Transcript Control of Gene Expression

Control Mechanisms
for Gene Expression
Genes Gone Wild?!?!
Remember, it takes energy to do make
proteins and if they are not needed at
that moment, you shouldn’t waste
energy making them.
Some proteins are vital to cell survival
and are needed all the time –
housekeeping genes. Others are only
needed in specific circumstances.
Gene regulation is vital to an
organism’s survival. A cell must be
able to turn specific genes on or off
depending on that cell’s specific
requirements.
Eukaryotic Gene Control
Genes in eukaryotes are controlled in
four manners:
Transcriptional – factors can control
whether a gene is transcribed or not.
Post-transcriptional – the mRNA
modifications must be made before it can be
released.
Translational – Factors determine how often
and frequently the mRNA is translated by
ribosomes before the cytoplasmic enzymes
destroy it.
Post-translational – Even after the protein is
made it may not be activated right away or
release from the cell may be delayed if
necessary.
Prokaryotic Gene Control
Operons are clusters of genes that operate under the
control of a promoter and an operator. They usually look
after a specific job in the cell that requires several
enzymes in order to get the job done.
Operons are found mainly in prokaryotic cells but there
are instances in which lower eukaryotes possess
operons.
The promoter is a region within the DNA that controls
the transcription of a gene. It usually lies just upstream
of the gene it regulates.
The operator is an region within the DNA to which
regulatory proteins can bind. The operator lies just next
to (or overlaps) with the promoter.
Regulatory proteins bind to the operator to either
proceed with or stop transcription.
Parts of an Operon
The promoter is a region within the DNA that
controls the transcription of a gene. It usually
lies just upstream of the gene it regulates.
The operator is an region within the DNA to
which regulatory proteins can bind. The
operator lies just next to (or overlaps) with the
promoter.

The operator has a direct impact on the promoters
ability to allow transcription to proceed (or not).
Regulatory proteins bind to the operator to
either proceed with or stop transcription.

Examples of regulatory proteins are repressors and
corepressors – we will see what they do shortly!
The lac Operon
The lac operon looks after the
breakdown of lactose in bacterial
cells. The enzyme that breaks down
the lactose is called β-galactosidase.
As with any enzyme, it takes energy to
make, so it wouldn’t make much sense
for the bacterial cell to make this
enzyme unless lactose was present.
The cell uses a negative control
system to control the production of the
enzyme. Only a change within the cell
triggers the operon’s function.
The lac Operon
The successful breakdown of lactose depends on three genes
– lac Z, lac Y and lac A. These genes are located on the same
stretch of DNA along with the operon’s promoter and operator
regions, which overlap just a bit.
When lactose is not present in the cell, a repressor protein
called the LacI protein binds to the operator and covers part of
the promoter – they do overlap. This stops the RNA polymerase
from binding from the promoter and transcribing their codes.
The gene products are not made and the cell saves energy.
When the bacterial cell takes in some lactose, the lactose
acts as an inducer and binds to the LacI repressor and
changes its shape so it can no longer bind to the operator and
promoter. With the repressor no longer acting as a roadblock,
the RNA polymerase makes the copies of the genes and the
enzymes are made to breakdown the lactose.
Once the lactose is all used up (including the one acting as an
inducer), the LacI repressor goes back and binds to the
operator and covers part of the promoter and the genes are
basically shut down.
The lac Operon
The trp Operon
The trp operon controls the production of tryptophan – an
amino acid – and it consists of five genes, an operator and a
promoter.
It differs from the lac operon in that it shuts down when high
levels of tryptophan are present. There is no need to make if
it the cell already has it. This makes tryptophan the effector.
When tryptophan enters the cell is binds to a repressor
molecule to form the trp repressor-tryptophan complex. This
complex then binds to the operator and does not allow the
RNA polymerase to transcribe the genes of the operon.
Tryptophan is termed a corepressor because it is needed to
bind to the repressor in order to activate it.
When the tryptophan taken in by the cell is all used up –
including the one acting as a corepressor – the cell must
begin to make its own tryptophan again. Once the
corepressor is gone, the trp repressor falls off the operator
and the genes needed to make tryptophan can begin to
function again.
The trp Operon
That’s All I Got…