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16.2 Prokaryotic Gene Regulation
The nucleus of the cell's cytoplasm is the location of the circular chromosomes.
All of the genes needed to use lactose as an energy source arecoded next to each other in the lac operon and transcribed into a single mRNA.
There are three types of regulatory molecules that can affect the expression of operons.
The cell makespressors and activators.
The genes they control are regulated by both repressors and activators.
The promoter site is where the activators bind to.
Inducers can be produced by the cell or in the environment.
Depending on the needs of the cell,ducers can either turn on or off transcription.
Escherichia coli are able to synthesise many of the amino acids they need to survive.
The genes are transcribed into a single messenger RNA, which is then translated to produce all five enzymes.
If there is tryptophan in the environment, E. coli doesn't need to synthesise it and the trp operon is turned off.
The switch controlling the operon is turned on when the availability of tryptophan is low.
There are five genes located next to each other in the trp operon.
Two tryptophan molecules bind to the operator sequence when tryptophan is plentiful.
This blocks the genes from being transcribed.
The genes are transcribed when tryptophan is not present.
The coding region, the trp operator and the trp promoter are important parts of the trp operon.
The promoter sequence is before the start of the transcription process.
The operator region is between the promoter and the start site.
The operator cannot be bound by the repressor alone.
Two tryptophan molecules bind to the trp repressor, which changes the shape of the repressor to a form that can bind to the trp operator.
The operator's binding of the tryptophan-repressor complex prevents the RNA polymerase from binding to the promoter.
The repressor does not bind to the operator when tryptophan is not present in the cell.
This video will show you more about the trp operon.
E. coli can use other sugar sources for fuel when there is a shortage.
New genes must be transcribed to process alternate sugars.
CAMP accumulates in the cell when there is a drop in sugar levels.
The cAMP molecule is involved in energy metabolism in E. coli.
When cAMP binding to CAP, the complex binding to the promoter region of the genes that are needed to use alternate sugar sources.
A CAP-binding site is located upstream of the promoter in these operons.
CAP binding increases the activity of the associated genes.
New genes are needed to use other sugars for fuel when glucose levels fall.
cAMP levels increase when the supplies of glucose are limited.
This cAMP binding to the CAPProtein is a positive regulator that binding to a promoter region upstream of the genes required to use other sugar sources.
The third type of gene regulation in prokaryotic cells occurs through inducible operons, which bind to the local environment and the needs of the cell.
A typical inducible operon is the lac operon.
E. coli is able to use other sugars as energy sources.
The Z gene of the lac operon breaks down lactose into sugars.
Two conditions must be met for the lac operon to be activated.
It must be very low or not at all.
The lac operon will only be transcribed when there is a lack of glucose and a lack of Lactose.
The lac operon is more effective in the absence of sugar.
Allolactose can bind to the lacor and change its shape so that it cannot bind to the lac operator.
It would be wasteful to synthesise the enzymes to process lactose if there were not enough sugars in the cell.
The lac operon is transcribed at a very low rate even when there is sugar in the air.
The expression of the lac operon is regulated so that it only occurs when there is a restriction on the amount of sugar in the blood.
By default, the trp operon is on in E. coli.
CAP fails to bind to the promoter sequence if there is sugar present.
If there is no Lactose, the repressor will bind to the operator to prevent transcription.
If either of the conditions is met, transcription is off.
The lac operon is only transcribed when there is a lack of glucose and a lack of Lactose.
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