the gene
• When lactose is present it binds to the
repressor protein and changes its shape
• The inactivated repressor protein is then unable
to bind to the operator region of the operon
• Since the inactive repressor protein is unable to
bind to the operator region, RNA polymerase
(the enzyme responsible for the transcription of
genes) is now able to bind to the promoter
region of the operon
• RNA polymerase is now able to transcribe the
three enzyme genes (Z, Y, and A) into mRNA.
• With the transcription of these genes, the three
enzymes needed for the bacterium to utilize the
sugar lactose are now synthesized.
– The Z gene codes for lactase, an enzyme that breaks
down lactose into glucose and galactose.
– The Y gene codes for permease, an enzyme which
transports lactose into the bacterium.
• The transcription of the structural
genes allows the breakdown of
lactose to glucose and galactose
• As the lactose is broken down,
releasing energy, the repressor
protein is released and can bind to
the operator region again
• Transcription of the genes is once
again stopped
In summary:
• When lactose is not present, the genes in the lac operon
are not expressed.
• A repressor, which is always present in the cell, binds to
the lac operon and prevents transcription by blocking the
passage of RNA polymerase.
• However, when lactose is present, it binds to the
repressor and changes its shape, such that the repressor
can no longer bind to the operon.
• In this case, RNA polymerase proceeds along the operon
and transcribes all 3 genes.
• The product of these genes metabolises lactose,
releasing the repressor and allowing it to once again
block RNA polymerase.
• The lac operon is an inducible system, meaning that the
system is turned off until an inducer – lactose – arrives on
the scene
Control of gene expression in
eukaryotes
• This is much more complicated due
to the numbers and arrangement of
genes within the genome
• Several genes may be responsible
for one characteristic, and while
they may be clustered together, the
mechanisms that control them may
be located on different
chromosomes
• Eukaryotic genomes are much
larger
• They contain repetitive sequences
of DNA and introns (non-coding
regions which RNA splices out
during transcription)
• Each gene in eukaryotes has its
own set of regulatory sequences
that control its expression
Transcription control is based
on 2 fundamental control
sequences
• PROMOTERS
– Found before the binding site for RNA
polymerase, and allows it to bind to its
recognition site
• ENHANCERS
– Can be located at a great distance
from the gene, and can be before or
after the promoter region
Transcription factors
• In addition to promoters and
enhancers, there are a large
number of proteins called
TRANSCRIPTION FACTORS
involved in control
– Some bind to enhancers, some to the
promoter to increase transcription
• Most models of control of
transcription in eukaryotes involve
the formation of a complex between
enhancers, transcription factors, the
promoter and RNA polymerase
resulting in the formation of a loop
of DNA that initiates transcription
Eukaryotic gene expression
• http://highered.mcgrawhill.com/sites/0072437316/student_
view0/chapter18/animations.html#