2. Prokaryotic Regulation of Genes
Regulating Biochemical
Pathway for Tryptophan
Synthesis.
1. Produce something that
will interfere with the
function of the enzyme
in the pathway.
2. Produce a gene
regulator that can inhibit
the transcription of one
biochemical pathway
enzymes.
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3. 1. Eukaryotic cells have many more genes (i.e.
23,000 in human cells) in their genomes than
prokaryotic cells (i.e. average 3000).
2. Physically there are more obstacles to regulate
eukaryotic genes because there is so much
more DNA to manage. For example, eukaryotic
chromatin is wrapped around histone proteins.
3. In addition there are other nonhistone proteins
that are used in eukaryotic gene expression
that are not used in prokaryotic gene
expression.
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4. Operon and Prokaryotic Gene Expression
• Operon- A group of prokaryotic genes with
a related function that are often grouped
and transcribed together. In addition, the
operon has only one promoter region for
the entire operon.
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5. Operon and Prokaryotic Gene Expression
An operon is composed of the following:
• Structural genes- genes that are related and used in
a biochemical pathway.
• Promoter-The nucleotide sequence that can bind
with RNA polymerase to start transcription. This
sequence also contains the operator region.
• Operator-The nucleotide sequence that can bind
with repressor protein to inhibit transcription. 5
6. Regulator Genes and Repressors
• Regulator gene- This gene produces a
protein called a repressor that can inhibit
the transcription of an operon by attaching
to the operator.
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7. Interaction of Modulators and Repressors
• Repressors have allosteric properties.
Modulators can bind to the repressor at an
allosteric site changing the conformation of the
repressor, thereby activating or deactivating the
repressor. Usually the modulator is a product of
the biochemical pathway.
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8. Lactose and the Inducible lac Operon
• The lac operon is an example of an
inducible operon.
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Negative Gene Regulation
1. Inducible operon- the lac operon. This operon has
the ability to convert lactose into glucose and galactose.
This involves three structural genes
10. Absence of Lactose and the lac Operon
• If no lactose or allolactose is present, the repressor
protein is active, binding to the operator site. This
prohibits the RNA polymerase from transcribing the
operon.
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In all cells, there are genes that code for proteins. It would be wasteful if genes were expressed when there is not a need for the protein product.
Example - why make enzymes used to break glycogen into glucose if the cell has a reservoir of glucose molecules? OR if the cell has no glycogen?
Regulating Prokaryotic Genes
Prokaryotic regulation is different from eukaryotic regulation.
Prokaryotic gene regulation tends to be negative. Meaning that the gene is usually activated unless some regulator inhibits it or deactivates it.
Eukaryotic gene regulation is usually positive. Meaning that the gene is usually deactivated unless a regulator activates it or turns it “on”.
Prokaryotic Gene Expression
Prokaryotic genes with related function are often grouped together. This grouping is called an operon. All the related genes will be transcribed together. In addition, the operon has only one promoter region for the entire operon. An operon is composed of the following:
-Structural genes- genes that are related and used in a biochemical pathway.
-Regulator gene- This gene will produce a repressor protein that will turn the operon off or inhibit the transcription of the operon. Repressors have allosteric properties. Modulators can bind to the repressor at an allosteric site changing the conformation of the protein. The modulator is usually a product of the biochemical pathway.
Inducible Operon-Is an operon that is induced, or turned on, by a particular modulator that inhibits the repressor. Inhibiting the repressor will allow for the transcription of the operon. i.e. inducible operons are “off” unless something turns them “on” – inducing them to act.
Example lac operon
The products of the lac operon produces enzymes necessary to break down lactose into glucose and galactose. This allows the prokaryote to use the disaccharide lactose as an energy source when necessary. There is no need to use this gene unless lactose is present, so the gene is “off” unless lactose induces it to be “on.”
Three structural genes of the lac operon
lacZ-makes b-galactosidase an enzyme that breaks lactose into glucose and galactose
lacY-makes permease which increases the cell's permeability for lactose.
lacA-makes transacetylase whose function is unknown.
The purpose of the trp operon is to synthesize the amino acid tryptophan when needed.
Unlike inducible operons, repressible operons are “on” unless the presence of something actively turns them “off” – represses them to make them inactive.
Repressible operon-the trp operon. This is a repressible operon because it is “on” unless excess tryptophan is present. When the cell does not have tryptophan, the repressor enzyme is inactive. RNA polymerase can attach to the promoter region and transcribe the genes necessary for the synthesis of tryptophan. When tryptophan is present, however, it represses the operon or inhibits the transcription of the operon it by activating the repressor protein. The trp operon includes five structural genes that make five enzymes necessary for the synthesis of tryptophan. The repressor protein that controls this operon is inactive without tryptophan present. Tryptophan is called a corepressor as it attaches to the repressor.
The repressor protein becomes active when tryptophan is present. The repressor protein binds to the operator region blocking RNA polymerase from attaching to the promoter region.
